J H Ouma

Kenya Medical Research Institute, Nairoba, Nairobi Area, Kenya

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Publications (124)484.3 Total impact

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    ABSTRACT: Despite ongoing preventive chemotherapy campaigns, intestinal schistosomiasis is hyper-endemic in shoreline communities living along Lake Albert, Uganda. To provide a deeper insight into the local epidemiology of Schistosoma mansoni, a variety of field-based studies were undertaken focusing upon schistosome-snail interactions and confirmation of transmission foci. Cercarial shedding patterns of field-caught Biomphalaria spp., as identified by morphology, were hourly observed over a ten day period and showed that Biomphalaria stanleyi produced significantly more cercariae than Biomphalaria sudanica. Peak production times in both species were between 12.00 and 14.00h indicating greatest infection risk from lake water exposure is during the early afternoon. Laboratory-bred snails were exposed to locally hatched miracidia and susceptibility of Biomphalaria spp. was confirmed experimentally. Biomphalaria stanleyi was a more permissive host. After ascertaining appropriate conditions for infection of laboratory mice, 28 groups of between 5 and 6 naïve mice were placed in floatation cages at four suspected shoreline transmission sites for a 30 minute period of exposure. Eight weeks later, mice (n=142) were culled and S. mansoni adult worms were retrieved from 10 animals. Taken as a whole, these observations highlight the local importance of B. stanleyi in transmission of intestinal schistosomiasis and clearly demonstrate the risk of infection on the Lake Albert shoreline. To mitigate this risk local environmental modification(s), i.e. improvement in sanitation and hygiene and control of snail populations, is needed to bolster the impact of chemotherapy-based interventions.
    Parasitology International 10/2009; 59(1):49-53. · 2.30 Impact Factor
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    ABSTRACT: In sub-Saharan Africa, chronic hepatosplenomegaly, with palpable firm/hard organ consistency, is common, particularly among school-aged children. This morbidity can be caused by long-term exposure to malaria, or by Schistosoma mansoni, and it is exacerbated when these two occur together. Although immunological mechanisms probably underlie the pathogenic process, these mechanisms have not been identified, nor is it known whether the two parasites augment the same mechanisms or induce unrelated processes that nonetheless have additive or synergistic effects. Kenyan primary schoolchildren, living in a malaria/schistosomiasis co-transmission area, participated in cross-sectional parasitological and clinical studies in which circulating immune modulator levels were also measured. Plasma IL-12p70, sTNF-RII, IL-10 and IL-13 levels correlated with relative exposure to malaria, and with hepatosplenomegaly. Soluble-TNF-RII and IL-10 were higher in children infected with S. mansoni. Hepatosplenomegaly caused by chronic exposure to malaria was clearly associated with increased circulating levels of pro-inflammatory mediators, with higher levels of regulatory modulators, and with tissue repair cytokines, perhaps being required to control the inflammatory response. The higher levels of regulatory modulators amongst S. mansoni infected children, compared to those without detectable S. mansoni and malarial infections, but exposed to malaria, suggest that S. mansoni infection may augment the underlying inflammatory reaction.
    Parasite Immunology 03/2009; 31(2):64-71. · 2.21 Impact Factor
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    ABSTRACT: African freshwater snails of the genus Bulinus act as intermediate hosts for schistosomes, trematode parasites responsible for medical and veterinary forms of schistosomiasis. The relationship between these snails and their parasites is an intricate one, with particular species of snail susceptible to infection only by certain species of schistosome. In common with other self-fertile hermaphrodite gastropods, Bulinus consists of a number of closely related species complexes with restricted gene flow between populations of each taxon. Consequently, despite their medical and veterinary importance as intermediate hosts, unambiguous identification and differentiation of planorbid snails such as these remains problematic, often confounding attempts to define the distribution and evolutionary relationships of conchologically similar taxa. Here we consider how morphological methods of discrimination can be used in conjunction with molecular based approaches to improve snail identification, thereby achieving a better understanding of the epidemiology of schistosomiasis. Data are presented from Central and East African taxa which illustrate how PCR-based methods have begun to be used in combination with traditional analyses in an integrated approach to characterize the genus Bulinus, specifically the B. forskalii species group. Particular emphasis is given to the analyses of Random Amplified Polymorphic DNA (RAPDs) and the mitochondrial gene cytochrome oxidase I (COI).
