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Highly efficient dry season transmission of malaria in Thailand
Abstract
Man-biting collections were made for 7 consecutive nights per month for 24 months at 2 sites in a Thai village regularly treated with DDT and fenitrothion yet hyperendemic for Plasmodium falciparum and P. vivax. Only Anopheles dirus was incriminated as a vector: 1.6% were infective and 2.4% were infected (median numbers of oocysts = 3.5). Transmission occurred within the village, which was located in groves of rubber and fruit trees, during the dry months of November to May only, when rates of parity (64%) and biting (2/man-night) were higher than during the monsoon (38% and 0.8%/man-night). Vectorial capacity and inoculation rates surged and then fell during 30 d at the end of the monsoon, quickly reinitiating transmission. Sporozoite species were identified using indirect fluorescent antibody tests or enzyme-linked immunosorbent assays: 76% were P. falciparum, compared to 78% of gametocytes; one mosquito was infected with both species. Vector survival and inoculation rates differed between similar sites 800 m apart. Dry season breeding occurred at the bottom of a deep, concrete-lined well. Much of the natural forest habitat of An. dirus in south-eastern Thailand that was once destroyed for farming is now being replaced with orchards; this ecological change may reintroduce malaria to a wide area.
... In Thailand, seven species have been implicated as primary malaria vectors, in the Minimus and Dirus complexes and the Maculatus Group [11][12][13][14]. These species include Anopheles dirus Peyton & Harrison and Anopheles baimaii Sallum & Peyton of the Dirus Complex [15,16]; Anopheles minimus Theobald of the Minimus Complex [15,17]; Anopheles aconitus Dönitz of the Aconitus Subgroup (Funestus Group) [16,18]; and Anopheles maculatus Theobald, Anopheles pseudowillmori Theobald, and Anopheles sawadwongporni Rattanarithikul & Green of the Maculatus Group [15,19,20]. Additionally, Anopheles campestris Reid, Anopheles barbirostris van der Wulp (Barbirostris Group) [21,22], and Anopheles epiroticus Linton & Harbach (Sundaicus Complex) [23] have also been identified as potential malaria vectors in Thailand. ...
... In Thailand, seven species have been implicated as primary malaria vectors, in the Minimus and Dirus complexes and the Maculatus Group [11][12][13][14]. These species include Anopheles dirus Peyton & Harrison and Anopheles baimaii Sallum & Peyton of the Dirus Complex [15,16]; Anopheles minimus Theobald of the Minimus Complex [15,17]; Anopheles aconitus Dönitz of the Aconitus Subgroup (Funestus Group) [16,18]; and Anopheles maculatus Theobald, Anopheles pseudowillmori Theobald, and Anopheles sawadwongporni Rattanarithikul & Green of the Maculatus Group [15,19,20]. Additionally, Anopheles campestris Reid, Anopheles barbirostris van der Wulp (Barbirostris Group) [21,22], and Anopheles epiroticus Linton & Harbach (Sundaicus Complex) [23] have also been identified as potential malaria vectors in Thailand. ...
... In Thailand, seven species have been implicated as primary malaria vectors, in the Minimus and Dirus complexes and the Maculatus Group [11][12][13][14]. These species include Anopheles dirus Peyton & Harrison and Anopheles baimaii Sallum & Peyton of the Dirus Complex [15,16]; Anopheles minimus Theobald of the Minimus Complex [15,17]; Anopheles aconitus Dönitz of the Aconitus Subgroup (Funestus Group) [16,18]; and Anopheles maculatus Theobald, Anopheles pseudowillmori Theobald, and Anopheles sawadwongporni Rattanarithikul & Green of the Maculatus Group [15,19,20]. Additionally, Anopheles campestris Reid, Anopheles barbirostris van der Wulp (Barbirostris Group) [21,22], and Anopheles epiroticus Linton & Harbach (Sundaicus Complex) [23] have also been identified as potential malaria vectors in Thailand. ...
Background
The entomological inoculation rate (EIR) is one of the key indices used to evaluate malaria transmission and vector control interventions. One of the components of the EIR is the sporozoite rate in Anopheles vectors. A systematic review and meta-analysis was performed to identify the prevalence of Plasmodium spp. in field-collected Anopheles species across Thailand.
