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Epidemiology of soil‐transmitted nematode infections in Ha Nam Province, Vietnam
Tropical Medicine & International Health
Abstract
Baseline epidemiological data are presented from a parasitological survey conducted in Thuy Loi commune, Ha Nam province, Vietnam; a farming community where night soil is routinely used as fertilizer for crops. 177 households were visited and 543 individuals (aged 1-88 years) recruited to the study. Helminth infection intensity was assessed by Kato-Katz to determine the density of parasite eggs per gram of stool (epg). Ascaris lumbricoides, Trichuris trichiura and hookworm infections were the only species detected. 83% of individuals were infected with A. lumbricoides (mean epg = 11,971), 94% with T. trichiura (mean epg = 793) and 59% with hookworm (mean epg = 302). Age-dependent patterns of infection prevalence and intensity were similar for A. lumbricoides and T. trichiura, but markedly different for hookworm infection. Similarly, age-dependency in the k-values for the three infections was due to covariance with the respective mean intensities with age rather than to independent age effects, with similar patterns for A. lumbricoides and T. trichiura, and a different pattern for hookworm. Three major conclusions can be drawn from the multiple-species analyses: There is positive interaction between A. lumbricoides and T. trichiura infections; high-intensity A. lumbricoides infections are significantly associated with high-intensity T. trichiura infections; and there is positive interaction between these two species such that infection intensity of A. lumbricoides is higher in individuals with concurrent T. trichiura infection than in individuals without and vice versa.
... Intestinal parasitic infections from soil-transmitted helminths such as hookworm (Necator americanus and Ancylostoma duodenale) , whipworm (Trichuris trichiura) and roundworm (Ascaris lumbricoides) are common in Vietnam (Needham et al., 1998 ;van der Hoek et al., 2003 ;Verle et al., 2003 ). Infections from these parasites cause diarrhoea, loss of appetite, reduced absorption of micronutrients, abdominal pain and fatigue. ...
... Vietnamese adults generally have a higher prevalence of helminths than infants (Trang, Mølbak, Cam, & Dalsgaard, 2007 ;Verle et al., 2003 ). Roundworm is the most common helminth affecting children under 5 years of age (Trang et al., 2007 ;Verle et al., 2003 ), although one study found a very high frequency of whipworm and roundworm in children by 5 years of age (Needham et al., 1998 ), while hookworm was present in lower rates in young children but at increasing levels with increasing age (Needham et al., 1998 ;Verle et al., 2003 ). ...
... Vietnamese adults generally have a higher prevalence of helminths than infants (Trang, Mølbak, Cam, & Dalsgaard, 2007 ;Verle et al., 2003 ). Roundworm is the most common helminth affecting children under 5 years of age (Trang et al., 2007 ;Verle et al., 2003 ), although one study found a very high frequency of whipworm and roundworm in children by 5 years of age (Needham et al., 1998 ), while hookworm was present in lower rates in young children but at increasing levels with increasing age (Needham et al., 1998 ;Verle et al., 2003 ). ...
In many ways, the current staged approach for exploring the issue of care in the past proposed in the bioarchaeology of care model can be used with any individual, from any time period and/or cultural background, regardless of their final age-at-death. Indeed, the care model can be trained on children, adults and the very old to great effect. This was demonstrated in the first published case study of Man Bac Burial 9 (MB9) (Journal of Paleopathology, 1(1):35–42; 2011), in as much as MB9’s care commenced in childhood and continued into his teens and until his death in his mid to late twenties. The question is whether the bioarchaeology of care model can be used to explore possible provision of health-related care for children when disease and its manifestations are rather less dramatic. Might this model – or an adaptation of it – be useful in exploring health-related care needs and provision for children at a population level, particularly in the contexts of elevated rates of childhood mortality and morbidity often found in prehistoric and early historic lifeways? This chapter explores this issue, focusing on behaviour within the same population that raised and cared for MB9. The value of returning to the Man Bac population is that care has already been clearly demonstrated for one severely physically disabled individual, thus establishing a capacity for caregiving among some members of this community, at least. This chapter considers whether, in a situation where skeletal evidence suggests elevated rates of subadult morbidity and mortality, this capacity for intensive levels of care provision was invested in all children.
... Those with high worm burden tend to be clustered within households or families (Bethony et al., 2006;Dold and Holland, 2011). Infection with 1 type of STH increases the likelihood of infection with others, most commonly between T. trichiura and A. lumbricoides (Needham et al., 1998). Although age is 1 of the key factors associated with STH exposure, with most infections occurring in school-aged children (Galvani, 2005), many studies also document high levels of infection among adults, amongst whom infection also shows an overdispersed pattern (Needham et al., 1998;de Silva et al., 2003;Blackwell et al., 2011;Dold and Holland, 2011). ...
... Infection with 1 type of STH increases the likelihood of infection with others, most commonly between T. trichiura and A. lumbricoides (Needham et al., 1998). Although age is 1 of the key factors associated with STH exposure, with most infections occurring in school-aged children (Galvani, 2005), many studies also document high levels of infection among adults, amongst whom infection also shows an overdispersed pattern (Needham et al., 1998;de Silva et al., 2003;Blackwell et al., 2011;Dold and Holland, 2011). However, few studies (Fitton, 2000;Godoy and Cardenas, 2000;Tanner et al., 2009) have been conducted that test these patterns among indigenous populations experiencing social and cultural change associated with rapid economic development and market integration (MI; the emergence of and increased dependence on market-based systems of exchange, resulting in increased consumer goods ownership, processed food consumption, and changes to housing structure and materials). ...
... We predict that as a group, Shuar will have moderate to high prevalence and intensity of STH infection based on standard definitions (Montresor et al., 1998), because of their local ecology and geographic location within NTD hot spots (Hotez et al., 2008). We also expect to see an overdispersed distribution with a few individuals harboring most of the population's worm burden, because of the proposed nature of STH infection (Needham et al., 1998;Bethony et al., 2006;Dold and Holland, 2011). ...
Abstract Soil-transmitted helminth (STH) infections can result in a variety of negative health outcomes (e.g., diarrhea, nutritional deficiencies). Market integration (MI; participation in market-based economies) has been suggested to alter levels of STH exposure due to associated changes in diet, sanitation, and behavior, but the effects are complicated and not well understood. Some effects of economic development result in decreased exposure to certain pathogens, while other factors can lead to higher pathogen exposure. Using geographic location as a proxy, the present study investigates the effects of economic development on parasite load among an indigenous population at multiple points along the spectrum of MI. This research has many implications for public health, including an increased understanding of how social and economic changes alter disease risk around the world and how changing parasite load affects other health outcomes (i.e., allergy, autoimmunity). Specifically, this study examines the prevalence of intestinal helminths among the Shuar, an indigenous group in the Morona-Santiago region of Ecuador, from 2 geographically/economically separated areas, with the following objectives: 1) report STH infection prevalence and intensity among Shuar; 2) explore STH infection prevalence and intensity as it relates to age distribution in the Shuar population; 3) compare STH infection patterns in geographically and economically separated Shuar communities at different levels of MI. Kato-Katz thick smears were made from fresh stool samples and examined to determine STH presence/intensity. Results indicate that 65% of the 211 participants were infected with at least one STH. Twenty-five percent of the sample had coinfections with at least 2 species of helminth. Infection was more common among juveniles (<15 yrs) than adults. Infection prevalence and intensity was highest among more isolated communities with less market access. This study documents preliminary associations between STH infection and exposure to MI, with implications for public health research and interventions.
... The main parasites that cause STH infection are the hookworms (Ancylostoma duodenale and Necator americanus), Ascaris lumbricoides and Trichuris trichiura. A. lumbricoides and T. trichiura infections are most common amongst school aged children and the prevalence usually decreases with age (Needham et al., 1998). Heavy hookworm infections can also occur in childhood, but frequency and intensity is usually higher in adulthood and is very common among older populations (Needham et al., 1998;Galvani, 2005). ...
... A. lumbricoides and T. trichiura infections are most common amongst school aged children and the prevalence usually decreases with age (Needham et al., 1998). Heavy hookworm infections can also occur in childhood, but frequency and intensity is usually higher in adulthood and is very common among older populations (Needham et al., 1998;Galvani, 2005). A recent systematic review shows that single dose albendazole has high cure rates for hookworm and A. lumbricoides infections. ...
... children, adults, pregnant women or women of reproductive age). Generally, studies in northern Viet Nam have found that A. lumbricoides and T. trichiura infections are more common in school children than adults (Verle et al, 2003) and women have a higher hookworm burden than men (Needham et al., 1998). We also found a very high prevalence of hookworm infections (76.2% at baseline) compared to A. lumbricoides (19.2%) and T. trichiura (29.1%) in women of reproductive age in Yen Bai province. ...
Soil-transmitted helminth (STH) infections are endemic in northern Viet Nam where the climate and agricultural practices, such as the use of human excreta as fertiliser and the use of wastewater for irrigation, favour transmission. An intervention was conducted in Yen Bai Province, north-west Viet Nam, to measure the effectiveness of single dose albendazole (400mg) administered every 4 months for reducing the prevalence of STH infections in women of reproductive age. Stool samples were collected from women before the intervention and 3 and 12 months post-intervention. Information on a range of demographic and socio-economic variables was also collected to measure the major risk factors for high STH burden in this area. The prevalence of hookworm, Ascaris lumbricoides and Trichuris trichiura infection in the baseline sample of 366 women were 76.2%, 19.2% and 29.1%, respectively. In the women who were surveyed at baseline and again at 3 and 12 months after the intervention (n=118) cure rates were 71.3% for hookworm, 87.0% for A. lumbricoides and 81.4% for T. trichiura by the end of the 12 month study period (i.e. after three doses of albendazole). The main risk factor for hookworm infection was if women worked outside (odds ratio (OR)=3.2 (95% Confidence Interval (CI) 1.6-6.2), P=0.001) and the major risk factor for A. lumbricoides and T. trichiura infection was a lack of education. Low educational attainment was also the strongest risk factor for co-infection with all three species of STH (OR=7.5 (95% CI 3.4-16.4), P<0.001). The high rates of hookworm infection in this area of Viet Nam and the high cure rates for all three species of STH with 4 monthly albendazole treatment suggest that this programme should be expanded to all endemic areas in Viet Nam. The study also highlights the important contribution of education to women's health.
... Experimental high-dose infections present a scenario in which primary STH infection is limited in duration, characterized by an immediate and potent Th2-polarized immune response, and generates sterilizing immunity to subsequent challenges. However, epidemiological evaluation of STH burden in human populations shows that in endemic areas, infected individuals suffer chronic parasitism throughout their lives (79)(80)(81)(82). This holds true for non-human primates (83)(84)(85), livestock (86,87), and wild rodent populations (88,89). ...
... This holds true for non-human primates (83)(84)(85), livestock (86,87), and wild rodent populations (88,89). In humans, infection burden correlates strongly with age following one of two patterns: (i) parasite burden builds rapidly during early childhood but peaks shortly before adolescence, burden then declines and plateaus at a low level throughout adulthood e.g., Trichuris trichiura, and Ascaris lumbricoides (79,90,91); or (ii) STH burden builds consistently throughout childhood and early adolescence but plateaus at a moderate level prior to adulthood e.g., Necator americanus and Ancylostoma duodenale (82). Both patterns indicate that protective immunity to infection develops with age. ...
Infection with soil-transmitted helminths (STH) remains a major burden on global health and agriculture. Our understanding of the immunological mechanisms that govern whether an individual is resistant or susceptible to infection is derived primarily from model infections in rodents. Typically, experimental infections employ an artificially high, single bolus of parasites that leads to rapid expulsion of the primary infection and robust immunity to subsequent challenges. However, immunity in natura is generated slowly, and is only partially effective, with individuals in endemic areas retaining low-level infections throughout their lives. Therefore, there is a gap between traditional model STH systems and observations in the field. Here, we review the immune response to traditional model STH infections in the laboratory. We compare these data to studies of natural infection in humans and rodents in endemic areas, highlighting crucial differences between experimental and natural infection. We then detail the literature to date on the use of “trickle” infections to experimentally model the kinetics of natural infection.
... People living in poor areas of the tropics commonly harbour multiple parasitic infections, including infection with multiple helminth species [1,2]. An increasing number of studies demonstrate that individuals infected with multiple helminth species tend to harbour the most intense infections [3][4][5][6][7][8][9][10][11] and can be at an increased risk of infection-related morbidity [12][13][14][15]. For example, a study of Brazilian school children showed those harbouring concomitant infection with Ascaris lumbricoides and Trichuris trichiura were at increased risk of stunting [16], whilst another Brazilian study found the risk of anaemia among school children infected with Schistosoma mansoni and two or three soil-transmitted helminth (STH) infections was significantly higher that those harbouring single STH species [12]. ...
... However, our understanding of the determinants of multiple helminth species infection patterns within communities remains poorly defined. For example, while recent studies have documented the prevalence of multiple helminth infections and their patterns by age and sex [3][4][5][6][7][8][9][10], little is known about spatial and household clustering of multiple helminth infection within communities or putative risk factors [20]. ...