    Biological Journal of the Linnean Society 01/2008; 68(1‐2):215 - 240. · 2.41 Impact Factor
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    ABSTRACT: In Lake Albert, an ecological study was conducted, between June 2000 and May 2003, which assessed snail population dynamics, parasite infection patterns and interplay of environmental factors upon Biomphalaria. Monthly sampling surveys were conducted at 29 sites monitoring populations of Biomphalaria stanleyi and Biomphalariasudanica. Altogether, a total of 21,715 B.stanleyi and 8452 B.sudanica were collected during the period. Both species could be found infected with Schistosomamansoni although infection prevalence was significantly higher (p < 0.01**) in B.stanleyi (4.4%) than in B.sudanica (3.5%). Each species occupied slightly different aquatic niches with B.stanleyi preferring deeper water habitats whilst B.sudanica was found along the shoreline in shallower water. B.stanleyi was more widely distributed among the sampling locations (19 sites) than B.sudanica (10 sites). Of the four villages included in the study area, snails from sites near Piida and Bugoigo villages had the highest schistosome infection rates, presumably attributable to the closer proximity of people with intestinal schistosomiasis. After inspection of cross-correlation plots which identified most suitable time lags, snail density dynamics could be associated with seasonal variations inclusive of: air temperature, rainfall, lake level, water temperature, water conductivity and water pH. These temporal observations better reveal the relationship between snail populations and environmental factors, providing important information concerning the relative roles of B.stanleyi and B.sudanica in transmission of S.mansoni and development of integrated strategies for disease control around Lake Albert.
    Hydrobiologia 01/2006; 568(1):433-444. · 1.99 Impact Factor
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    ABSTRACT: Schistosoma mansoni infection, associated morbidity and symptoms were studied in Piida fishing community at Butiaba, along Lake Albert, Uganda, from November 1996 to January 1997. The study revealed that S. mansoni is highly endemic with an overall prevalence of 72%, a mean intensity of 419.4 eggs per gram (epg) faeces (geometric mean for positives only), with 37.8% of males and 33.0% of females excreting over 1000 epg. Prevalence and intensity peaked in the 10-14 year old age group and decreased with increasing age. Females were less heavily infected than males. Differences were also shown between tribes. Diarrhoea and abdominal pain were commonly reported in Piida. However, no clear-cut correlation between intensity of S. mansoni infection and these conditions could be demonstrated, indicating that retrospective questionnaires concerning S. mansoni related-symptomatology are of limited value. Organomegaly, as assessed by ultrasonography, was frequent and hepatomegaly was associated with heavy S. mansoni infection. No correlation was demonstrated between splenomegaly and infection. This study emphasizes that schistosomiasis mansoni is a major public health problem in Piida fishing community and presumably also in many similar fishing communities. These observations call for immediate intervention and can help in planning long-term strategies for sustainable morbidity control.