Methods
This systematic review was registered under the PROSPERO number CRD42021297255. Studies that focused on the identification of Plasmodium spp. in Anopheles mosquitoes were identified from the electronic databases PubMed, Web of Science, and Scopus. The quality of the identified studies was determined using the Strengthening the Reporting of Observational Studies in Epidemiology approach. The proportion of Anopheles mosquitoes collected, Anopheles vectors for Plasmodium species, and specificity of Anopheles vectors for Plasmodium species were analyzed. The pooled prevalence of Plasmodium species among the primary vectors ( Anopheles dirus , Anopheles minimus , and Anopheles maculatus ) was estimated using the random-effects model.
Results
Of the 1113 studies identified, 31 were included in the syntheses. Of the 100,910 Anopheles mosquitoes identified for species and sibling species, An. minimus (40.16%), An. maculatus (16.59%), and Anopheles epiroticus (9.18%) were the most prevalent Anopheles species. Of the 123,286 Anopheles mosquitoes identified, 566 (0.46%) were positive for Plasmodium species. The highest proportions of Plasmodium species were identified in Anopheles hodgkini (2/6, 33.3%), Anopheles nigerrimus (2/24, 8.33%), Anopheles balabacensis (4/84, 4.76%), An. dirus (114/4956, 2.3%), Anopheles annularis (16/852, 1.88%), Anopheles kochi (8/519, 1.54%), Anopheles vagus (3/215, 1.4%), and Anopheles baimaii (1/86, 1.16%). The pooled prevalence of Plasmodium species identified in the main Anopheles vectors was 0.4% of that of Plasmodium species identified in An. dirus was 2.1%, that of Plasmodium species identified in An. minimus was 0.4%, and that of Plasmodium species identified in An. maculatus was 0.4%.
Conclusions
We found a low prevalence of Plasmodium infection in Anopheles mosquitoes across Thailand. Therefore, the use of EIR to determine the impact of vector control intervention on malaria parasite transmission and elimination in Thailand must be undertaken with caution, as a large number of Anopheles specimens may be required.
Graphical Abstract
... Plusieurs études ont mesuré les indicateurs entomologiques de la transmission du paludisme en ASE continentale (Ismail et al., 1974, Ismail et al., 1978, Baker et al., 1987, Harbach et al., 1987, Rosenberg et al., 1990b, Gingrich et al., 1990, Rahman et al., 1993, Somboon, 1993, Somboon et al., 1998, Rattanarithikul et al., 1996, Limrat et al., 2001, Toma et al., 2002, Sithiprasasna et al., 2003, Sidavong et al., 2004, Muenworn et al., 2009, Zollner et al., 2016, Manh et al., 2010, Wang et al., 2015, Sriwichai et al., 2017, Sriwichai et al., 2016b . L'index sporozoïtaire (IS), c'est-à-dire la prévalence du portage de sporozoïtes de plasmodium dans la population anophélienne, est en moyenne inférieur à 1%. ...
... La détection de moustiques infectés par P. falciparum pendant la saison sèche est rare dans la plupart de foyers de transmission. Certains auteurs ont associé cette saisonnalité à des variations importantes de la capacité vectorielle (Rosenberg et al., 1990b, Gingrich et al., 1990, Rattanarithikul et al., 1996. Dans un travail de modélisation mathématique, Pongtavornpinyo et al. ont identifié des différences entre les facteurs qui déterminent la prévalence en zone de forte et de faible endémicité respectivement. ...
... Rosenberg et al. ont documenté un foyer de transmission particulier où la transmission pendant la saison sèche, assurée par An. dirus (s.l.), était particulièrement intense (Rosenberg et al., 1990a, Rosenberg et al., 1990b. Au contraire, la transmission de P. vivax (détection de moustiques infectés par plasmodium) a lieu pendant toute l'année. ...