... This study is, to our knowledge, the first epidemiological study to estimate the association between human schistosomiasis and the use of human waste as an agricultural fertilizer. Human waste is used as an agricultural fertilizer in many areas including Asia [31,32], West Africa [33], Central America and Northern Europe [1]. The global extent of this practice is not well documented. ...
... We found approximately half of households in our study applied human waste to their crops. A study in Vietnam estimated over 90% of farmers in the study region used night soil [32]. In our study, night soil was applied to all major crops, in both growing seasons and by household across the socio-economic spectrum. ...
Human waste is used as an agricultural fertilizer in China and elsewhere. Because the eggs of many helminth species can survive in environmental media, reuse of untreated or partially treated human waste, commonly called night soil, may promote transmission of human helminthiases.
We conducted an open cohort study in 36 villages to evaluate the association between night soil use and schistosomiasis in a region of China where schistosomiasis has reemerged and persisted despite control activities. We tested 2,005 residents for Schistosoma japonicum infection in 2007 and 1,365 residents in 2010 and interviewed heads of household about agricultural practices each study year. We used an intervention attributable ratio framework to estimate the association between night soil use and S. japonicum infection. Night soil use was reported by half of households (56% in 2007 and 46% in 2010). Village night soil use was strongly associated with human S. japonicum infection in 2007. We estimate cessation of night soil use would lead to a 49% reduction in infection prevalence in 2007 (95% CI: 12%, 71%). However, no association between night soil and schistosomiasis was observed in 2010. These inconsistent findings may be due to unmeasured confounding or temporal shifts in the importance of different sources of S. japonicum eggs on the margins of disease elimination.
The use of untreated or partially treated human waste as an agricultural fertilizer may be a barrier to permanent reductions in human helminthiases. This practice warrants further attention by the public health community.
... 21 When considering potential socio-demographic factors associated with adult STH prevalence, age ≥55 years, less education and women were associated with increased prevalence, while non-farming occupations were associated with lower prevalence. Previous studies have shown that hookworm prevalence was highest among children and the elderly, [22][23][24] while education level has also been reported as an independent risk factor for STH infection. 25 The reason for these risk factors is unclear, however older age is likely correlated with decreased immunity to infection, as well as decreased adoption of newer hygiene and sanitation interventions. ...
... It has been demonstrated that pregnant women are at increased risk of infection 26 and that women may have higher individual parasite burden compared to men, 14 but few studies have demonstrated a significant difference in STH prevalence between men and women. 14,22,24 One possible explanation for this finding in the current study is that women are more likely to be in direct contact with more heavily infected children, though this is likely the case in other study populations as well. This study showed moderately large associations for occupation and STH prevalence, but was not powered sufficiently to ensure that this was a valid finding. ...
Background:
Soil-transmitted helminth (STH) prevalence in children is high in rural southwestern Kenya, but adult prevalence data are scarce. A 2010 study of a village in Nyanza province found a pediatric STH prevalence of 44% using a direct stool-smear method. Adult STH prevalence and associated predictors was measured in the same village.
Methods:
Adults (≥18 years) presenting at the out-patient department of the small hospital or community outreach events completed a short questionnaire and provided stool samples. Light microscopy for ova and larvae was conducted using a stool concentration technique to improve sensitivity. Multivariable regression models were used to identify predictors of STH prevalence.
Results:
Among 344 adults, STH prevalence was 15.7% (54/344). Hookworm was most common (13.1%; 45/344), followed by Ascaris lumbricoides (6.1%; 21/344) and Trichuris trichiura (0.6%; 2/344). Twelve participants (3.5%; 12/344) had multiple STHs and three (0.9%; 3/344) had Schistosoma mansoni. Female sex, older age and lower education level were significant STH predictors.
Conclusions:
Adult STH prevalence was lower than previous studies of children from the same village. Adults with the identified risk factors had a prevalence of ≥20%, which may warrant periodic, targeted deworming of adults with these risk factors given the low cost and low toxicity of anthelmintic drugs.
... Significant co-infection rates, that is, the common occurrence of pairs or groups of parasites at higher frequency than independent probability events predict, result from a range of conditions (Howard et al., 2001;Petney and Andrews, 1998). Significant associations between different helminths, most notably, A. lumbricoides and T. trichiura (Booth and Bundy, 1992;Booth et al., 1998;Fleming et al., 2006;Howard et al., 2001;Needham et al., 1998;Raso et al., 2004;Tchuem Tchuenté et al., 2003) or intestinal protozoan species (Chunge et al., 1995;Utzinger et al., 1999) have been documented. They might be driven by common risk factors for infection with the implicated parasites (e.g. ...
... From Vietnam, a generally high prevalence of STHs leading to frequent double or triple infections, but low prevalences of intestinal protozoa, trematodes and cestodes have been reported (Goodrich, 1967;Kim et al., 1970). More recent data corroborate these findings (Needham et al., 1998;Nguyen et al., 2006;Olsen et al., 2006;Uga et al., 2005). Studies conducted in Vietnam which also include blood screening are very rare (Verle et al., 2003). ...
This review focuses on the issue of multiparasitism, with a special emphasis on its characteristics, its extent in eastern Asia and its significance for infectious disease control. Multiparasitism is pervasive among socially and economically disadvantaged or marginalised communities, particularly in tropical and subtropical areas. Intestinal parasites are the most numerous group, but an array of parasites is located elsewhere than in the human gastrointestinal tract. Although multiparasitism has been recognised for decades, in-depth studies are rare, and its public health and economic implications have yet to be fully elucidated. The assessment of multiparasitism is hampered by a lack of sensitive broad-spectrum diagnostic tools and the need to collect multiple biological samples for detailed appraisal. Non-specific symptoms and mainly subtle effects complicate the appreciation of its influence on cognitive and physical development, health, economic productivity and general well-being. Multiparasitism has been reported from virtually every eastern Asian country, and studies regarding the extent of multiparasitism and its effects on child health have been implemented in the region. However, new research is needed, as no comprehensive evaluations of multiparasitism in eastern Asia could be identified. Two case studies pertaining to multiparasitism at the local and regional scale are presented. Multiparasitism was rampant in an ethnic minority village in southern People's Republic of China where the challenges associated with its thorough evaluation are illustrated. The results from a cross-sectional survey covering 35 villages highlight the significance of its evaluation for the design of locally adapted and sustainable parasite control and poverty alleviation programmes. We conclude by listing a set of research needs for future investigations.
... An important outcome of our investigation is the observation that intestinal helminth infections were not independent of one another, and that some species co-occurred more frequently than might have been expected if their occurrences were by chance. Though this pattern of interaction, especially the co-occurrence of A. lumbricoides and T. trichiura have been described previously (Booth et al., 1998a,b;Howard et al., 2001;Keiser et al., 2002;Lwambo et al., 1999;Needham et al., 1998), this is the first time that a significant co-infection of hookworm and S. mansoni/T. trichiura, and A. lumbricoides and S. mansoni/T. ...
... Similarly, individuals co-infected with hookworm had higher egg burden and were four times more likely to have S. mansoni as individuals without. These findings are consistent with reports from other endemic areas (Fleming et al., 2006;Needham et al., 1998;Yatich et al., 2009). This synergy and dependent association has been suggested to be a result of impaired immunological status of individuals already infected with A. lumbricoides and T. trichiura (Correa-Oliveira et al., 2002;Yazdanbakhsh et al., 2001). ...
A cross-sectional study was conducted in a schistosome-endemic rural community in Southwestern Nigeria. We assessed prevalence and intensity of soil-transmitted nematodes and the co-occurrence with Schistosoma haematobium and Schistosoma mansoni. Urine and stool samples from 419 schoolchildren were examined, and a questionnaire was administered to obtain socio-demographic characteristics. In total, 78.3% (328/419) were infected with at least one helminth species, with a prevalence (mean egg-count) of 55.1% (3069.2) of Ascaris lumbricoides, 41.1% (127.5) of S. haematobium, 22.7% (98.6) of hookworms, 17.9% (161.3) of Trichuris trichiura, and 10.3% (12.9) of S. mansoni. Multiple infections were significantly more common among children from households with more playmates, absence of toilet facilities and low income level (all p<0.001). Children with heavy hookworm burden were at a significantly higher chance of acquiring S. mansoni (OR=36.35; 95% Cl: 13.22-100.97; p<0.0001). The risk of S. mansoni and A. lumbricoides infections was increased in co-infections with S. haematobium. Logistic regression analysis revealed infections by hookworms and S. mansoni (adjusted odds ratio [aOR]=3.90, 95% Cl: 2.03-7.46; p<0.0001), and by hookworms and T. trichiura (aOR=2.46, 95% Cl: 1.44-4.22; p=0.001) as significant risk factors for multiple infections. Our study shows that polyparasitism is common in the study area. Focused interventions such as mass treatment with anthelminthics and health education are needed to improve the well-being of the affected population.
... Significant co-infection rates, that is, the common occurrence of pairs or groups of parasites at higher frequency than independent probability events predict, result from a range of conditions (Howard et al., 2001;Petney and Andrews, 1998). Significant associations between different helminths, most notably, A. lumbricoides and T. trichiura (Booth and Bundy, 1992;Booth et al., 1998;Fleming et al., 2006;Howard et al., 2001;Needham et al., 1998;Raso et al., 2004;Tchuem Tchuenté et al., 2003) or intestinal protozoan species (Chunge et al., 1995;Utzinger et al., 1999) have been documented. They might be driven by common risk factors for infection with the implicated parasites (e.g. ...
... From Vietnam, a generally high prevalence of STHs leading to frequent double or triple infections, but low prevalences of intestinal protozoa, trematodes and cestodes have been reported (Goodrich, 1967;Kim et al., 1970). More recent data corroborate these findings (Needham et al., 1998;Nguyen et al., 2006;Olsen et al., 2006;Uga et al., 2005). Studies conducted in Vietnam which also include blood screening are very rare (Verle et al., 2003). ...
This review focuses on the issue of multiparasitism, with a special emphasis on its characteristics, its extent in eastern Asia and its significance for infectious disease control. Multiparasitism is pervasive among socially and economically disadvantaged or marginalised communities, particularly in tropical and subtropical areas. Intestinal parasites are the most numerous group, but an array of parasites is located elsewhere than in the human gastrointestinal tract. Although multiparasitism has been recognised for decades, in-depth studies are rare, and its public health and economic implications have yet to be fully elucidated. The assessment of multiparasitism is hampered by a lack of sensitive broad-spectrum diagnostic tools and the need to collect multiple biological samples for detailed appraisal. Non-specific symptoms and mainly subtle effects complicate the appreciation of its influence on cognitive and physical development, health, economic productivity and general well-being. Multiparasitism has been reported from virtually every eastern Asian country, and studies regarding the extent of multiparasitism and its effects on child health have been implemented in the region. However, new research is needed, as no comprehensive evaluations of multiparasitism in eastern Asia could be identified. Two case studies pertaining to multiparasitism at the local and regional scale are presented. Multiparasitism was rampant in an ethnic minority village in southern People's Republic of China where the challenges associated with its thorough evaluation are illustrated. The results from a cross-sectional survey covering 35 villages highlight the significance of its evaluation for the design of locally adapted and sustainable parasite control and poverty alleviation programmes. We conclude by listing a set of research needs for future investigations.
... People living in poor areas of the tropics commonly harbour multiple parasitic infections, including infection with multiple helminth species [1,2]. An increasing number of studies demonstrate that individuals infected with multiple helminth species tend to harbour the most intense infections [3][4][5][6][7][8][9][10][11] and can be at an increased risk of infection-related morbidity [12][13][14][15]. For example, a study of Brazilian school children showed those harbouring concomitant infection with Ascaris lumbricoides and Trichuris trichiura were at increased risk of stunting [16], whilst another Brazilian study found the risk of anaemia among school children infected with Schistosoma mansoni and two or three soil-transmitted helminth (STH) infections was significantly higher that those harbouring single STH species [12]. ...
... However, our understanding of the determinants of multiple helminth species infection patterns within communities remains poorly defined. For example, while recent studies have documented the prevalence of multiple helminth infections and their patterns by age and sex [3][4][5][6][7][8][9][10], little is known about spatial and household clustering of multiple helminth infection within communities or putative risk factors [20]. ...
BACKGROUND
Individuals living in areas endemic for helminths are commonly infected with multiple species. Despite increasing emphasis given to the potential health impacts of polyparasitism, few studies have investigated the relative importance of household and environmental factors on the risk of helminth co-infection. Here, we present an investigation of exposure-related risk factors as sources of heterogeneity in the distribution of co-infection with Necator americanus and Schistosoma mansoni in a region of southeastern Brazil.
METHODOLOGY
Cross-sectional parasitological and socio-economic data from a community-based household survey were combined with remotely sensed environmental data using a geographical information system. Geo-statistical methods were used to explore patterns of mono- and co-infection with N. americanus and S. mansoni in the region. Bayesian hierarchical models were then developed to identify risk factors for mono- and co-infection in relation to community-based survey data to assess their roles in explaining observed heterogeneity in mono and co-infection with these two helminth species.