    Transactions of the Royal Society of Tropical Medicine and Hygiene 01/2005; 98(12):711-8. · 1.82 Impact Factor
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    ABSTRACT: Peri-portal fibrosis can be a serious sequelae of Schistosoma mansoni infection. Age or duration of exposure have been identified as important risk factors, but their relative importance cannot be easily separated. Here, we have compared two cohorts, aged 6-50 years and resident for ten years or since birth, from two neighbouring villages (Booma and Bugoigo) on the eastern shore of Lake Albert, Uganda. Parasitological measurements were similar, whereas the prevalence of peri-portal fibrosis was 5-fold higher in Booma. Data from the cohorts were pooled to assess the relative contribution of age and duration of residency on the risk of disease. Amongst adults, duration of residency was the critical risk factor--individuals aged 17-31 years resident for more 22 years had an almost 12-fold increased risk of fibrosis than those resident for less than 15 years. Height-standardised Splenic Vein Diameter (SVD), Portal Vein Diameter (PVD), Para-sternal Liver Length (PLL) and Spleen Length (SL) values were all higher in Booma, and each organometric parameter except PLL increased with the severity of fibrosis. Our results clearly demonstrate that duration of exposure is a critical risk factor for the development of peri-portal fibrosis and its sequelae in adults. This parameter should therefore be a routine measurement during epidemiological surveys of S. mansoni.
    Transactions of the Royal Society of Tropical Medicine and Hygiene 03/2004; 98(2):125-36. · 1.82 Impact Factor
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    ABSTRACT: Circulating IgG antibody reactivity and excreted egg counts were investigated in 489 Kenyans given chemotherapy for schistosomiasis mansoni. Antibody reactivity was measured in ELISA, using either unfractionated aqueous soluble constituents of Schistosoma mansoni eggs (SEA) or CEF6 (a soluble fraction of S. mansoni eggs containing two cationic antigens) as the antigen source. Antibody reactivity for each antigen source was strongly associated with egg counts, both pre- and post-treatment. Approximately 6 months after chemotherapy, egg counts were zero in 84% of the subjects. The mean optical densities (OD) measured in the post-treatment ELISA were 60% (CEF6) or 45% (SEA) lower than the pre-treatment values, the reduction in the OD with CEF6 as antigen source being significantly greater than that observed with SEA (P <0.001). The usefulness of an assay for antibody reactivity in monitoring the effects of the treatment of schistosomiasis is discussed.
    Annals of Tropical Medicine and Parasitology 10/2003; 97(7):697-709. · 1.31 Impact Factor
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    ABSTRACT: To assess the effects of multi-micronutrient supplementation and multi-helminth chemotherapy on haemoglobin concentration (Hb), using schools as a health delivery system. STUDY AREA AND POPULATION: Nine hundred seventy-seven children between 9 and 18 y of age from 19 primary schools in Bondo District, western Kenya, were included in the trial. The 746 (76.4%) children on whom baseline Hb was available were included in this study. The study was a randomized, placebo-controlled, double-blind, two-by-two factorial trial of the effects of multi-micronutrient supplementation and multi-helminth chemotherapy on Hb after 8 months. Single treatment of infected children with albendazole (600 mg) for geohelminths and praziquantel (40 mg/kg) for Schistosoma mansoni and daily supplementation with 13 micronutrients. : Multi-micronutrient supplementation (3.5 g/l, 95% CI 1.7, 5.3; P=0.0002) and anthelminthic treatment (2.0 g/l, 95% CI 0.2, 3.9; P=0.03) increased Hb independently (interaction, P=0.33). The effects were also independent of baseline Hb and general nutritional status. The treatment effect was due to reductions in S. mansoni and hookworm intensities of infection, in that Hb increased by 0.4 and 0.2 g/l, respectively, per 100 epg reductions in egg output. Interestingly, among S. mansoni-infected children, the effect of treatment seemed stronger in those with compared to those without co-existing malaria parasitaemia (interaction, P=0.09). Multi-micronutrient supplementation and multi-helminth chemotherapy increased Hb among school children, irrespective of initial Hb and nutritional status.
    European Journal of Clinical Nutrition 05/2003; 57(4):573-9. · 2.76 Impact Factor
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    ABSTRACT: Sickle cell genotype prevalence was 26% in a malaria-holoendemic lowland area compared with 3% in a highland area of Kenya. The prevalence of glucose-6-phosphate dehydrogenase deficiency was 7% and 1% in holoendemic lowland and highland areas, respectively. Lack of protective polymorphisms may contribute to morbidity and mortality during outbreaks of malaria in the highlands.