La zone transfrontalière entre la Thaïlande et le Myanmar (ZTTM) est une zone d'endémicité palustre où l'intensité de la transmission est faible, saisonnière et instable. Par conséquent, le paludisme est symptomatique dans toutes les classe d'âge et responsable d'une mobi-mortalité élevée, en particulier chez les femmes enceintes et les enfants. Les résistances de Plasmodium falciparum aux médicaments antipaludiques constituent le principal obstacle à la lutte contre le paludisme dans la région. L'apparition de souches de P. falciparum résistantes aux derniers médicaments encore efficaces (notamment les résistances à l'artémisinine) est particulièrement inquiétante car elle menace les acquis fragiles toutefois très importants en terme de contrôle du paludisme (dans la ZTTM, l'incidence du paludisme a été divisée par 20 au cours des trente dernières années passant 2 cas/personne/an dans le milieu des années 1980 à <0.1 cas/personne/an après 2010). L'objectif principal de cette thèse est de d'évaluer l'impact d'une stratégie d'élimination de P. falciparum basée sur des moyens médicamenteux (accès universel au diagnostic et traitement précoce et élimination du réservoir asymtpomatique, généralement sous-microscopique, par le traitement de masse)sur les indicateurs entomologiques de la transmission du paludisme. Les objectifs secondaires sont multiples et visent à mieux décrire et comprendre les modalités et la dynamique de la transmission du paludisme.
... Anopheles kochi plays a significant role in malaria transmission on the Bangladesh-Indian border, as confirmed by the Plasmodium-positive specimens listed in previous studies [25][26][27], and An. kochi also acts as a potential vector of malaria transmission in Thailand [49][50][51], as the sporozoites of P. falciparum or P. vivax were identified in Thailand using the indirect fluorescent antibody (IFA) tests or enzymelinked immunosorbent assays (ELISA) [50]. Plasmodium infection was found in a high number of Anopheles species (e.g. ...
... Anopheles kochi plays a significant role in malaria transmission on the Bangladesh-Indian border, as confirmed by the Plasmodium-positive specimens listed in previous studies [25][26][27], and An. kochi also acts as a potential vector of malaria transmission in Thailand [49][50][51], as the sporozoites of P. falciparum or P. vivax were identified in Thailand using the indirect fluorescent antibody (IFA) tests or enzymelinked immunosorbent assays (ELISA) [50]. Plasmodium infection was found in a high number of Anopheles species (e.g. ...
Background
Anopheles maculatus , Anopheles minimus and Anopheles dirus are the major vectors of malaria transmission in the Greater Mekong Subregion (GMS). The malaria burden in this region has decreased significantly in recent years as all GMS countries progress towards malaria elimination. It is necessary to investigate the Anopheles diversity and abundance status and assess the Plasmodium infection rates to understand the malaria transmission potential of these vector species in GMS countries to guide the development of up-to-date vector control strategies and interventions.
Methods
A survey of mosquitoes was conducted in Stung Treng, Sainyabuli and Phongsaly Provinces on the Cambodia-Laos, Thailand-Laos and China-Laos borders, respectively. Mosquito collection was done by overnight trapping at sentinel sites in each province. After morphological identification, the 18S rRNA-based nested-PCR was performed to detect malaria parasites in the captured Anopheles mosquitoes.
Results
A total of 18 965 mosquitoes comprising of 35 species of 2 subgenera (Subgenus Anopheles and Subgenus Cellia ) and 4 tribes (Tribes Culicini, Aedini, Armigerini and Mansoniini) were captured. Tribe Culicini accounted for 85.66% of captures, followed by Subgenus Anopheles (8.15%). Anopheles sinensis dominated the Subgenus Anopheles by 99.81%. Plasmodium -infection was found in 25 out of the 1 683 individual or pooled samples of Anopheles . Among the 25 positive samples, 19, 5 and 1 were collected from Loum, Pangkhom and Siem Pang village, respectively. Eight Anopheles species were found infected with Plasmodium , i.e., An. sinensis , Anopheles kochi , Anopheles vagus , An. minimus, Anopheles annularis , Anopheles philippinensis , Anopheles tessellatus and An. dirus . The infection rates of Plasmodium falciparum , Plasmodium vivax and mixture of Plasmodium parasite species were 0.12% (2/1 683), 1.31% (22/1 683) and 0.06% (1/1 683), respectively.
Conclusions
Overall, this survey re-confirmed that multiple Anopheles species carry malaria parasites in the international border areas of the GMS countries. Anopheles sinensis dominated the Anopheles collections and as carriers of malaria parasites, therefore may play a significant role in malaria transmission. More extensive investigations of malaria vectors are required to reveal the detailed vector biology, ecology, behaviour, and genetics in GMS regions in order to assist with the planning and implementation of improved malaria control strategies.