PRINCIPAL FINDINGS
The majority of individuals had N. americanus (71.1%) and/or S. mansoni (50.3%) infection; 41.0% of individuals were co-infected with both helminths. Prevalence of co-infection with these two species varied substantially across the study area, and there was strong evidence of household clustering. Hierarchical multinomial models demonstrated that relative socio-economic status, household crowding, living in the eastern watershed and high Normalized Difference Vegetation Index (NDVI) were significantly associated with N. americanus and S. mansoni co-infection. These risk factors could, however, only account for an estimated 32% of variability between households.
CONCLUSIONS
Our results demonstrate that variability in risk of N. americanus and S. mansoni co-infection between households cannot be entirely explained by exposure-related risk factors, emphasizing the possible role of other household factors in the heterogeneous distribution of helminth co-infection. Untangling the relative contribution of intrinsic host factors from household and environmental determinants therefore remains critical to our understanding of helminth epidemiology.
... In our dataset, we identified a strong 3-way association between A. lumbricoides, T. trichiura, and hookworm, in which co-infection with all three species occurred more fre- quently than expected by random chance. This observation is in line with results from many other human populations [40][41][42][43]. Such positive associations between co-infecting helminths could reflect facilitative interactions between species, or may simply be a by-product of co- transmission given the similarity in the life cycle and transmission mode of soil-transmitted helminths in particular [44,45]. ...
Author summary
Over the last decade, an increasing number of studies have shown that co-infected individuals can show strikingly different disease symptoms than singly infected individuals. Among these studies, the potential protective effect of some helminth species on malaria severity has attracted considerable attention because of the possible public health impact. However, despite the fact that helminth species frequently co-occur together within an individual, no studies have investigated the role of within-host helminth community interactions on malaria severity. Taking the advantage of a cohort study in Thailand, we found that infection by Ascaris lumbricoides and Trichuris trichiura were both associated with direct reductions in cerebral malaria risk. However, the benefit of T. trichiura infection was halved in the presence of hookworm, revealing that interactions among helminth species can jeopardize the beneficial role of individual helminths. Our study shows that the outcome of interactions between two parasite species can be significantly modified by a third, emphasizing the critical role that parasite community interactions play in shaping infection outcomes and their impact on public health. These insights are especially relevant for neglected tropical diseases that occur in areas of high pathogen diversity.
... Age-dependency of overdispersion was reported to be an effect of co-variance with mean intensity (Guyatt et al., 1990;Needham et al., 1998). In this present study, it was also shown that age-dependency of overdispersion, was due mean intensity levels. ...
A longitudinal study of Ascaris lumbricoides reinfection over a 9-10 month period was carried out in a wastewater irrigated fanning region in Central Mexico (September 1989 - May 1991). The study assessed the relationship between different levels of exposure and its components, in the context of wastewater irrigation, and the prevalence and intensity of Ascaris infection. Three farming populations were studied: (1) those who irrigated with untreated or raw wastewater; (2) those who irrigated with wastewater that had sedimented during passage through a storage reservoir (a form of partial treatment); and (3) those who lived in a rain-fed area. Individuals' exposure to Ascaris eggs was estimated by in-depth interviews (characterising the frequency and type of contact with wastewater). Wastewater contact through agricultural activities was also estimated by structured observation. Predisposition to Ascaris infection was largely a consequence of behavioural and environmental factors that caused wastewater contact. Overdispersion of Ascaris intensity in the study population was not age- or gender-dependent. Over the 12-month monitoring period, untreated wastewater had a mean concentration of 96 Ascaris egg/litre and sedimented wastewater a concentration of <1 Ascaris egg/litre. Contact with untreated wastewater during various activities was associated with differing degrees of excess risk of Ascaris infection in the respective groups: -crop irrigation: 3-fold risk among children -chilli production: 5-fold risk among men and higher intensity infections -tending livestock: 4-fold risk among women -consumption of crops irrigated with wastewater: 2-fold risk in men and children and higher intensity infections in children -sweeping the yard: 5-fold risk in women Contact with sedimented wastewater during play was associated with more than a two-fold risk in Ascaris infection among children, and during maize production, with higher intensity infections among men. The nematode egg guideline of ~l egg/litre is adequate to protect the health of farmers using wastewater in agriculture, but is not sufficient to protect children. Any future modifications of the guideline must consider this.
... [65,66] This is thought to be because of the morbidity associated with each species, [5][6][7] and because co-infected individuals generally have heavier infections of each worm species. [65][66][67][68][69] Our maps suggest that the central area of Manufahi is likely to have a higher presence of co-infections. ...
Background:
In Timor-Leste there have been intermittent and ineffective soil-transmitted helminth (STH) deworming programs since 2004. In a resource-constrained setting, having information on the geographic distribution of STH can aid in prioritising high risk communities for intervention. This study aimed to quantify the environmental risk factors for STH infection and to produce a risk map of STH in Manufahi district, Timor-Leste.
Methodology/principal findings:
Georeferenced cross-sectional data and stool samples were obtained from 2,194 participants in 606 households in 24 villages in the Manufahi District as part of cross sectional surveys done in the context of the "WASH for Worms" randomised controlled trial. Infection status was determined for Ascaris lumbricoides and Necator americanus using real-time quantitative polymerase chain reaction. Baseline infection data were linked to environmental data obtained for each household. Univariable and multivariable multilevel mixed-effects logistic regression analysis with random effects at the village and household level were conducted, with all models adjusted for age and sex. For A. lumbricoides, being a school-aged child increased the odds of infection, whilst higher temperatures in the coolest quarter of the year, alkaline soils, clay loam/loam soils and woody savannas around households were associated with decreased infection odds. For N. americanus, greater precipitation in the driest month, higher average enhanced vegetation index, age and sandy loam soils increased infection odds, whereas being female and living at higher elevations decreased the odds of infection. Predictive risk maps generated for Manufahi based upon these final models highlight the high predicted risk of N. americanus infection across the district and the more focal nature of A. lumbricoides infection. The predicted risk of any STH infection is high across the entire district.
Conclusions/significance:
The widespread predicted risk of any STH infection in 6 to 18 year olds provides strong evidence to support strategies for control across the entire geographical area. As few studies include soil texture and pH in their analysis, this study adds to a growing body of evidence suggesting these factors influence STH infection distribution. This study also further supports that A. lumbricoides prefers acidic soils, highlighting a potential relatively unexplored avenue for control.
Trial registration:
ClinicalTrials.gov ACTRN12614000680662.
... The high prevalence in A. lumbricoides and T trichiura are indicative of the similar transmission pattern reported by different authors [5,[23][24][25]. The occurrence of ascariasis and trichuriasis was observed to peak in the young age group of 11-20 years, and it was peculiar to other studies in several localities [26,27]. ...
Prevalence of malaria and soil-transmitted helminth infections, and the burden of disease are enormous in sub-Saharan Africa. Co-infections aggravate the clinical outcome, but are common due to an overlap of endemic areas. A cross-sectional survey was conducted to assess prevalence, intensity of infection and association between malaria and soil-transmitted helminth infections in a typical periurban community in Kwara State. Fresh blood and faecal samples were examined using thick blood film and Kato-Katz smear techniques.
... When male are getting incorporation of soil and get the touch of human stool as they try to increase the strength of house structure. Conversely, in few areas like China (Sengchanh et al., 2011;Gandhi et al., 2001;Bethony et al., 2002) and Vietnam (Needham et al., 1998) ...
Hookworm infection, a global problem of human, is caused by both Ancylostoma duodenale and Necator americanus. These parasites are considered as important blood feeding nematodes. Hookworm is usually prevalent among the people who live with low socio-economic status. However, this tropical disease is neglected in most cases. This review was aimed at investigating the insights of hookworm infection of human considering its epidemiology, pathogenesis, clinical manifestation, diagnosis, treatment, immune mechanism, prevention, and control. This study showed how hookworm infection poses a global burden and infects 438.9 million people around the word annualy, and causes significant morbidity among the children and adult in endemic countries. Also, this review discussed how hookworm infection threatens the mankind causing serious health hazards. [J Adv Vet Anim Res 2016; 3(4.000): 297-320]
... Given the distance of the plot from the home and the absence of latrines at the plot (personal observation), adults also likely defecated at the plot, as reported in other studies (Kightlinger et al. 1998, Needham et al. 1998. Furthermore, agricultural land is generally considered suitable for survival of the eggs because agricultural produce is often contaminated with Ascaris eggs (Amoah et al. 2005, Adenusi et al. 2015. ...
This longitudinal study explored whether aspects of subsistence agriculture were associated with presence and intensity of
Ascaris
and hookworm in preschool children in rural Panama. Questionnaires were used to collect data on household socio-demographics, child exposure to agriculture and household agricultural practices. Stool samples were collected from children (6 months–5 years) at 3 time points, with albendazole administered after each to clear infections, resulting in 1 baseline and 2 reinfection measures. A novel Agricultural Activity Index (AAI) was developed using principal components analysis to measure the intensity of household agricultural practices. Zero-inflated negative binomial regression models revealed baseline hookworm egg counts were higher if children went to the agricultural plot and if the plot was smaller. Baseline and reinfection
Ascaris
egg counts were higher if children went to the plot and households had higher AAI, and higher at baseline if the plot was smaller. Caregiver time in the plot was negatively associated with baseline
Ascaris
egg counts, but positively associated with baseline hookworm and
Ascaris
reinfection egg counts. Children who spent more time playing around the home were less likely to be infected with
Ascaris
at baseline. We conclude that preschool child exposure to subsistence agriculture increased
Ascaris
and hookworm intensity.
... Mann-Whitney U tests, which have also previously been employed in the assessment of risk factors for helminth infection [27][28][29], were used to compare arithmetic mean infection intensities of S. mansoni and the STHs between schools with and without evidence of open defecation in the compound. Once more, P values lower than 0.05 were considered statistically significant. ...
Background
It is thought that improving water, sanitation, and hygiene (WASH) might reduce the transmission of schistosomes and soil-transmitted helminths, owing to their life cycles. However, few large-scale studies have yet assessed the real extent of associations between WASH and these parasites.
Methodology/Principal Findings
In the 2013–2014 Ethiopian national mapping of infections with these parasites, school WASH was assessed alongside infection intensity in children, mostly between 10 and 15 years of age. Scores were constructed reflecting exposure to schistosomes arising from water collection for schools, from freshwater sources, and the adequacy of school sanitation and hygiene facilities. Kendall’s τb was used to test the WASH scores against the school-level arithmetic mean intensity of infection with each parasite, in schools with at least one child positive for the parasite in question.
WASH and parasitology data were available for 1,645 schools. More frequent collection of water for schools, from open freshwater sources was associated with statistically significantly higher Schistosoma mansoni infection intensity (Kendall’s τb = 0.097, 95% confidence interval, CI: 0.011 to 0.18), better sanitation was associated with significantly lower Ascaris lumbricoides intensity (Kendall’s τb = -0.067, 95% CI: -0.11 to -0.023) and borderline significant lower hookworm intensity (Kendall’s τb = -0.039, 95% CI: -0.090 to 0.012, P = 0.067), and better hygiene was associated with significantly lower hookworm intensity (Kendall’s τb = -0.076, 95% CI: -0.13 to -0.020). However, no significant differences were observed when comparing sanitation and infection with S. mansoni or Trichuris trichiura, or hygiene and infection with A. lumbricoides or T. trichiura.
Conclusions/Significance
Improving school WASH may reduce transmission of these parasites. However, different forms of WASH appear to have different effects on infection with the various parasites, with our analysis finding the strongest associations between water and S. mansoni, sanitation and A. lumbricoides, and hygiene and hookworm.
... The highest prevalence of trichuriasis has been confined to Central Africa, Southern India, and Southeast Asia. Infections in children in some endemic areas may surpass 90% [2][3][4][5][6]. Although the majority of infected individuals remain asymptomatic, a significant number of trichuriasis patients, especially children with longstanding massive infections, have dysenteric syndrome presenting with chronic mucous diarrhea, rectal prolapse, anemia from chronic blood loss and iron deficiency, clubbing of fingers, protein-energy malnutrition, and growth retardation [7][8][9][10]. ...
Background: Trichuriasis is an important soil-transmitted helminth infection caused by Trichuris trichiura. About one-tenth of the world population may be infected. Incidentally, T. vulpis or dog whipworm has been reported to infect humans based on the egg size. However, an overlapping egg dimension occurs between T. trichiura and T. vulpis leading to the potential for misdiagnosis. Objective: Develope a PCR method to differentiate T. trichiura and T. vulpis eggs in stool samples and to investigate the prevalence of both whipworms in humans and dogs in a rural community in Thailand. Materials and methods: We determined and compared the small subunit ribosomal RNA sequences of both species of whipworms for developing species-specific PCR diagnosis. After validation of the method, we conducted a cross-sectional survey at Ta Song Yang District in Tak Province, northwestern Thailand in 2008. Stool samples were randomly recruited from 80 schoolchildren (36 males, 44 females) and 79 dogs in this community. Results: Fifty-six individuals harbored Trichuris eggs in their stools. The PCR-based diagnosis revealed that 50 cases were infected with T. trichiura and six (10.7%) were co-infected with both T. trichiura and T. vulpis. Although the dimension of Trichuris eggs provided some diagnostic clues for species differentiation, a remarkable variation in the length of these whipworm eggs was observed among samples that could lead to misdiagnosis. Conclusion: Both T. trichiura and T. vulpis eggs were detected in stool samples of dogs that roamed around this community, highlighting the potential reservoir role of dogs in the transmission of both human and dog whipworms in this population.