    Transactions of the Royal Society of Tropical Medicine and Hygiene 01/2003; 97(5):513-4. · 1.82 Impact Factor
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    ABSTRACT: The aim of the study was to assess the efficacy and side effects following single and repeated (6 weeks apart) praziquantel treatment (40 mg/kg) in a Schistosoma mansoni-endemic focus with long-standing transmission at Lake Albert in Uganda between December 1996 and January 1997. The results were based on 482 individuals, randomly representing all age and both gender groups. The cure rate following the first and second treatments was 41.9% and 69.1%, respectively. The cure rate was higher in adults than in children, irrespective of intensity of infection. In addition, the cure rate declined markedly with increasing intensity of infection. The reduction in intensity of infection was marked, being 97.7% and 99.6% after the first and second treatments, respectively. A pre- and post-treatment symptom questionnaire revealed a broad range of side effects, including abdominal pain and diarrhoea. However, no serious or long-lasting complications affecting compliance were observed. The marked reductions in faecal egg excretion and the acceptable level of side effects point to a single praziquantel treatment (40mg/kg) as the strategy of choice in such a highly endemic S. mansoni focus.
    Transactions of the Royal Society of Tropical Medicine and Hygiene 01/2003; 97(5):599-603. · 1.82 Impact Factor
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    ABSTRACT: To assess the effects of multi-micronutrient supplementation and multi-helminth chemotherapy on serum retinol concentration, using schools as a health delivery system. STUDY AREA AND POPULATION: From 19 primary schools in Bondo District, western Kenya, 977 children between 9 and 18 y were included in the trial. The 644 (65.9%) children on whom baseline serum retinol was available were included in this study. A randomised, placebo-controlled, double-blind, two-by-two factorial trial on the effects of multi-micronutrient supplementation and multi-helminth chemotherapy on serum retinol after 8 months. Single treatment with albendazole (600 mg) and praziquantel (40 mg/kg of body weight) and daily multi-micronutrient supplementation with tablet containing 1000 microg vitamin A. Micronutrient supplementation (0.08 micromol/l, 95% CI 0.01, 0.14; P=0.025), but not treatment (0.03 micromol/l, 95% CI -0.04, 0.10; P=0.38), increased serum retinol. However, treatment did increase serum retinol in S. mansoni-infected (0.09, 95% CI 0.02, 0.16; P=0.009), but not in uninfected children (-0.07, 95% CI -0.18, 0.03; P=0.18; interaction, P=0.01). Similarly, reduction in egg output of S. mansoni, but none of the geohelminth, was a predictor, corresponding to a 0.008 micromol/l (95% CI 0.00002, 0.02; P=0.049) increase in serum retinol per 100 epg reduction. Interestingly, interactions were found between age and sex (P=0.046), and malaria parasitaemia and sickle cell phenotype (P=0.04). Multi-micronutrient supplementation and reduction in S. mansoni egg output increased serum retinol, irrespective of initial serum retinol. The Danish International Development Assistance.
    European Journal of Clinical Nutrition 08/2002; 56(7):666-73. · 2.76 Impact Factor
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    ABSTRACT: To determine the relative susceptibility of Schistosoma mansoni infections to treatment with the oxamniquine (OXA) and praziquantel (PZQ). Three separate cross sectional studies were performed in six primary schools located in two Schistosoma mansoni endemic areas in Eastern Kenya: Kangundo (low morbidity) and Kibwezi (high morbidity). One thousand two hundred and fourteen infected children aged 6-20 years were involved. Each child received either 15-mg OXA/kg body weight twice within an interval of six hours or a single dose of 40 or 60 mg PZQ/kg body weight. Three duplicate Kato stool examinations were done before and four or five weeks after treatment to assess treatment efficacy. The cure rates in different schools with OXA were 71.7-79.7% in Kangundo and 56.7-61.9% in Kibwezi. In children treated with PZQ, the 40-mg/kg-dose regimen achieved cure rates of 77.6-87.2% in Kangundo and 67.1-81.1% in Kibwezi, whereas the 60-mg/kg dose regimen attained cure rates of 93.2% in Kangundo and 76.3% in Kibwezi. Both OXA and PZQ efficacy declined significantly with age in Kangundo, whereas the age effect was not seen in Kibwezi. The poorer cure rates in Kibwezi than in the Kangundo children were not due to known previous drug exposure to either OXA or PZQ. The varying efficacy may be attributed to innate low drug susceptibility, possibly related to schistosome strain differences between the two areas.