Graphical Abstract
... Rainforests and secondary forests or plantations (rubber, fruits) provide appropriate breeding sites for An. dirus [30,31]. The farm and forest sites in both communes are around 3 km and 10 km (IKD and IDR, respectively) away from the villages, and outside the flight range (approximately 1.5 km) for An. ...
... Anopheles dirus was observed to be the most anthropophilic species in this study and is consistent with other studies conducted in SEA [13,30,31,33,49]. However, there are reports of An. dirus with strong zoophilic behaviour in Kanchanaburi Province, Thailand [50] suggesting some variance in behaviour across SEA. ...
Background
In 2018, the National Malaria Control Programme in Vietnam switched from prioritizing malaria control to elimination. However, with the ongoing elimination programme, there are still areas where residual malaria transmission persists, including the central highlands. This entomological survey was conducted to evaluate Anopheles diversity and host-seeking activity of Anopheles vectors in two communes with very low malaria transmission in Gia Lai Province.
Methods
Anopheles species were collected in Ia DReh commune and Ia KDam commune, Gia Lai Province in the central highlands of Vietnam. Collections were conducted using human-baited double net trap, light trap and manual aspiration collections around cattle sheds, in the dry and rainy season. Mosquito specimens were identified morphologically, and members of species complexes were distinguished molecularly. Mosquito night-feeding patterns were investigated during the dry and rainy seasons.
Results
Overall, 18,835 specimens including 19 taxa were collected in Ia KDam and Ia DReh communes. These included the primary malaria vectors, Anopheles dirus and Anopheles minimus , and other secondary vector species. Anopheles dirus was observed to be an anthropophilic species, whereas An. minimus and a number of secondary vectors were observed to be zoophilic. Anopheles vagus was the dominant species, followed by Anopheles sinensis and Anopheles peditaeniatus. The majority of specimens were collected in the rainy season due to the relatively large number of An. vagus , while An. peditaeniatus , An. dirus, Anopheles kochi , Anopheles monstrosus and Anopheles tessellatus were collected in greater numbers during the dry season. The peak of host-seeking activity for An. dirus , An. sinensis , and An. vagus was between 18.00 and 19.00 h.
Conclusion
This study provided information on the diversity, seasonal prevalence and behaviour of Anopheles at the study sites. Identifying the diverse mosquito fauna in the central highlands of Vietnam allows species-specific control measures to be implemented by the National Programme to reduce malaria in areas of very low malaria transmission. The peak Anopheles host-seeking activity observed in this study was between 18.00 and 23.00 h, which highlights the need to better characterize Anopheles behaviour in this region of Vietnam to inform on vector control strategies.
... Of the 79 Anopheles taxa, generally only seven are regarded as important malaria vectors in Thailand [10], while some other species circumstantially may play more minor or secondary roles as vectors. The primary vector species include (i) Anopheles baimaii Sallum & Peyton (previously Anopheles dirus D) [34], (ii) An. dirus Peyton & Harrison [35], (iii) An. minimus Theobald (previously species A) [29], (iv) An. maculatus Theobald [36], (v) Anopheles sawadwongporni Rattanarithikul & Green [37], (vi) Anopheles pseudowillmori (Theobald) [38], and (vii) Anopheles aconitus Doenitz [39]. ...
... baimaii, based on the circumsporozoite protein enzymelinked immunosorbent assay (CSP-ELISA), has been made on molecularly identified populations [56,58]. Overall, published infection rates (Table 2) for the Dirus Complex have ranged between 0.8% and 6.4% [35,38,[58][59][60][61]. ...
There are seven Anopheles species incriminated as important (primary) malaria vectors in Thailand. These vectors belong to species complexes or are in closely related groups that are difficult to separate morphologically. Precise species identification, using molecular methods, enables control operations to target only important vectors and to increase understanding of their specific ecological requirements, bionomic characteristics, and behavioral traits. This review focuses on adult mosquito behavior, vector transmission capacity, and geographical distribution of malaria vectors in Thailand identified using genetic and molecular identification methods between 1994 and 2019. A better understanding of Anopheles biodiversity, biology, behavior, vector capacity, and distribution in Thailand and neighboring countries in the Greater Mekong Subregion (GMS) will facilitate more effective and efficient vector-control strategies and consequently contribute to a further decrease in the malaria burden.