... Parasitological studies consistently demonstrate that, in natural conditions, an individual host is typically infected by multiple parasites (Petney and Andrews, 1998;Cox, 2001). The health impact of multiple infections has been known for some time, with most information from human studies (Haswell-Elkins et al. 1987;Ferreira et al. 1994;Needham et al. 1998;Brooker et al. 2000;Pullan and Brooker, 2008). However, few studies have investigated the impact on wild vertebrate host health (for a review see Bordes and Morand, 2011). ...
SUMMARY Most animals are concurrently infected with multiple parasites, and interactions among them may influence both disease dynamics and host fitness. However, the sublethal costs of parasite infections are difficult to measure and the effects of concomitant infections with multiple parasite species on individual physiology and fitness are poorly described for wild hosts. To understand the costs of co-infection, we investigated the relationships among 189 European eel (Anguilla anguilla) from Mar Menor, parasites (richness and intensity) and eel's 'health status' (fluctuant asymmetry, splenic somatic index and the scaled mass index) by partial least squares regression. We found a positive relationship with 44% of the health status variance explained by parasites. Contracaecum sp. (Nematoda: Anisakidae) was the strongest predictor variable (44·72%) followed by Bucephalus anguillae (Platyhelminthes: Bucephalidae), (29·26%), considered the two most relevant parasites in the analysis. Subsequently, 15·67 and 12·01% of the response variables block were explained by parasite richness and Deropristis inflata (Platyhelminthes: Deropristiidae), respectively. Thus, the presence of multiple parasitic exposures with little effect on condition, strongly suggests that eels from Mar Menor tolerate multiparasitism.
... Citrobacter freundii, Eschérichia coli, Klebsiella levinea, Shigella, Salmonella, giardia and Pseudomonas sp were the organisms we isolated on the stool samples [Nguendo Yongsi, 2007]. Then, bacteria, viruses and parasites are the types of pathogens in standing water that are hazardous to humans here as in some Asian and Central America urban settings (Xu et al., 1995;Needham et al., 1998). ...
Context: Like many sub Saharan African cities, Yaoundé is experiencing a faster growth of its population and urban perimeter. The urban population has grown from 812 000 inhabitants in 1987 to 2 100 000 inhabitants in 2006. However, this population growth has not been monitored by the city planners and decision makers. Accordingly, the city is lacking basic urban facilities. such as a good sewage system to evacuate urban waste water. Objective: This paper aims at addressing health consequences resulting from inadequate management of rainwater in Yaoundé. Material and methods: From the data gathered by us in the framework of the PERSAN programme focused on urban health, a cross sectional study has been carried out in 2002 and 2006 across the city. Based on socio-environmental and medical surveys, the study covered neighborhoods and 3 034 households in Yaoundé. Results: It comes out that that the present urban draining network is outdated and ineffective. This has led to increasing fl oods in several sectors of the city, with health hazards. It has been noted that many diarrheal diseases in Yaoundé are related to the poor sanitation resulting from urban waste coupled with standing waters. Conclusion: We are of the opinion that to solve this problem, there is urgent need to set up a new town-planning mechanism which takes into account the city’s demographic and space dynamics. Contexto: Como muchas ciudades africanas secundarias del Sahara, Yaoundé está experimentando un crecimiento rápido de su población y perímetro urbano. La población urbana ha crecido de 812 000 habitantes en 1987 a 2 100 000 habitantes en 2006. Sin embargo, este crecimiento de la población no ha sido supervisado por los planificadores de la ciudad y los que toman decisión. Por consiguiente, la ciudad está careciendo de instalaciones urbanas básicas tales como un buen sistema de las aguas residuales para evacuar las aguas negras urbanas. Objetivo: Este papel tiene como objetivo exponer las consecuencias de la salud resultando de la gerencia inadecuada del agua de lluvia en Yaoundé. Material y métodos: De los datos recopilados por nosotros en el marco del programa de PERSAN centrado en salud urbana, un estudio representativo se ha realizado en 2002 y 2006 a través de la ciudad. Basado en los exámenes socio-ambientales y médicos, se estudia vecindades cubiertas y 3 034 casas en Yaoundé. Resultados: Se puede concluir que la actual red de drenaje urbana es anticuada e ineficaz. Esto ha conducido a varias inundaciones en varios sectores de la ciudad, con los peligros para la salud. Se ha observado que muchas enfermedades diarreicas en Yaoundé están relacionadas con el saneamiento pobre resultando de la basura urbana juntada con aguas estancadas. Conclusión: Somos de la opinión que para solucionar este problema, hay necesidad urgente de instalar un nuevo mecanismo de la ciudad-planeamiento que considere la ciudad dinámica demográfi ca y del espacio.
... observaron que la prevalencia de Ascaris lumbricoides (2.1%) era baja en comparación con el resto de los geohelmintos. Tampoco hallaron ninguna asociación entre Trichuris trichiura y Ascaris lumbricoides como lo señalan otros autores 14 . En nuestro estudio no se detectó la presencia de Ascaris lumbricoides, probablemente debido a su baja prevalencia en esta población. ...
Resumen Se investigó la presencia de protozoarios y helmintos intestinales en una comunidad aborigen ubicada a 6 km de Tartagal, provincia de Salta, Argentina. La edad de los individuos estudiados es-taba comprendida entre 1 y 49 años. Ciento doce muestras de materia fecal se recogieron en solución acética formolada (SAF). Cada muestra se recolectó en tres días, en forma alternada. Todas las muestras se procesaron por el método de concentración bifásico de Ritchie y la técnica de flotación de Faust. Para la investigación de Enterobius vermicularis, se recolectaron 68 muestras seriadas de 6 días en formol al 5% por escobillado anal. Para la búsqueda de Dientamoeba fragilis se utilizó la coloración tricrómica modificada de Gomori-Wheatley. Ciento seis individuos (94.6%) fueron positivos para parásitos entéricos. Cuarenta y cuatro sujetos (41.5%) estaban poliparasitados, con más de cuatro especies. Los parásitos más frecuentes entre los protozoarios fueron Blastocystis hominis (58.9%), Entamoeba coli (51.8%), Giardia lamblia (27.7%) y Entamoeba histolytica/E. dispar (24.1%). Entre los helmintos, los hallados con mayor frecuencia fueron uncinarias (58.0%), Hymenolepis nana (31.2%) y Strongyloides stercoralis (24.1%). Por primera vez se informa Entamoeba histolytica/E. dispar (24.1%) y Dientamoeba fragilis (2.7%) en una comunidad aborigen de los alrededores de Tartagal. Este estudio revela el problema de salud pública que constituyen las parasitosis intestinales en la comunidad indígena estudiada, en la que coexisten la falta de saneamiento y de abastecimiento de agua potable. Abstract Survey of intestinal parasites among an aboriginal community in Salta. The prevalence of in-testinal parasitoses by protozoans and helminths was determined in an aboriginal community located 6 km from Tartagal, province of Salta, Argentina. The age of the inhabitants studied ranged from 1 to 49 years old. A total of 112 stool samples were collected in sodium acetate-acetic acid-formalin solution (SAF). Each sample was obtained in three different days, alternatively. Ritchie biphasic concentration method and Faust flotation technique were applied. For survey of Enterobius vermicularis, 68 samples were collected during six consecutive days using anal swabs. For the diagnosis of Dientamoeba fragilis, the modified trichrome Gomori-Wheatley staining technique was used. One hundred and six (94.6%) subjects resulted positive for enteric parasites. Forty-four (41.5%) individuals were positive for four or more parasites. The parasites more frequently found were, among the protozoans Blastocystis hominis (58.9%), Entamoeba coli (51.8%), Giardia lamblia (27.7%) and Entamoeba histolytica/E. dispar (24.1%). Among the helminths, the most frequent were hookworms (58.0%), Hymenolepis nana (31.2%) and Strongyloides stercoralis (24.1%). This is the first time that Entamoeba histolytica/E. dispar (24.1%) and Dientamoeba fragilis (2.7%) have been registered in the suburbs of Tartagal. This study reveals how intestinal parasitoses constitute a critical problem of public health in aboriginal communities like the one examined, where the lack of adequate sanitation conditions and unsuitable supply of water coexist.
... In areas where human excreta is used as a fertilizer for crops, a high prevalence and intensity of Ascaris infection has often been reported (for example, in Iran 47 and China 48 ); hookworm infection is also highly prevalent in wetter climates where excreta is used (for example, Vietnam 49 and Southern China 48 ). Blum and Feachem 4 include description of some of the older, descriptive studies of the prevalence of helminth infections in areas where excreta is used as fertilizer. ...
... . The knowledge of school pupils about the proper use of latrines (98 per cent), safe water supplies (98 per cent) and the prevention of worm infection (95 per cent) was found to be very high in one study covering four provinces (Trinh et al., 1999). However, a study of worm infection in adults and children found rates for round-worm, thread-worm and hook-worm to be 83 per cent, 94 per cent and 59 per cent respectively (Needham et al., 1998). Worm infection rates are felt to be a reliable indicator of hygiene practice and sanitary conditions. . ...
This paper provides a description of the present structure and main channels of information, education and communication in Vietnam: the mass media; face-to-face communication; print materials; and opportunistic activities. These communication channels have traditionally been used separately, with limited inter-sectoral collaboration for planning and limited resource allocation. This paper introduces an approach that has been designed to build on and strengthen existing capacity, quality and delivery of information, education and communication in Vietnam. The approach is based on the understanding that information, education and communication is more effective when a combination of channels is used as a part of the same intervention. The paper discusses some of the key challenges to transforming information, education and communication into a more sophisticated communication model. The paper may be of particular interest to practitioners in countries that are undergoing a similar transition and use similar structures and channels for communication.
... We therefore base our analyses on a small number of available well-designed studies that record prevalence and intensity of infection, stratified by age and sex, before and after various treatment programmes. The age-profiles used are from studies of A. lumbricoides in Myanmar [33], India [24] and Iran [34]; T. trichiura in St Lucia [35]; hookworm in Uganda [36] and Vietman [37] and, for comparison, studies of Schistosoma mansoni in Uganda [38] and Brazil [39]. ...
The London Declaration on neglected tropical diseases was based in part on a new World Health Organization roadmap to "sustain, expand and extend drug access programmes to ensure the necessary supply of drugs and other interventions to help control by 2020". Large drug donations from the pharmaceutical industry form the backbone to this aim, especially for soil-transmitted helminths (STHs) raising the question of how best to use these resources. Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective. However, the impact of school-based deworming on transmission in the wider community remains unclear.
We first estimate the proportion of parasites targeted by school-based deworming using demography, school enrolment, and data from a small number of example settings where age-specific intensity of infection (either worms or eggs) has been measured for all ages. We also use transmission models to investigate the potential impact of this coverage on transmission for different mixing scenarios.
In the example settings <30% of the population are 5 to <15 years old. Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of worms (the highest proportion of parasites for our examples). In addition, it is estimated that from 40-70% of these children are enrolled at school.
These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates. We discuss the consequences for transmission for a range of scenarios, including when infective stages deposited by children are more likely to contribute to transmission than those from adults.
... Prevalence of and risk factors for helminth infections have been studied in Vietnam [18,19,23,[25][26][27][28][29]. However, only a few studies focused on the relationship between helminth infection and exposure to wastewater such as handling practices and use of wastewater and excreta in agriculture, environmental factors, and personal hygiene practices. ...
... The most recent comprehensive estimation of the prevalences of the soil-transmitted helminthiases (STH) documents a global prevalence of 17% for Trichuris trichiura infection, with approximately 800 million persons infected at any one time [1,2]. Community-wide prevalences are frequently over 30-40% and it is not uncommon to observe prevalences exceeding 80% in community sub-groups like school-age children and preschool-age children [3][4][5][6][7]. T. trichiura infections contribute to the STHattributable burden of disease by adversely affecting the growth and cognitive development of children and the health and productivity of adults [8,9]. ...
The intensity categories, or thresholds, currently used for Trichuris trichiura (ie. epg intensities of 1-999 (light); 1,000-9,999 epg (moderate), and ≥10,000 epg (heavy)) were developed in the 1980s, when there were little epidemiological data available on dose-response relationships. This study was undertaken to determine a threshold for T. trichiura-associated anemia in pregnant women and to describe the implications of this threshold in terms of the need for primary prevention and chemotherapeutic interventions.
In Iquitos, Peru, 935 pregnant women were tested for T. trichiura infection in their second trimester of pregnancy; were given daily iron supplements throughout their pregnancy; and had their blood hemoglobin levels measured in their third trimester of pregnancy. Women in the highest two T. trichiura intensity quintiles (601-1632 epg and ≥1633 epg) had significantly lower mean hemoglobin concentrations than the lowest quintile (0-24 epg). They also had a statistically significantly higher risk of anemia, with adjusted odds ratios of 1.67 (95% CI: 1.02, 2.62) and 1.73 (95% CI: 1.09, 2.74), respectively.