    East African medical journal 02/2002; 79(1):29-33.
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    ABSTRACT: This paper reports the detailed results of a study of the impact of the Health Workers for Change (HWFC) workshop series on clients' perceptions of health services, relationships within the health centre and relations between the health facility and the district health system. The study was carried out in three stages: baseline, intervention and evaluation over a period of 20 months. Data, both qualitative and quantitative, were collected at three levels: client, facility and system. Results indicate that relations between health workers and clients improved a great deal after the intervention while those between the facility and the system remained to a large extent unchanged. The paper concludes that, with external support and help, especially from the health system level, health workers can work towards improving health services and their job satisfaction, which can lead to better health worker-client relations.
    Health Policy and Planning 10/2001; 16 Suppl 1:33-9. · 3.06 Impact Factor
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    F W Thiong'o, A Luoba, J H Ouma
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    ABSTRACT: To determine the extent of intestinal schistosomiasis, ascariasis, trichuriasis and hookworm infections among school children of Usigu and Bondo divisions of Bondo District. A cross sectional study. Fifty out of 130 primary schools in Usigu and Bondo divisions, Bondo District. Randomly selected school children (n= 3158) aged five to 20 years, were examined for intestinal helminths and schistosomiasis using Kato thick smear technique. The overall prevalence and geometric mean egg counts per gram/faeces for Schistosoma mansoni were 31.6% and 3.1; hookworm 36.8% and 4.1; Trichuris trichiura 21.8% and 1.5, and Ascaris lumbricoides 16.5% and 2.5. More girls (34.9%) than boys (28.6%) were infected with S. mansoni whereas more boys (39.0%) than girls (34.5%) were infected with hookworm. The prevalence of S. mansoni and hookworm infections increased with age but Ascaris and Trichuris infections decreased with age without any sex differences. Children under ten years of age tended to be more heavily infected with ascariasis, trichuriasis and hookworm than the older ones, while the intensity of S. mansoni increased gradually with age. There were positive relationships between different infections except for a significant negative correlation between Schistosoma mansoni and hookworm infections. Only four cases out of 789 had S. haematobium infection. CONCLUSION. Schistosoma mansoni and geohelminths were endemic in Bondo District, where two thirds of the school children suffered from these parasites. Polyparasitism was also common. There was a little overlap in the distribution of Schistosoma mansoni and hookworm, whereas ascariasis and trichuriasis were fairly distributed in the district.
    East African medical journal 07/2001; 78(6):279-82.
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    ABSTRACT: Severe periportal fibrosis is not an inevitable consequence of infection with Schistosoma mansoni. Genetic predisposition may be a deciding factor in the development of disease. To assess the contribution of genetic factors in the severity of hepatic fibrosis, the degree of familial aggregation was determined in a Kenyan population. Schistosomal fibrosis was identified with hepatic ultrasound and newly proposed World Health Organization criteria, which include both qualitative and quantitative observations. These 2 aspects of the criteria correlated well with one another. The peak prevalence of ultrasound proven fibrosis trailed 5-10 years behind peak prevalence of infection and declined sharply after age 50 years. This pattern was consistent with either resolution of severe fibrosis over 10-20 years or early death of those severely affected. Genetic predisposition appears to be a weak factor in the development of severe disease in this population, since no household or familial aggregation could be identified.