... In the present study, only a few females of An. dirus were collected but one captured on human bait outdoors in the wet season was positive for a mixed infection of P. vivax (Pv-210 and Pv-247) based on ELISA and confirmation by PCR. This finding was consistent with the suggestions of Rosenberg et al. (1990) and Trung et al. (2004) that in areas where An. dirus is not dense, the species can maintain malaria transmission. Anopheles dirus is a very important vector in forested and hilly-forested areas of the country. ...
... Numerous investigators have reported variation in human-biting activity in different regions of Thailand. For example, An. dirus in the central region showed a peak of biting activity between 2000 and 2400 hours (Wilkinson et al. 1970;Baimai et al. 1988), whereas it exhibited a peak between 2200 and 0100 hours in northwestern (Harbach et al. 1987) and southeastern Thailand (Rosenberg et al. 1990), and between 2400 and 0300 hours in southern areas of the country (Scanlon and Sandhinand 1965). Recently, Tananchai et al. (2012) reported short periods of indoor (1900-2000 hours) and outdoor (2300−0100 hours) biting activity for An. ...
There was recently an outbreak of malaria in Ubon Ratchathani Province, northeastern Thailand. In the absence of information on malaria vector transmission dynamics, this study aimed to identify the anopheline vectors and their role in malaria transmission. Adult female Anopheles mosquitoes were collected monthly by human-landing catch in Na Chaluai District of Ubon Ratchathani Province during January 2014-December 2015. Field-captured mosquitoes were identified to species using morphology-based keys and molecular assays (allele-specific polymerase chain reaction, AS-PCR), and analysed for the presence of Plasmodium falciparum and Plasmodium vivax using an enzyme-linked immunosorbent assay (ELISA) for the detection of circumsporozoite proteins (CSP). A total of 1,229 Anopheles females belonging to 13 species were collected. Four anopheline taxa were most abundant: Members of the Anopheles barbirostris complex, comprising 38% of the specimens, species of the Anopheles hyrcanus group (18%), Anopheles nivipes (17%) and Anopheles philippinensis (12%). The other nine species comprised 15% of the collections. Plasmodium infections were detected in two of 668 pooled samples of heads/thoraces, Anopheles dirus (1/29) and An. philippinensis (1/97). The An. dirus pool had a mixed infection of P. vivax-210 and P. vivax-247, whereas the An. philippinensis pool was positive only for the latter protein variant. Both positive ELISA samples were confirmed by nested PCR. This study is the first to incriminate An. dirus and An. philippinensis as natural malaria vectors in the area where the outbreak occurred. This information can assist in designing and implementing a more effective malaria control programme in the province.
... It is interesting to note that the malaria decline in Thailand based on reports 60-70 years ago until today coincides with large historical land use changes. In fact, it has been suggested that the reduction of malaria could have been as much an effect of deforestation as of malaria control efforts (Rosenberg et al. 1990 ). Deforestation and environmental change can reduce availability of breeding habitats, thus affecting mosquito propagation of anopheline species that breed in shaded water bodies, such as many of the malaria vectors in Southeast Asia (Yasuoka and Levins 2007 ). ...
... aconitus ) and diversity (Overgaard et al. 2003 ). Based on previous observations (Scanlon and Sandhinand 1965 ;Rosenberg et al. 1990 ;Kondrashin et al. 1991 ), it was hypothesized that malaria mosquito density would be lower in more developed, urbanized locations with a high landscape diversity than in less developed forest locations with a low landscape diversity. Adult malaria mosquitoes were collected by monthly outdoor human landing catches during the wet and dry seasons from October 1997 to December 1999. ...