This analysis provides support for categorizing a T. trichiura infection ≥1,000 epg as 'moderate', as currently defined by the World Health Organization. Because this 'moderate' level of T. trichiura infection was found to be a significant risk factor for anemia in pregnant women, the intensity of Trichuris infection deemed to cause or aggravate anemia should no longer be restricted to the 'heavy' intensity category. It should now include both 'heavy' and 'moderate' intensities of Trichuris infection. Evidence-based deworming strategies targeting pregnant women or populations where anemia is of concern should be updated accordingly.
... geohelminth has been reported to be associated with higher levels of anaemia compared to an independent infection of helminth parasites (Ezeamama et al., 2008). A number of epidemiological studies have indicated that individuals infected with multiple species of helminth often harbour heavier infection than individuals infected with a single helminth species (Booth et al., 1998;Needham et al., 1998). ...
A study of the prevalence, intensity and risk factors of geohelminth infections was investigated among preschool children aged 1-5 years old in Ibilo, Akoko-Edo Local Government Area of Edo State, Nigeria. Two hundred and ninety eight children (94.30%) out of 316 volunteers examined were infected with one or more geohelminth parasites. The predominant geohelminth was Ascaris lumbricoides (85.7%, 104.4 epg), followed by hookworm (65.7%, 172.2 epg) and Trichuiris trichuira (2.9%, 305 epg). The difference in the prevalence of infection of the geohelminths among the preschool children was statistically significant (F=6.708, p <0.05). Infection rate of A. lumbricoides and hookworm (35.6%) was more than for either Ascaris and Trichuris (2.7%) or Hookworm and Trichuris (6.0%). The infection rate of multiple parasites of A. lumbricoides, hookworm and Trichuris was 3.4%. The difference in the mean pre treatment parasite load (369.8 epg) and post treatment parasite load (17 epg) was statistically signi-ficant (2 =13.92, p < 0.05). The mean difference between children who were anaemic pre treatment (7.9 g/dL) and the improvement of the haemoglobin profile post treatment (10.5 g/dL) was not statistically significant (t=2.65, p > 0.05). Of the three anthelmintic drugs administered, namely, Albendazole, Mebendazole and Pyrantel pamoate, children treated with Pyrantel pamoate had the lowest parasitic load post treatment and highest cure rate. Places of defecation such as pit latrines, bushes and water closets as well as mothers'/caregivers' occupation have been identified as potential risk factors contributing to the high infection rates of helminth parasites among preschool age children studied.
... People in villages are therefore at higher risk of infections because of their greater exposure to contaminated soils. Needham et al. (1998) reported that the environmental and climatic conditions in a community, and also certain farming practices, favour the transmission and distribution of soil helminths. In 1988, the WHO reported a prevalence of 82% of A. lumbricoides in poor peri-urban and urban communities in Nairobi. ...
A cross-sectional study of the prevalence, intensity and effects of soil-transmitted helminth and protozoan infections was undertaken among patients at the Buea Hospital Annex located in Buea sub-division of Cameroon. Stool samples from 356 subjects (174 males and 182 females) were collected and processed using standard concentration methods. Our results showed that 31.0% of subjects were infected with intestinal helminths and the prevalence was higher in females (32.4%) than in males (30.5%). A significantly higher prevalence was observed in rural (47.2%) than in urban areas (21.0%); significance < 0.1%. Prevalence was highest among those aged between 6 and 12 years (41.4%). The total prevalence of intestinal helminth infections were 19.3% for Ascaris lumbricoides, 14.0% for hookworm and 11.8% for Trichuris trichiura. The intensity of infection was unevenly distributed, with very heavy loads concentrated in a few individuals. Data also showed that 28.1% (100/356) of the subjects were infected with protozoans. Females showed a higher prevalence (28.6%; 52/182) than males (20.7%; 36/174). Also, there was a significantly higher prevalence in rural (34.0%; 49/144) than urban areas (18.4%; 39/212); significance < 0.1%. The age group 6-12 years again had a higher prevalence (37.1%; 26/70). The total prevalence of intestinal protozoans was: Entamoeba histolytica (24.4%), Entamoeba coli (11.2%) and Giardia lamblia (0.6%). These relatively heavy prevalences in patients may be reduced by appropriate medication and maintaining strict personal hygiene. Health education, clean water supply, good sewage management and a congenial environment will all help to minimize infection.
... Current status of soil-transmitted helminthiases among pre-school and schoolchildren previous studies (Kightlinger et al., 1995; Udonsi et al., 1996; Needham et al., 1998 ). This study has shown that soiltransmitted helminth infections are endemic and are of public health concern among schoolchildren in Ile-Ife. ...
A cross-sectional survey was conducted to determine the prevalence and intensity of soil-transmitted helminths among pre-school and school-aged children attending nursery and primary schools in Ile-Ife. Single stool samples were collected between January and March, 2009 from 352 children randomly selected from a total of 456 children attending both private and government schools. The stool samples were processed using the modified Kato-Katz technique, and then examined for the eggs of soil-transmitted helminths (STHs). One hundred and twenty-one (34.4%) samples were positive for STH eggs. The overall prevalences of Ascaris lumbricoides, Trichuris trichiura and hookworm were 33.2%, 3.7% and 0.9%, respectively. The prevalence of STH infection in government schools (47.8%) was significantly higher than in private schools (16.1%) (P < 0.001). The most common type of mixed infection was the combination of A. lumbricoides and T. trichiura (6.8%). The prevalence and intensity of A. lumbricoides rose with age. The lowest prevalence and intensity (7.7%; 0.240 ± 0.136 eggs per gram (epg)) were recorded in the 2- to 3-year-old age group, while the highest prevalence and intensity (58.7%; 1.820 ± 0.237 epg) were recorded in children aged 10 years and above. A questionnaire survey indicated that 73% of the children attending private school had been treated with anthelminthics less than 2 months prior to the collection of stool specimens, while 43% of the children attending government school received anthelminthic treatment during the same period. The findings indicate that STH infections are endemic among schoolchildren in Ile-Ife and that the burden of parasitic infections is greater in government schools than in private schools.
... This prevalence of multi-infections is affected by malnutrition [7], by differences in the behavior of children, by irregular distributions of infecting stages in the environment, by differences in the ability to generate an adequate immunological response, by basic biological differences between parasites, and by host genetic differences [2]. Previous studies have shown that individuals with multiple infections tend to display higher intensities of infection than that expected for each infection separately [8,9]. Thus, infection by one of the pathogens may be influenced by concurrent or earlier infections with the other. ...
The prevalence of multi-infections with helminthes, protozoans and Campylobacter spp. in Guatemalan children is a reflection of differences in the risk factors related to pathogen transmission.
Two hundred and eighty-nine fecal samples were collected from children of the Guatemalan highlands and patterns of pathogen occurrences were evaluated using an immunoassay for Campylobacter spp., a formalin-ether concentration followed by observation of unstained slides for helminthes and trichome stains of fecal smears for protozoans. Specimens were examined microscopically using 100, 400 and 1000x magnification.
Prevalence of Ascaris lumbricoides, Campylobacter spp., Giardia duodenalis, Entamoeba histolytica/E. dispar and Trichuris trichiura were 55.1%, 30.8%, 21.5%, 19.8% and 19.4%, respectively. Overall, the prevalence of at least one intestinal pathogen was 85.5%. Multi-infections were found in 43% of the children harboring pathogens.
Infections with Campylobacter spp., E. histolytica/E. dispar, T. trichiura and G. duodenalis were closely associated with the presence of co-infection with A. lumbricoides. T. trichiura infection was related to co-infection with A. lumbricoides and Campylobacter spp. Infections with G. duodenalis and T. trichiura were related to co-infections with either Campylobacter spp. or E. histolytica/E. dispar. The prevalence of multi-gastrointestinal infections with helminthes, protozoans and Campylobacter spp. in children was found to be related to age and gender.
... Numerous epidemiological studies indicate that individuals infected with multiple helminths, particularly soil-transmitted helminths (STH; especially hookworms and Ascaris lumbricoides) and schistosomes, harbour heavier worm burdens for each species than those infected with a single species (Booth et al., 1998; Needham et al., 1998; Brooker et al., 2000; Tchuem Tchuente et al., 2003; Fleming et al., 2006 ). Presence (and intensity) of coinfection may result from environmental and/or behavioural factors of the human host; for example, factors specific to the peri-domiciliary environment (Holland et al., 1988; Bethony et al., 2001; Raso et al., 2005; Hotez, 2007) or to family-specific behaviours (Bethony et al., 2001; Clennon et al., 2004). ...
Strong statistical associations between soil-transmitted helminths and schistosomes are frequently observed in co-endemic human populations, although the underlying explanations remain poorly understood. This study investigates the contribution of host genetics and domestic environment to hookworm and Schistosoma mansoni infection intensity and evaluates the role of genetic and non-genetic factors in co-variation of infection intensity. Detailed genealogical information allowed assignment of 1303 individuals living in the Brazilian community of Americaninhas, Minas Gerais state, to 25 pedigrees (containing between two and 1159 members) residing in 303 households. The prevalence of co-infection with both hookworms and schistosomes was high (38.5%), with significant correlation between Necator americanus and S. mansoni faecal egg counts. Bivariate variance component analysis demonstrated a modest but significant species-specific heritability for intensity of N. americanus (h(2)=0.196) and S. mansoni infection (h(2)=0.230). However, after accounting for demographic, socio-economic and household risk factors, no evidence for common genetic control of intensity of hookworm and schistosome infection was observed. There was some evidence for residual clustering within households but the majority (63%) of the covariance between N. americanus and S. mansoni infection intensity remained specific to the individual and could not be explained by shared genes, shared environment or other shared demographic, socio-economic or environmental risk factors. Our results emphasize the importance of exposure to hookworm and schistosome infection in driving the association between levels of infection with these species in hosts resident in areas of high transmission and suggest that much of this common exposure occurs outside the home.
... nana. Such sinergism has been observed in rural populations, where the worm burden of A. lumbricoides is higher in individuals infected with T. trichiura and vice versa (Needham et al., 1998). A. lumbricoides is regarded as an indicator both of fecal contamination in soil and of coprofagic habits of canines (Traub et al., 2003). ...
We analyzed intestinal parasitic infections in children aged 1-12 years from a poor neighborhood in La Plata, Argentina, and determined the correlations with their nutritional status and socio-environmental conditions. We performed parasitological analyses with anal brushed technique (for Enterobius vermicularis eggs) and fecal samples, employing the techniques of Ritchie, Carles Barthelemy and Willis. The worm burdens of nematodes were estimated by means of Kato Katz technique. Low weight-for-age (underweight), height-for-age (stunting) and weight-for-height (wasting) were calculated based on the 5th centile of the WHO 2006 (children under 5) and CDC 2000 (older children and adolescents) growth references. We also analyzed samples of soil, water, and canine feces and surveyed other domestic and environmental data using structured questionnaires to each child's parents. To associate the parasitological, anthropometric and socio-environmental data, a categorical analysis of principal components (catPCA) was conducted. In the first axis of catPCA, the correlations among socio-environmental variables showed a gradient of "relative welfare". The eigenvectors showed the most influential variables in the analysis were promiscuity (0.0765), father's education (-0.741), crowding (0.727), wastewater disposal (-0.658), mother's education (-0.574), and flooding (-0.409). The 85% of children were parasitized and 79.6% polyparasitized. The 27.7% of children had deficit in some nutritional status indicator, being the stunting the most prevalent deficit (16.8%). There also found parasites in 42% of the dog feces, 53% of the soil samples, and non-pathogenic amoebae in the water samples. The SEV was mainly associated with geohelminths and stunting, especially among the poorest children. The study evidences that living conditions are variable within this population. Part of these variations could be linked to the differences in the extent to which parents are able to use their scant resources to influence their children's morbidity. Further studies need to be done from a qualitative approach.
... Individuals living in areas where helminths are endemic often carry more than one species of worm infection; egg output for an individual helminth species is often higher in individuals carrying mixed infections than in individuals carrying singlespecies infections (3,5,20,38,45), which may reflect higher intensities of infection and so a higher risk of morbidity. Geographical and environmental factors are known to be of significance in facilitating this type of polyparasitism: for example, similarities in the transmission pathways for certain soil-transmitted nematodes are likely to account for some of the observed associations. ...
Mixed-parasite infections are common in many parts of the world, but little is known of the effects of concomitant parasite
infections on the immune response or on disease progression. We have investigated the in vivo effects of a chronic gastrointestinal
nematode infection on the infectivity and development of the immune response against the common trematode helminth Schistosoma mansoni. The data show that mice carrying an established chronic Trichuris muris infection and coinfected with S. mansoni, had significantly higher S. mansoni worm burdens than mice without coinfection. The increase in S. mansoni worm burden was accompanied by a higher egg burden in the liver. Kinetic analysis of S. mansoni establishment indicate reduced trapping of S. mansoni larvae during skin-to-lung migration, with T. muris-induced alterations in lung cytokine expression and inflammatory foci surrounding lung-stage schistosomula, suggesting that
the immunomodulatory effects of chronic T. muris infection elicited at the gut mucosal surface extend to other organs and perhaps specifically to other mucosal surfaces.