    The Journal of Infectious Diseases 04/2001; 183(6):960-6. · 5.85 Impact Factor
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    ABSTRACT: A clinical trial to compare the effectiveness of 4- and 6-mo repeated treatment with albendazole 600 mg (Zentel, SmithKline Beecham) or mebendazole 600 mg (Vermox, Janssen) on geohelminth infections was carried out on children in 6 primary schools; the study included 1,186 children, ages 4 to 19 yr. Kato-Katz examination was performed on stool samples before and after treatment. Overall, albendazole produced better cure rates and egg reduction rates for geohelminths. The cure rates for albendazole were 92.4% for hookworm infection, 83.5% for Ascaris lumbricoides, and 67.8% for Trichuris trichiura. Mebendazole given either 2 or 3 times in a year had cure rates of 50 and 55.0% (respectively) for hookworm, 79.6 and 97.5% for A. lumbricoides, and 60.6 and 68.3% for T. trichiura infection. The geometric mean intensity of hookworm eggs per gram (epg) of stool decreased by 96.7% after albendazole treatment compared with 66.3 and 85.1%, respectively, for 2 or 3 doses of mebendazole (P < 0.05) over the same period. Reductions in epg for A. lumbricoides and T. trichiura were comparable for both drugs. Our results indicate that treatment with albendazole at a 6-mo interval was more effective than mebendazole regimens and may be the best choice for use in the control of the 3 geohelminths.
    Journal of Parasitology 04/2001; 87(2):413-8. · 1.32 Impact Factor
  • Trends in Parasitology 04/2001; 17(3):117-8. · 5.51 Impact Factor
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    ABSTRACT: Freshwater snails of the Bulinus forskalii group are one of four Bulinus species complexes responsible for the transmission of schistosomes in Africa and adjacent regions. The species status of these conchologically variable and widely distributed planorbids remains unclear, and parasite compatibility varies considerably amongst the eleven taxa defined, making unambiguous identification and differentiation important prerequisites for determining their distributions and evolutionary relationships. Random Amplified Polymorphic DNA (RAPD) analyses were used to investigate relationships between taxa, with particular emphasis on Central and West African representatives. RAPD-derived phylogenies were compared with those from other independent molecular markers, including partial sequences of mitochondrial cytochrome oxidase subunit I (COI) gene, and the nuclear ribosomal RNA internal transcribed spacer 1 region (ITS1). The phylogenetic reconstructions from the three approaches were essentially congruent, in that all methods of analysis gave unstable tree topologies or largely unresolved branches. There were large sequence divergence estimates between species, with few characters useful for determining relationships between species and limited within species differentiation. Nuclear and mtDNA sequence data from Central and East African representatives of the pan-African B. forskalii showed little evidence of geographical structuring. Despite the unresolved structure within the phylogenies, specimens from the same species clustered together indicating that all methods were capable of differentiating taxa but could not establish the inter-specific relationships with confidence. The limited genetic variation displayed by B. forskalii, and the evolution and speciose nature of the group, are discussed in the context of the increasingly arid climate of the late Miocene and early Pliocene of Africa.