This chapter presents results on the effect of land use on malaria vector density, species diversity, and insecticide resistance in northern Thailand. Entomological data from 1977 to 1999 was analysed to understand spatio-temporal variations in vector density and potential effects on malaria transmission. Landscape analysis can potentially be used as a tool in vector control strategy development, particularly to understand local vector behaviour and habitat characteristics for stratification of vector control areas. Our results show that an increase in landscape diversity, particularly forest fragmentation, reduces malaria vector diversity and density. Such land use changes in non-malaria transmission areas were the potential cause for reductions in malaria mosquito densities. Similar land use changes were not observed in areas with permanent malaria transmission. The landscape analysis further shows that agricultural intensification increases the likelihood of insecticide resistance development in malaria mosquitoes. We therefore argue for intersectoral collaboration, especially between the public health and agricultural sectors, to develop and implement integrated insecticide resistance management plans and integrated control. Integration of pest and vector control strategies should build on the integrated pest and vector management (IPVM) strategies described in this chapter. Such a strategy should be implemented through the Farmer Field School approach. For future research topics we suggest studies on the bionomics of the sibling species of the malaria vectors in the region to more effectively implement vector control actions. Furthermore, the effect of IPVM strategies on vector populations and disease outcome should be evaluated through large-scale trials.
... Vector survival rates have been estimated by parous rates (85,87,98,115,140), mark-release-recapture experiments (37,80,148), multiparous age grading (36, 72, 100), and from mosquito infection rates (20,76,149). All of these methods estimate survival rate per feeding cycle. ...
... dirus and 3.0 for An. minimus (140). Studies involving An. gambiae infected with P. faldparum from gametocytemic individuals were unable to demonstrate an increased mortality in malaria-infectedmosquitoes (38,136). ...
The human malaria parasite life cycle appears deceptively simple because it involves only one species of vertebrate host (humans) and anopheline mosquitoes to transmit the pathogen. However, a number of host, vector, and pathogen factors have evolved that interact with ecological and logistic considerations to determine whether transmission of the parasite from a mosquito to a human, or the converse, will successfully occur.
... The concept of transmission level in the region is, however, problematic from a spatial perspective. Even in low transmission regions there are small pockets of high transmission, and transmission can vary seasonally [24,28,29]. When transmission statistics are aggregated at subnational, national, or regional units this heterogeneity is lost. ...
... While they are typically considered exophilic, biting can occur as frequently indoors as it does outdoors in open houses in forests [43,47,50,51]. Feeding patterns (early versus late) are sometimes contradictory, even in the same site and species across different years or locations [28,43]. ...
Background
Malaria in Southeast Asia frequently clusters along international borders. For example, while most of Thailand is malaria free, the border region shared with Myanmar continues to have endemic malaria. This spatial pattern is the result of complex interactions between landscape, humans, mosquito vectors, and malaria parasites. An understanding of these complex ecological and socio-cultural interactions is important for designing and implementing malaria elimination efforts in the region. This article offers an ecological perspective on the malaria situation along the Thailand–Myanmar border.
Discussion
This border region is long (2000 km), mountainous, and the environment ranges from thick forests to growing urban settlements and wet-rice fields. It is also a biologically diverse region. All five species of malaria known to naturally infect humans are present. At least three mosquito vector species complexes, with widely varying behavioural characteristics, exist in the area. The region is also a hub for ethnic diversity, being home to over ten different ethnolinguistic groups, several of which have been engaged in conflict with the Myanmar government now for over half a century. Given the biological and ethnic diversity, as well as the complex socio-political context, malaria control and elimination in the region is challenging.
Conclusion
Despite these complexities, multipronged approaches including collaborations with multiple local organizations, quick access to diagnosis and treatment, prevention of mosquito bites, radical cure of parasites, and mass drug administration appear to be drastically decreasing Plasmodium falciparum infections. Such approaches remain crucial as the region moves toward elimination of P. falciparum and potentially Plasmodium vivax.
... Vector survival rates have been estimated by parous rates (85,87,98,115,140), mark-release-recapture experiments (37,80,148), multiparous age grading (36, 72, 100), and from mosquito infection rates (20,76,149). All of these methods estimate survival rate per feeding cycle. ...
... dirus and 3.0 for An. minimus (140). Studies involving An. gambiae infected with P. faldparum from gametocytemic individuals were unable to demonstrate an increased mortality in malaria-infectedmosquitoes (38,136). ...
Malaria is the most important parasitic disease in the tropics, with indigenous transmission in over 100 countries or areas of the world (122, 123). The World Health Organization (WHO) estimates the global incidence of this mosquito-transmitted disease to be approximately 100 million cases annually (67); other estimates are in the 200 to 400 million range (57, 101). In 1984, nearly 400 million people, one-twelfth of the world’s population, were living in areas where malaria is highly endemic and where no specific antimalarial measures were being applied. An additional 2700 million people, 56% of the world’s population, live in areas where malaria is still endemic but where control measures have reduced its level of endemicity. In many of these areas malaria is increasing dramatically (67, 122, 123).