The data show that a preexisting chronic gastrointestinal nematode infection facilitates the survival and migration of S. mansoni schistosomula to the portal system, and as a result, increases the egg burden and associated pathology of S. mansoni infection.
... Other studies have found that there is a positive association between A. lumbricoides and T. trichiura infections (Udonsi et al. 1996). Kightlinger et al. (1995) and Needham et al. (1998) have found a dual species intensity correlation for A. lumbricoides and T. trichiura infections; high intensity infections of A. lumbricoides were associated with high intensity infections of T. trichiura. The reasons for this are unclear, but it may be explained by the similar route of infection, the fecaloral, for both parasites. ...
Between January and March 1998, a cross-sectional survey was carried out in four rural communities in Honduras, Central America. We examined the prevalence and intensity of Ascaris lumbricoides and Trichuris trichiura infections among 240 fecal specimens, and the association between selected socio-demographic variables and infection for 62 households. The overall prevalence of A. lumbricoides and T. trichiura was 45% (95% CI 39.0-51.9) and 38% (95% CI 31.8-44.4) respectively. The most intense infections for Ascaris and Trichuris were found in children aged 2-12 years old. By univariate analysis variables associated with infections of A. lumbricoides were: number of children 2-5 years old (p=0.001), level of formal education of respondents (p=0.01), reported site of defecation of children in households (p=0.02), households with children who had a recent history of diarrhea (p=0.002), and the location of households (p=0.03). Variables associated with both A. lumbricoides and T. trichiura infection included: number of children 6-14 years old (p=0.01, p=0.04, respectively), ownership of a latrine (p=0.04, p=0.03, respectively) and coinfection with either helminth (p=0.001, p=0.001, respectively). By multivariate analysis the number of children 2-5 years living in the household, (p=0.01, odds ratio (OR)=22.2), children with a recent history of diarrhea (p=0.0, OR=39.8), and infection of household members with T. trichiura (p=0.02, OR=16.0) were associated with A. lumbricoides infection. The number of children 6-14 years old in the household was associated with both A. lumbricoides and T. trichiura infection (p=0.04, p=0.01, OR=19.2, OR=5.2, respectively).
Background
Individual helminth infections are ubiquitous in the tropics; geographical overlaps in endemicity and epidemiological reports suggest areas endemic for multiple helminthiases are also burdened with high prevalences of intestinal protozoan infections, malaria, tuberculosis (TB), and human immunodeficiency virus (HIV). Despite this, pathogens tend to be studied in isolation, and there remains a need for a better understanding of the community ecology and health consequences of helminth polyparasitism to inform the design of effective parasite control programs.
Methodology
We performed meta-analyses to (i) evaluate the commonality of polyparasitism for helminth-helminth, helminth-intestinal protozoa, helminth-malaria, helminth-TB, and helminth-HIV co-infections, (ii) assess the potential for interspecies interactions among helminth-helminth and helminth-intestinal protozoan infections, and (iii) determine the presence and magnitude of association between specific parasite pairs. Additionally, we conducted a review of reported health consequences of multiply-infected individuals compared to singly- or not multiply-infected individuals.
Principal findings
We found that helminth-helminth and helminth-intestinal protozoan multiple infections were significantly more common than single infections, while individuals with malaria, TB, and HIV were more likely to be singly-infected with these infections than co-infected with at least one helminth. Most observed species density distributions significantly differed from the expected distributions, suggesting the potential presence of interspecies interactions. All significant associations between parasite pairs were positive in direction, irrespective of the combination of pathogens. Polyparasitized individuals largely exhibited lower hemoglobin levels and higher anemia prevalence, while the differences in growth-related variables were mostly statistically insignificant.
Conclusions
Our findings confirm that helminth polyparasitism and co-infection with major diseases is common in the tropics. A multitude of factors acting at various hierarchical levels, such as interspecies interactions at the within-host infra-parasite community level and environmental variables at the higher host community level, could explain the observed positive associations between pathogens; there remains a need to develop new frameworks which can consider these multilevel factors to better understand the processes structuring parasite communities to accomplish their control.
The links between socioeconomic status, nutritional status, and parasitic infection are difficult to clarify. This study investigated the correlations between household building materials, amenities, and head-of-household education with vitamin A status, hematocrit and hemoglobin (Hb) values in Indonesian preschool children infected with Ascaris lumbricoides and/or Trichuris trichiura. Children (N = 125, aged 3.3 ± 1.2 year) were enrolled at health posts in West Java. Anthropometric measures (i.e., gender, age, height, weight) and blood samples were collected. Vitamin A status was assessed with the modified relative-dose-response (MRDR) test and serum retinol concentrations, and iron status was assessed with Hb concentrations and hematocrit. A standard socioeconomic questionnaire was administered to the subjects' mothers by oral interview. Statistical analysis used multiple linear regression models for MRDR value, serum retinol concentration, hematocrit, and Hb with physical and socioeconomic factors as independent variables. Hb and hematocrit were negatively associated with the quality of walls (p = 0.032 and p = 0.0021, respectively) and floors (p = 0.0046 and p = 0.0032, respectively). Hb was positively associated with the quality of drinking water (p = 0.030) and serum retinol was positively associated with a higher level of head-of-household education (p = 0.035). Nutritional indicators correlated with household material quality and head-of-household education in Indonesian children and causality should be further investigated.
Rapid urban population growth that induces health problems constitutes one of the major and current challenges in developing countries. In 2002, the French agency Institut de Recherches pour le Développement led an interdisciplinary study in Yaoundé entitled "Population, Environment and Health" (PEH) in Yaoundé. In 2005, a supplemented study was undertaken focusing on health issues in urban environment in Cameroon. The study had two main objectives: (i) identifying risk factors of the diarrhea diseases in Yaoundé and measuring their prevalence, (ii) and evaluating their spatial distribution. The cross-sectional epidemiologic study across the city revealed a diarrhea prevalence of 14.4 percent (437 cases of diarrhea out of 3 034 tested children). Among various risk factors examined, wastewater handling and disposal methods were statistically associated to diarrhea cases. Moreover, it was noted that levels of diarrhea attacks varied considerably from one suburb to the other, partly because of the discrepancy character of the urbanization process in Yaoundé.
For most of human evolution, man lived in widely dispersed, small nomadic groups that subsisted on hunting and gathering.
While this lifestyle undoubtedly exposed individuals to the risk of occasional zoonotic disease transmission, population size
likely minimized the effect of exposure to such diseases. The Neolithic period, which saw the advent of the agricultural revolution
and the eventual domestication of certain animal species, not only brought about a rapid increase and concentration in human
population but also fostered an environment in which substantial increases in infectious and nutritional diseases could rapidly
spread (Armelagos, 1991). In addition, man’s descent from the trees, subsequent eccrine evolution, and the development of
agriculture forced him to become “water-bound,” thereby putting him in contact with a group of infectious agents that can
be described as “water associated” (Desowitz, 1981). It is within this context that many zoonotic diseases, including those
of nematode origin that are discussed in this chapter, may have gained a foothold in human populations.
The meaning of sustainability in water resources management has changed through the time. Initially, meeting water demand was the dominant concern. While later quality issues became more important followed by wider water reuse, today sustainability must include a whole range of aspects (e. g., energy, pollution, persistent chemicals) on various spatial and time scales. New approaches to define sustainability metrics are needed that include three essential elements: the sphere of sustainability, its time horizon and its metric.
Malnutrition and parasitic infections are common public health problems of children in developing countries. The prevalence of malnutrition and parasitic infections in school children in the Adamawa region of Cameroon was carried out. Anthropometric measurements consisting of height and weight were measured according to WHO guide lines (WHO, 1983, 1987). Body Mass Index (BMI) which is weight/height<SUP>2</SUP> for age was used as indicator to determine nutritional status. Examination of stool specimens was done using direct smear examination and concentration techniques and malaria parasitemia was determined microscopically from Giemsa stained blood films. The nutritional status and parasitosis was studied in 1200 (715 boys and 485 girls) school children aged 6 to 17 years. For malnutrition, severe acute malnutrition prevalence and moderate acute malnutrition was recorded respectively as 15.5% and 35.9%. 14.2% of which 64.3% were boys while 35.6% girls were found positive for various parasitic infections with malaria parasite accounting for the highest (64.9%) and hookworm the lowest (18.7%) prevalence rates. The difference in the level of parasitism was found to be statistically significant (p<0.05, chi square = 15.5) in the region. Severe and mild malnutrition was more prevalence in girls (12.9% and 46.7% respectively) than in boys (11% and 44.9% respectively). The relationship between the parasitic infection prevalence and nutritional status of the children showed that 98 (57.2%) of the infected children were malnourished (p<0.05) and there was no significant difference (p>0.05) of malnutrition between infected and non-infected children. The relationship between malnutrition and parasitic infection revealed that the correlation coefficient was 0.85, indicating a moderately strong relationship between the variables. The study confirmed that malnutrition and parasitosis were important child health problems. Therefore, it is recommended that lunch meals, nutrition education, sanitation education, treatment of parasitic infections be added to the school curriculum of school children in the Adamawa region of Cameroon.
There are remarkably few contemporary, population-based studies of intestinal nematode infection for sub-Saharan Africa. This paper presents a comprehensive epidemiological analysis of hookworm infection intensity in a rural Ugandan community. Demographic, kinship, socioeconomic and environmental data were collected for 1,803 individuals aged six months to 85 years in 341 households in a cross-sectional community survey. Hookworm infection was assessed by faecal egg count. Spatial variation in the intensity of infection was assessed using a Bayesian negative binomial spatial regression model and the proportion of variation explained by host additive genetics (heritability) and common domestic environment was estimated using genetic variance component analysis. Overall, the prevalence of hookworm was 39.3%, with the majority of infections (87.7%) of light intensity (<or=1000 eggs per gram faeces). Intensity was higher among older individuals and was associated with treatment history with anthelmintics, walking barefoot outside the home, living in a household with a mud floor and education level of the household head. Infection intensity also exhibited significant household and spatial clustering: the range of spatial correlation was estimated to be 82 m and was reduced by a half over a distance of 19 m. Heritability of hookworm egg count was 11.2%, whilst the percentage of variance explained by unidentified domestic effects was 17.8%. In conclusion, we suggest that host genetic relatedness is not a major determinant of infection intensity in this community, with exposure-related factors playing a greater role.
Background:
Individuals living in areas endemic for helminths are commonly infected with multiple species. Despite increasing emphasis given to the potential health impacts of polyparasitism, few studies have investigated the relative importance of household and environmental factors on the risk of helminth co-infection. Here, we present an investigation of exposure-related risk factors as sources of heterogeneity in the distribution of co-infection with Necator americanus and Schistosoma mansoni in a region of southeastern Brazil.
Methodology:
Cross-sectional parasitological and socio-economic data from a community-based household survey were combined with remotely sensed environmental data using a geographical information system. Geo-statistical methods were used to explore patterns of mono- and co-infection with N. americanus and S. mansoni in the region. Bayesian hierarchical models were then developed to identify risk factors for mono- and co-infection in relation to community-based survey data to assess their roles in explaining observed heterogeneity in mono and co-infection with these two helminth species.
Principal findings:
The majority of individuals had N. americanus (71.1%) and/or S. mansoni (50.3%) infection; 41.0% of individuals were co-infected with both helminths. Prevalence of co-infection with these two species varied substantially across the study area, and there was strong evidence of household clustering. Hierarchical multinomial models demonstrated that relative socio-economic status, household crowding, living in the eastern watershed and high Normalized Difference Vegetation Index (NDVI) were significantly associated with N. americanus and S. mansoni co-infection. These risk factors could, however, only account for an estimated 32% of variability between households.
Conclusions:
Our results demonstrate that variability in risk of N. americanus and S. mansoni co-infection between households cannot be entirely explained by exposure-related risk factors, emphasizing the possible role of other household factors in the heterogeneous distribution of helminth co-infection. Untangling the relative contribution of intrinsic host factors from household and environmental determinants therefore remains critical to our understanding of helminth epidemiology.
A baseline epidemiological survey for parasite infections was conducted between December 2007 and January 2008 in 155 villagers in a rural commune in Hoa Binh province, Vietnam. The prevalence of Ascaris lumbricoides, Trichuris trichiura and hookworm infection was 13.5%, 45.2% and 58.1%, respectively. At least one of the parasites was detected in 72.3% of the samples. We found no association between infection with A. lumbricoides or T. trichiura and engagement in agriculture, while hookworm infection was more prevalent in populations having frequent contact with soil. Agricultural use of human faeces was not correlated with any of the infections. We suggest that the consumption of vegetables that are commonly fertilized with human faeces in the community has led to the high infection rates with A. lumbricoides and T. trichiura, rather than the manipulation of faeces in farming activity. This also explains the high infection prevalence, despite high latrine coverage (98.1%) in the study population. The presence of latrines alone is not sufficient to reduce the prevalence of helminthiasis in a rural agricultural community if fresh faeces are used as fertilizer.