    Parasitology 02/2001; 123 Suppl:S277-92. · 2.36 Impact Factor
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    ABSTRACT: Seasonal epidemics of malaria occur in highland areas of western Kenya where transmission intensity varies according to rainfall. This study describes the seasonal changes in cytokine responses to Plasmodium falciparum liver-stage antigen 1 (LSA-1) by children (< or =17 years old) and adults (> or =18 years old) living in such a highland area. Fourteen- to 24-mer peptides corresponding to the N- and C-terminal nonrepeat regions of LSA-1 stimulated production of interleukin-5 (IL-5), interleukin-10 (IL-10), gamma interferon (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha) by peripheral blood mononuclear cells (PBMC) from 17 to 73% of individuals in both age groups in both seasons. IL-10 and TNF-alpha responses were more frequent during the high-transmission, rainy season than during the low-transmission, dry season (73 and 67% versus 17 and 25% response rates, respectively). In contrast, there was no seasonal change in the proportion of LSA-1-driven IFN-gamma and IL-5 responses. Children produced less IFN-gamma than adults, but IL-5, IL-10, and TNF-alpha levels were similar for both age groups. Depletion of CD8(+) cells from PBMC decreased IFN-gamma but increased IL-10 production. Individuals with LSA-1-stimulated IL-10 responses in the dry season were less likely to become reinfected in the subsequent rainy season than those without IL-10 responses (25% versus 49%; P = 0.083). These data support the notion that maintenance of LSA-1-driven IL-10 and TNF-alpha responses requires repeated and sustained exposure to liver-stage P. falciparum. In contrast, IFN-gamma responses increase slowly with age but persist once acquired. CD8(+) T cells are the major source of IFN-gamma but may suppress production or secretion of IL-10.
    Infection and Immunity 09/2000; 68(9):5198-204. · 4.07 Impact Factor
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    ABSTRACT: The prevalence of malaria infection in 102 paired maternal-blood and umbilical cord-blood samples was assessed by microscopy and polymerase chain reaction (PCR) in a holoendemic area in Kenya. Plasmodium falciparum single-species infection was detected in maternal peripheral blood (3.4%), whereas microscopy indicated that no Plasmodium species were in cord blood. In contrast, maternal-blood samples showed a PCR prevalence of 48% for P. falciparum, 25% for P. malariae, and 24% for P. ovale, and cord-blood samples showed a PCR prevalence of 32%, 23%, and 21%, respectively. Although mothers with mixed-species infections were more likely to have offspring infected with mixed species, the specific malaria species were discordant in paired maternal- and cord-blood samples. Triple-species infections were observed in 11 cord- and maternal-blood samples at a 5.5-fold greater frequency than expected. These findings indicate that Plasmodium species infections in cord blood are common, occur at lower densities, and may be acquired before parturition.
    The Journal of Infectious Diseases 09/2000; 182(2):558-63. · 5.85 Impact Factor

Publication Stats

4k Citations
484.30 Total Impact Points


  • 1993–2009
    • Kenya Medical Research Institute
      • • Centre for Global Health Research
      • • Centre for Clinical Research
      Nairoba, Nairobi Area, Kenya
  • 1987–2009
    • University of Cambridge
      • Department of Pathology
      Cambridge, ENG, United Kingdom
    • Addis Ababa University
      Ādīs Ābeba, Ādīs Ābeba, Ethiopia
    • The University of York
      • Department of Biology
      York, ENG, United Kingdom
  • 2003
    • Bangor University
      • School of Biological Sciences
      Bangor, WLS, United Kingdom
  • 2000
    • Centers for Disease Control and Prevention
      Atlanta, Michigan, United States
    • University of Oxford
      • Department of Zoology
      Oxford, ENG, United Kingdom
  • 1978–2000
    • Case Western Reserve University
      • • Department of Medicine (University Hospitals Case Medical Center)
      • • School of Medicine
      Cleveland, OH, United States
  • 1999
    • Hospital for Tropical Diseases, Ho Chi Minh City
      Thành phố Hồ Chí Minh, Ho Chi Minh City, Vietnam
  • 1998–1999
    • Leiden University Medical Centre
      • Department of Parasitology
      Leiden, South Holland, Netherlands
    • KEMRI-Wellcome Trust Research Programme
      Kilifi, Kilifi, Kenya
  • 1990–1997
    • London School of Hygiene and Tropical Medicine
      Londinium, England, United Kingdom
  • 1987–1996
    • Imperial College London
      Londinium, England, United Kingdom
  • 1992
    • Case Western Reserve University School of Medicine
      Cleveland, Ohio, United States
  • 1989–1992
    • Hebrew University of Jerusalem
      • Hadassah Medical School
      Jerusalem, Jerusalem District, Israel