... The most important malaria vectors are in the subgenus Cellia, and include members in the Leucosphyrus Group (Neomyzomyia Series), Maculatus Group (Neocellia Series), and the Minimus Subgroup (Myzomyia Series) [11][12][13][14][15]. ...
... Collectively, these include Anopheles baimaii Sallum & Peyton [4,12,16], Anopheles dirus Peyton & Harrison [12,15], Anopheles minimus Theobald [4,17], Anopheles maculatus Theobald [12,17], Anopheles pseudowillmori (Theobald) [12], Anopheles sawadwongporni Rattanarithikul & Green [14,17,18], and Anopheles aconitus Doenitz [11][12][13]. However, some species far exceed others in their overall capacity to transmit malaria, and their relative importance and role may vary depending on locality, season, and epidemiological circumstances. ...
In Thailand, seven Anopheles species within three species assemblages have been incriminated as important malaria vectors. Comprehensive maps showing the countrywide geographical distribution of primary and secondary malaria vector species have not yet been developed; the maps that do exist are typically restricted to specific areas or are out of date. In addition, with the advent of molecular-based species-identification tools, the geographical locations of various sibling species have been more clearly defined in the country. This review has compiled published data on the known distribution of important malaria vectors in Thailand based on genetic and molecular identification assays from 1994 to 2014, with maps of relevant species distribution provided herein.
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... Molecular techniques based on polymerase chain reaction (PCR) have allowed for precise and reliable differentiation and identification of the sibling species of medical importance , Sinka et al. 2011). Among these 73 species, five to six species, depending on the literature, are incriminated as primary malaria vectors in Thailand (Rattanarithikul et al. 2006), including Anopheles baimaii (previously An. dirus D) (Green et al. 1991) and Anopheles dirus (Rosenberg et al. 1990, Green et al. 1991 of the Dirus Complex, Anopheles minimus (previously An. minimus A) (Rattanarithikul et al. 1996a) of the Minimus Complex, and Anopheles pseudowillmori (Green et al. 1991), An. maculatus and An. sawadwongporni of the Maculatus Group (Saeung 2012). ...
... baimaii) and secondary forest (An. latens) (Gingrich et al. 1990, Rosenberg et al. 1990. Vanwambeke et al. (2007a,b) found that land-use changes could selectively influence the diversity and likely density of mosquito species, thus having a direct impact on pathogen transmission. ...
We determined the species diversity, blood-feeding behavior, and host preference of Anopheles mosquitoes in
two malaria endemic areas of Tak (Mae Sot District) and Mae Hong Son (Sop Moei District) Provinces, located along the Thai border with Myanmar, during a consecutive two-year period. Anopheline mosquitoes were collected using indoor and outdoor human-landing captures and outdoor cow-baited collections. Mosquitoes were initially identified using morphological characters, followed by the appropriate multiplex AS-PCR assay for the identification of sibling species within Anopheles (Cellia) complexes and groups present. Real-time PCR was performed for parasite-specific detection in mosquitoes (Plasmodium spp. and Wuchereria bancrofti). A total of 7,129 Anopheles females were captured, 3,939 from Mae Sot and 3,190 from Sop Moei, with 58.6% and 37% of all anophelines identified as An. minimus, respectively. All three malaria vector complexes were detected in both areas. One species within the Minimus Complex (An. minimus) was present along with two related species in the Funestus Group, (An. aconitus, An. varuna), two species within the Dirus Complex (An. dirus, An. baimaii), and four species within the Maculatus Group (An. maculatus, An. sawadwongporni, An. pseudowillmori, and An. dravidicus). The trophic behavior of An. minimus, An. dirus, An. baimaii, An. maculatus, and An. sawadwongporni are described herein. The highest An. minimus densities were detected from February through April of both years. One specimen of An. minimus from Mae Sot was found positive for Plasmodium vivax.