Two surveys of hookworm (Necator americanus) infections, conducted three years apart (December 1994 and January 1998) in a village in the Sikasso region of Mali, revealed that overall prevalence of infection was 68.7% and 53%, respectively. In both years there was a highly significant difference between the sexes in the prevalence and abundance of infection, with male subjects carrying heavier infections than females. Both prevalence and abundance of infection increased with age, although in 1998 there was a strong interaction between sex and age, arising from the declining egg counts among 16-20-year-old females and the continuing increase among males, reinforced by the subsequent reduction among the older males (> or = 61 years) and concomitant increase among females. After controlling for the effects of age, sex and their interaction, a highly significant positive relationship was detected between faecal egg counts of individuals who were examined in both 1994 and 1998 (n = 134), indicating predisposition to infection. This relationship remained significant in each of 4 age classes spanning 7-79 years. The members of some family compounds were shown to carry heavier infections than expected whilst others were less infected, suggesting compound-related clustering of hookworm infections. The use of footwear increased with age but there was no significant relationship between the extent of use of footwear and the abundance of hookworm infection. Eyesight deteriorated with age and impaired vision was particularly prominent among the older sectors of the community, a legacy from the time when onchocerciasis was widely prevalent in the region. Although men with partially damaged eyes carried lower infections than expected for their age, no overall significant relationship was found between quality of vision and hookworm infections. These results are discussed in relation to hookworm epidemiology in general and in Mali in particular.
Human populations are often infected with more than one species of parasite, especially in developing countries where overall rates of parasitism are high. Infections with multiple parasite species may not necessarily be independent within an individual as physiological, immunological or ecological factors may result in positive or negative associations between infections with different parasite species. A general framework for estimation of these associations is presented. Data from over 215000 individuals are analysed and the associations between geohelminth (Ascaris lumbricoides, Trichuris trichiura and hookworm) and malaria species are investigated. A method is presented for analysing data from multiple communities and testing whether the associations in different communities are equal. Overall estimates of the associations between species are obtained for each country and continent where data were available. Associations between geohelminth species were, in general, found to be positive whilst both positive and negative associations were found between the different Plasmodium species. There was evidence for significant geographical heterogeneity between the associations. A method for using these parameter estimates to predict the distribution of multiple infections when only marginal prevalence data are available is described and demonstrated.
Parasitic worm infections are amongst the most widespread of all chronic human infections. It is estimated that there are more than 3 billion infections in the world today. In many low income countries it is often more common to be infected than not to be. Indeed, a child growing up in an endemic community can expect be infected soon after weaning, and to be infected and constantly reinfected for the rest of her or his life. Infection is most common amongst the poorest and most disadvantaged communities, and is typically most intense in children of school going age. As the risk of morbidity is directly related to intensity of infection, it follows that children are the most at risk from the morbid effects of disease. Multiparasite infections are also common in such communities and there is evidence that individuals harbouring such infections may suffer exacerbated morbidity, making children even more vulnerable. Thus, these infections pose a serious threat to the health and development of children in low income countries. For many years, the need to control these infections has lain uncontested, and with the advent of broad-spectrum anthelminthic drugs that are cheap, safe and simple to deliver, control has at last become a viable option for many communities. Furthermore, there is now increased emphasis being placed on a multispecies approach as a cost-effective mechanism to control the morbidity of virtually all the major helminthic infections of humans.
In an earlier study Ascaris-specific IgG4 antibody was found to be elevated in cases of ascariasis. However, the usefulness of the elevated levels of this antibody in Ascaris infection as a diagnostic marker has not been well established. In India, in early 1999, blood samples of 83 cases of Ascaris infection, 35 cases of other nematode infection and 53 control subjects (without any helminth infection) were tested for anti-Ascaris IgG4 by ELISA. Further anti-Ascaris IgG4 levels in the blood of Ascaris-infected patients were determined, after eradication of the worms with drugs, at regular intervals to ascertain the duration of elevation of titre of the serological marker following initial infection. This information would indicate the sensitivity of the test as a diagnostic marker for recent infection. Blood samples of 422 rural people were also tested for anti-Ascaris IgG4 titre to ascertain the prevalence of ascariasis in the community. High levels of anti-Ascaris IgG4 antibody (OD 1.246 +/- 0.212) were found in all the 83 Ascaris-infected subjects compared to controls (OD 0.158 +/- 0.047). Anti-Ascaris IgG4 antibody levels of other nematode-infected subjects were comparable to the controls. Anthelmintic treatment of 8 Ascaris-infected subjects caused sequential fall of IgG4 level in their blood, and its titre reached control level within 6 months of deworming. Of 422 individuals from the rural community 229 (54.3%) had significantly high levels of specific IgG4 antibody against Ascaris excretory-secretory antigen, suggesting that they were infested with Ascaris. Thus, this study demonstrated that anti-Ascaris IgG4 antibody is a very sensitive and specific marker for the diagnosis of Ascaris infection. Utilizing this test, a significant number of a rural population could be diagnosed with Ascaris infection in West Bengal, India.
More than thirty years ago Stoll (1947) noted that helminth parasites were amongst the most prevalent of all human infections within many tropical, subtropical and temperate regions of the world. Today the global pattern of infection has changed little. World Health Organization statistics, for example, suggest that approximately one billion people are currently infected with the directly transmitted nematode Ascaris lumbricoides (roundworms). The picture is similar for other nematode parasites such as Trichuris trichiura (whipworms), Enterobius vermicularis (pinworms) and the hookworms Ancylostoma duodenale and Necator americanus (Table 3.1).
An epidemiological survey of intestinal helminthiases was conducted on 766 primary school children aged 5-16 years from Ile-Ife, Nigeria. On the basis of stool examinations, the prevalence of Ascaris lumbricoides, Trichuris trichiura, hookworm and Strongyloides stercoralis was 88.5, 84.5, 33.1 and 3% respectively. Intensity of infection was measured indirectly by egg counts for each species of helminth and also by counting worms passed after chemotherapy in the case of A. lumbricoides. The influence of host age and sex on infection levels was assessed. Relationships between the intensities of A. lumbricoides, T. trichiura and hookworm in individual children were identified. After anthelmintic treatment with levamisole, the frequency distribution of A. lumbricoides per host and the relationship between parasite fecundity and worm burden were investigated. Reinfection patterns of A. lumbricoides were assessed at two 6-monthly intervals and even within the narrow age range described, differences were found. In addition, evidence was obtained for predisposition of individuals to heavy or light infection with A. lumbricoides.
Studies of patterns of reinfection with four species of intestinal nematodes (Ascaris, hookworm, Trichuris and Enterobius) in 174 individual patients following chemotherapeutic treatment revealed statistical evidence for predisposition to heavy or light infection (relative to the average level in the overall population). Analyses of associations between the abundances of the four species of nematodes within a combined sample of 525 worm burdens showed significant correlations between 5 out of the 6 possible pair-wise comparisons between species. The relevance of these results to the design of control programmes based on chemotherapeutic application is discussed.
A striking feature of lymphatic filariasis is the considerable heterogeneity in infection burden observed between hosts, which greatly complicates the analysis of the population dynamics of the disease. Here, we describe the first application of the moment closure equation approach to model the sources and the impact of this heterogeneity for macrofilarial population dynamics. The analysis is based on the closest laboratory equivalent of the life cycle and immunology of infection in humans--cats chronically infected with the filarial nematode Brugia pahangi. Two sets of long-term experiments are analysed: hosts given either single primary infections or given repeat infections. We begin by quantifying changes in the mean and aggregation of adult parasites (inversely measured by the negative binomial parameter, kappa in cohorts of hosts using generalized linear models. We then apply simple stochastic models to interpret observed patterns. The models and empirical data indicate that parasite aggregation tracks the decline in the mean burden with host age in primary infections. Conversely, in repeat infections, aggregation increases as the worm burden declines with experience of infection. The results show that the primary infection variability is consistent with heterogeneities in parasite survival between hosts. By contrast, the models indicate that the reduction in parasite variability with time in repeat infections is most likely due to the 'filtering' effect of a strong, acquired immune response, which gradually acts to remove the initial variability generated by heterogeneities in larval mortality. We discuss this result in terms of the homogenizing effect of host immunity-driven density-dependence on macrofilarial burden in older hosts.
Males and females differ physiologically, but they also do different things. Thus, we when we note sex differences in average rates of parasite prevalence, incidence or intensity of infection, or in clinical outcome, we may attribute such differences either to the different physiology, or to different activities, or to some combination of differences between the sexes. Hence the subject of this month's debate-to explore the question of ‘why do certain males or females have higher or lower incidence or prevalence or severity of parasitic infection?’ Don Bundy opens with a discussion of the epidemiological patterns, while Jim Alexander and Bill Stimson explore some of the physiological effects of sex and sex hormones on the host's immune response to infection.
In order to estimate the public health impact of helminth infections, and to provide baseline data for interventions, parasitological and morbidity surveys were conducted among inhabitants of three villages in Dongting Lake region, Hunan Province, China. Ascaris lumbricoides was found to be the most common helminth infection, followed by Trichuris trichiura and Schistosoma japonicum. Left liver enlargement was the most common indicator of morbidity. Observed numbers of multiple species infections closely correlated with expected figures generated from a simple probabilistic model. Heterogeneity was observed in age and sex-standardized infection and morbidity prevalences among the villages and occupations. Males had higher levels of infection, were more likely to suffer morbidity, and were more likely to have been treated for schistosomiasis than women. The prevalence of each morbidity indicator was positively correlated with the number of times of treatment for schistosomiasis, and negatively correlated with number of years since last treatment. The results imply that treatment history for S. japonicum infection may be a good indicator of current morbidity risk.
Hookworms are parasites of mammals, being most frequent in primates, carnivores, and ungulates, with a few species in other groups, including two aquatic mammals. Their natural distribution, which is determined primarily by the temperature requirements for development of their free-living stages, is equatorial, tropical, or sub-tropical. Three species of hookworm occur in man: Ancylostoma duodenale, Necator americanus and Ancylostoma ceylanicum. The two principal hookworms, A. duodenale and N. americanus, have different geographical distributions. A. duodenale is sub-tropical and does not occur in the northern hemisphere above 52˚ latitude. Above 47˚ latitude its larvae require artificial shelter from the climate since the free-living stages require temperatures in excess of 22˚C. The optimum temperatures for N. americanus are higher, in the tropical range of 25–28˚C. The distributions of both species overlap, although one or other usually predominates. Whenever A. ceylanicum has been recovered from man, it has been found that it usually constitutes only a small proportion of mixed hookworm burdens. The world prevalence of hookworm infection in man exceeds the prevalence of almost all other helminthic infections and of other individual diseases, with the exception of only the common cold.
The assessment of morbidity caused by chronic parasitic infections in the populations of endemic areas has remained difficult and controversial. Contributing to this predicament is the frequent occurrence of multiple infections with agents that can cause a wide range of clinical manifestations, from the frequent symptomless carrier state to overt disease with more or less specific clinical manifestations. In the interpretation of the complex morbidity patterns found in rural populations of tropical countries, it is often difficult to make a clear determination of cause and effect. The situations is further complicated by the low degree of pathognomicity of the clinical manifestations of even the advanced stages of certain parasitic diseases. The paper gives examples that illustrate the interaction between endemic malaria and schistosomiasis as important causes of hepatosplenomegaly. Also shown in the paper are the inter-relationships between the nutritional status and the number of multiple infections with parasites found in African villages as well as the association between habitual coca leaf chewing, malnutrition and hookworm disease in a Peruvian community of mixed ethnic origin. The paper describes micro-epidemiological features of poly-parasitism by comparing the prevalence and intensity of infection with Onchocerca volvulus, Schistosoma mansoni and S; haematobium between sub-groups in the village population who have different sources of domestic water supply. In two African villages with endemic schistosomiasis where mass treatment will be administered, only 25% of the residents with parasitologically confirmed S. haematobium infection and 12% of those with S. mansoni had single infection; the remaining majority had at least one additional patent parasitic infection of public health importance.
Baseline data from an epidemiological study of hookworm infection in a rural community in Zimbabwe are presented. The infection status of an age-stratified sample of the community was assessed using anthelmintic expulsion techniques. Necator americanus was the only helminth parasite found to be present. The age-prevalence and intensity profiles rose asymptotically to an adult prevalence of about 80% and adult mean burden of 7.7 worms per host. The overall mean burden was 4.8 worms per host. The frequency distribution of N. americanus was overdispersed and well described by the negative binomial distribution with a value for the aggregation parameter, k, of 0.346. Separate estimates of k were lower in males and older hosts. The distribution patterns were difficult to reconcile with any simple process of age-dependent acquisition of an effective immune response. A significant negative correlation was recorded between per caput fecundity and worm burden, providing evidence for a density-dependent regulation of female worm fecundity. The basic reproductive rate (R0 congruent to 2) was found to be similar to estimates from other geographical areas.
A survey for intestinal parasites was carried out in a homogenous rice cultivation area, in which people had equal opportunities of acquiring the local endemic helminthiases, including schistosomiasis mansoni. The numbers of Schistosoma mansoni eggs excreted in faeces were counted. Infections with S. mansoni, Ascaris lumbricoides, ancylostomes and Trichuris trichiura were not randomly distributed, but were correlated, depending on the species of worms present. The S. mansoni egg counts were positively correlated with ancylostome infection but inversely correlated with A. lumbricoides and T. trichiura. Patients with schistosomiasis associated with 2 other helminth infections excreted more S. mansoni eggs than the patients with S. mansoni plus only one other helminth infection.
Observed field data from a range of geographically distinct human communities suggest a consistent non-linear relationship between prevalence and mean intensity of Ascaris lumbricoides infection. Utilizing the negative binomial distribution as a description of observed aggregation, maximum-likelihood analysis reveals that the degree of aggregation is a negative linear function of mean worm burden. The factors responsible for this relationship in human populations require further study but may involve some combination of (i) density-dependent reduction in worm numbers within individuals, (ii) density-dependent parasite-induced host mortality or (iii) self-treatment by heavily infected hosts. Variability in the degree of aggregation appears dependent on the level of infection in a community and independent of geographical differences in the host or parasite populations.
Baseline data from an immuno-epidemiological study of hookworm infection in a rural village in Madang Province, Papua New Guinea are reported. Necator americanus was found to be the commonest helminth infection, with a prevalence of near 100% and intensity of 40 worms per host in adults. Enterobius vermicularis, Ascaris lumbricoides and Trichuris trichiura were also present, at prevalences of 53, 10 and 3% respectively; Ancylostoma duodenale was absent. The frequency distribution of N. americanus was highly over-dispersed, and was well described by a negative binomial distribution with aggregation parameter, k, of 0.370. Intensity of infection was significantly related to host age, but did not differ between the sexes. Haemoglobin levels and haematocrit values were indicative of anaemia in the community, but were unrelated to hookworm infection. Levels of antibodies (IgG, IgA and IgM combined) against adult Necator cuticular collagen and excretory-secretory (ES) products were determined. Serum concentrations of the two types of antibody were significantly correlated with each other. Significant positive correlations were found between anti-ES antibody levels and hookworm egg production, and between anti-collagen antibody levels and host age. It is suggested that the level of anti-collagen antibodies may reflect cumulative exposure to infection, whereas levels of anti-ES antibodies may be more dependent on current worm burden. No evidence was found to suggest that either antibody response is important in regulating parasite population growth. Similarly, the presence of a positive correlation between eosinophil concentration and infection intensity in adults indicates that eosinophilia reflects, rather than determines, the host's worm burden.
A survey was carried out to determine values for the prevalence and intensity of infection of intestinal helminths in children living in Cocle Province, Republic of Panama. The data set consisted of the results of microscopic examinations (modified Kato Katz technique) of stool samples collected in November 1987 from 661 children attending primary schools in 4 communities in the province. The overall prevalences of Ascaris lumbricoides, hookworm and Trichuris trichiura infections were found to be 18.2, 12.0 and 27.5% respectively. There were significant differences between the infection prevalence values for children attending the different schools, but not with respect to age or sex. Positive associations were detected between particular pairs of infections and these were most evident when Trichuris trichiura was involved. The data revealed evidence of a positive association between siblings at one school only for each helminth infection. There was extreme variation in the values for intensity of infection (20-126180 epg for A. lumbricoides). Highly significant, and unexplained, differences were observed in the intensity of A. lumbricoides infection by primary school; this finding did not apply to hookworm or T. trichiura. The analysis also revealed that children heavily infected with A. lumbricoides were also likely to be heavily infected with T. trichiura. Comparisons with the results of surveys carried out in Cocle Province in 1926 and 1974 indicate that these infections are declining in this region of Panama.
This chapter focuses on human infection and discusses a reassessment of the place of T. trichiura in medical helminthology. It examines the life cycle of T. trichiura––much of which, in the absence of appropriate research, remains controversial––and emphasizes that T. trichiura differs in significant ways from the other major geohelminths: there is no pulmonary migration; the adult is located in the large bowel; the adult is essentially a tissue parasite. The chapter discusses the geographical distribution of the parasite, which is shown to occur in temperate regions and to have a prevalence exceeding that of A. lurnbricoides in some areas. It also focuses on pathological, clinical, and epidemiological studies to develop the thesis that trichuriasis morbidity is currently underestimated in some regions and unrecognized in others. The population biology, one area in which the understanding of T. trichiura rivals that of A. lumbricoides and exceeds that of the hookworms, is described and the prospects for control are examined.
The gastrointestinal helminth infection status of 1574 children living in a slum area of Kuala Lumpur, Malaysia was assessed by quantitative coprology. Almost two-thirds were infected with Trichuris trichiura, 49.6% with Ascaris lumbricoides, and 5.3% with hookworm. Infection prevalence rose rapidly to a stable asymptote at 7 years of age, and the age-intensity profile was convex with maximal values in the 5-10 year age classes. This pattern was the same for males and females, but differed markedly between different ethnic groups. The frequency distributions of A. lumbricoides and T. trichiura were highly overdispersed (k values were 0.21 and 0.27, respectively), and age-dependent over the 0-8 year age classes. This suggests that the force of infection with these nematodes is lower in infants than in older children.
A survey of 428 households in a shanty town in Coatzacoalcos, Mexico, revealed high prevalences of Ascaris lumbricoides and Trichuris trichiura. The data were analysed separately for A. lumbricoides and T. trichiura in order to investigate the spatial distribution of heavy infections through the town. Within each age class, those individuals with egg counts in the upper 20% of the range for that age were classified as "heavily infected". When the data were stratified by household size, it was found that the distribution of "heavily-infected" individuals was not random in the community. "Heavily-infected" individuals were found together in households; fewer household units had a single heavily-infected individual than would be expected by chance. Such a pattern could result either from genetic similarities among family members influencing their ability to mount an effective immunological response to infection, or focal transmission in the vicinity of the home, or both. This result may have important implications for the development of community control programmes.
The gastrointestinal helminth infection status of an age-stratified sample from a single Caribbean community was assessed using anthelmintic expulsion techniques. The same sample was re-assessed in a similar manner after a 17 month period of re-infection. The age-prevalence profile of Ascaris lumbricoides was convex while that of Trichuris trichiura was asymptotic. The age-intensity profiles of both species were convex. These differing patterns are attributed to differences in the absolute worm burdens of the 2 species. The frequency distributions of infection intensity were similar for both species, and largely independent of host age. The basic reproductive rate of A. lumbricoides (Ro = 1-1.8) was similar to that recorded elsewhere and much lower than that of T. trichiura (Ro = 4-6), implying that the latter is intrinsically more resistant to control. Individual hosts were predisposed to high (or low) intensity infection with either species, although predisposition to both species simultaneously was not conclusively demonstrated. Further studies are required to determine the cause of these observations.
The study design of a project to investigate the epidemiology, population dynamics and control of intestinal nematode infections in fishing village communities in Southern India is described. The paper focuses on Ascaris lumbricoides infection and describes changes in prevalence and intensity (worm burdens) with host age, the aggregated frequency distributions of parasite numbers per person, a density-dependent relationship between parasite fecundity and worm burden and rates of reinfection following chemotherapeutic treatment. The age-intensity of infection profile is convex in form, where maximum worm burdens are attained in children in the age range five to nine years. On the basis of juvenile to adult worm ratioos, the life expectancy of Ascaris in man is estimated to be of the order of one year. Rates of reacquisition of worms after chemotherapy are shown to be dependent on host age. Wormy individuals with heavy infections are shown to be predisposed to this state such that they reacquire heavier than average worm burdens following treatment.
The relationships between mean intensities of infection, the aggregation of infection among hosts, and host age are analysed using data from 2 large (> 3000 individuals) field studies of Schistosoma haematobium infection. The data show a convex relationship between mean intensity and age, a convex relationship between an inverse index of aggregation and age, and an age-dependent relationship between the mean and aggregation of infection intensity when levels of infection are high. These patterns are qualitatively compared with the output of mathematical models based on an immigration-death process (model I), and incorporating age-dependent changes in the distribution of exposure to infection as suggested by field data (model II), or reductions in the rate of infection as a function of either current (model III) or cumulative (model IV) parasite burdens, that is, density-dependent processes without or with memory, respectively. Models II and III were able to reproduce observed patterns, but model IV, which is a possible representation of acquired immunity, was not. These results are consistent with the following assumptions: (i) age-related patterns of aggregation can be generated without recourse to density-dependent processes; (ii) the epidemiological impact of density-dependent processes depends on whether these act with or without memory; and (iii) any acquired immunity to human S. haematobium infection may be significantly less than life-long.
Infections with Ascaris lumbricoides, Trichuris trichiura and the hookworm species are often found in the same communities and individuals. Hosts infected by more than one species are potentially at risk of morbidity associated with each infection. This paper describes the use of a probabilistic model to predict the prevalence of multiple-species infections in communities for which only overall prevalence data exist. The model is tested against field data, using log-linear analysis, and is found to be more effective at estimating the numbers of multiple infections involving hookworms than those involving only A. lumbicoides and T. trichiura. This latter combination of infections is found, in half the communities examined, to be more common than expected by chance. An age-stratified analysis reveals that the degree of interaction between these two infections does not alter significantly with age in the child age classes of a Malaysian population.
The characteristically aggregated frequency distribution of macroparasites in their hosts is a key feature of host-parasite population biology. We begin with a brief review of the theoretical literature concerning parasite aggregation. Though this work has illustrated much about both the sources and impact of parasite aggregation, there is still no definite analysis of both these aspects. We then go on to illustrate the use of one approach to this problem--the construction of Moment Closure Equations (MCEs), which can be used to represent both the mean and second moments (variances and covariances) of the distribution of different parasite stages and phenomenological measures of host immunity. We apply these models to one of the best documented interactions involving free-living animal hosts--the interaction between trichostrongylid nematodes and ruminants. The analysis compares patterns of variability in experimental infections of Teladorsagia circumcincta in sheep with the equivalent wildlife situation--the epidemiology of T. circumcincta in a feral population of Soay sheep on St Kilda, Outer Hebrides. We focus on the relationship between mean parasite load and aggregation (inversely measured by the negative binomial parameter, k) for cohorts of hosts. The analysis and empirical data indicate that k tracks the increase and subsequent decline in the mean burden with host age. We discuss this result in terms of the degree of heterogeneity in the impact of host immunity or parasite-induced mortality required to shorten the tail of the parasite distribution (and therefore increase k) in older animals. The model is also used to analyse the relationship between estimated worm and egg counts (since only the latter are often available for wildlife hosts). Finally, we use these results to review directions for future work on the nature and impact of parasite aggregation.
In order to estimate the public health impact of helminth infections, and to provide baseline data for interventions, parasitological and morbidity surveys were conducted among inhabitants of three villages in Dongting Lake region, Hunan Province, China. Ascaris lumbricoides was found to be the most common helminth infection, followed by Trichuris trichiura and Schistosoma japonicum. Left liver enlargement was the most common indicator of morbidity. Observed numbers of multiple species infections closely correlated with expected figures generated from a simple probabilistic model. Heterogeneity was observed in age and sex-standardized infection and morbidity prevalences among the villages and occupations. Males had higher levels of infection, were more likely to suffer morbidity, and were more likely to have been treated for schistosomiasis than women. The prevalence of each morbidity indicator was positively correlated with the number of times of treatment for schistosomiasis, and negatively correlated with number of years since last treatment. The results imply that treatment history for S. japonicum infection may be a good indicator of current morbidity risk.
Epidemiology of concurrent intestinal nematode infections in laboratory mice and human communities Effect of periodic half year deworming on prevalence of soil transmitted helminthiases The interpretation of intensity and aggregation data for infections of Schistosoma haematobium
- Slade
- Nf
Slade NF (1991) Epidemiology of concurrent intestinal nematode infections in laboratory mice and human communities. Unpublished PhD thesis, Department of Applied Biology, Imperial College, University of London. Toan ND (1991) Effect of periodic half year deworming on prevalence of soil transmitted helminthiases. APCO 13th Meeting, Tokyo. Woolhouse MEJ, Ndamba J & Bradley DJ (1994) The interpretation of intensity and aggregation data for infections of Schistosoma haematobium. Transactions of the Royal Society of Tropical Medicine and Hygiene 88, 520–526.
Are there interactions between schistosomes and intestinal nematodes? Transactions of the
- Marques Ca Gs Atuncar
- Pereira
- Pereira
Chamone M, Marques CA, Atuncar GS, Pereira ALA & Pereira LH (1990) Are there interactions between schistosomes and intestinal nematodes? Transactions of the Royal Society of Tropical Medicine and Hygiene 84, 557–558.
Epidemiology of concurrent intestinal nematode infections in laboratory mice and human communities
- N F Slade
Slade NF (1991) Epidemiology of concurrent intestinal nematode
infections in laboratory mice and human communities.
Effect of periodic half year deworming on prevalence of soil transmitted helminthiases
- N D Toan
Toan ND (1991) Effect of periodic half year deworming on
prevalence of soil transmitted helminthiases. APCO 13th Meeting,
Tokyo.
Unpublished PhD thesis
- N F Slade