... Dietz et al., 1974;Garrett-Jones and Shidrawi, 1969;Afrane et al., 2008 Asia 0.014-6.5 0.0011-0.52 Rattanarithikul et al., 1996;Rosenberg et al., 1990;Toma et al., 2002;Vythilingam et al., 2003;Zhou et al., 2010;Gunasekaran et al., 2014;Edalat et al., 2016 Oceania 1.60-9.64 0.13-0.77 ...
The establishment and spread of anti-malarial drug resistance vary drastically across different biogeographic regions. Though most infections occur in Sub-Saharan Africa, resistant strains often emerge in low-transmission regions. Existing models on resistance evolution lack consensus on the relationship between transmission intensity and drug resistance, possibly due to overlooking the feedback between antigenic diversity, host immunity, and selection for resistance. To address this, we developed a novel compartmental model that tracks sensitive and resistant parasite strains, as well as the host dynamics of generalized and antigen-specific immunity. Our results show a negative correlation between parasite prevalence and resistance frequency, regardless of resistance cost or efficacy. Validation using chloroquine-resistant marker data supports this trend. Post discontinuation of drugs, resistance remains high in low-diversity, low-transmission regions, while it steadily decreases in high-diversity, high-transmission regions. Our study underscores the critical role of malaria strain diversity in the biogeographic patterns of resistance evolution.
... Overall, the sporozoite rate was 2.3%, with the highest sporozoite rate (13.8%) surprisingly recorded during a collection carried out during the dry season in Zilé. Similar results were reported in Thailand by Rosenberg et al. [38] and were attributed to higher vector survival rates of mosquitoes during the dry season. ...
Background
Vector control is considered to be the most successful component of malaria prevention programs and a major contributor to the reduction of malaria incidence over the last two decades. However, the success of this strategy is threatened by the development of resistance to insecticides and behavioural adaptations of vectors. The aim of this study was to monitor malaria transmission and the distribution of insecticide resistance genes in Anopheles populations from three rural areas of the Moyen Ogooué Province of Gabon.
Methods
Anopheles spp. were collected using human landing catches in Bindo, Nombakélé and Zilé, three villages located in the surroundings of Lambaréné, during both the rainy and dry seasons. Mosquitoes were identified morphologically, and DNA was extracted from heads and thoraces. Members of the Anopheles gambiae complex were identified by molecular methods using the PCR SINE200 protocol and by sequencing of the internal transcribed spacer 2 region. Taqman assays were used to determine Plasmodium infection and the presence of resistance alleles.
Results
Anopheles gambiae sensu lato (97.7%), An. moucheti (1.7%) and An. coustani (0.6%) were the three groups of species collected. Anopheles gambiae sensu stricto (98.5%) and An. coluzzii (1.5%) were the only species of the An. gambiae complex present in the collection. Of the 1235 Anopheles collected, 1193 were collected during the rainy season; these exhibited an exophagic behaviour, and consistently more mosquitoes were collected outdoor than indoor in the three study areas. Of the 1166 Anopheles screened, 26 (2.2%) were infected with Plasmodium species, specifically Plasmodium falciparum (66.7%), P. malariae (15.4%), P. ovale curtisi (11.5%) and P. ovale wallikeri (3.8%). Malaria transmission intensity was high in Zilé, with an average annual entomological inoculation rate (aEIR) of 243 infective bites per year, while aEIRs in Bindo and Nombakélé were 80.2 and 17 infective bites per year, respectively. Both the L1014F and L1014S mutations were present at frequencies > 95% but no Ace1G119S mutation was found.
Conclusion
Our results demonstrate that malaria transmission intensity is heterogeneous in these three rural areas of Moyen Ogooué Province, with areas of high transmission, such as Zilé. The exophagic behaviour of the mosquitoes as well as the high frequency of resistance mutations are serious challenges that need to be addressed by the deployment of control measures adapted to the local setting.
Graphical Abstract
... Anopheles dirus (An. dirus) is one of the most efficient and geographically extensive malaria vectors (Rosenberg et al., 1990;Meek, 1995). This effectiveness is demonstrated by the fact that An. dirus is a long-lived and anthropophilic malaria vector. ...
Malaria occurrence is highly related to the geographical distribution of Anopheles dirus (An. dirus) in the South-East Asia Region and Western Pacific Region (SEAR/WPR). Future climate change has been shown to alter the geographical distribution of malaria vectors. However, few studies have investigated the impact of climate change on the potential distribution of An. dirus in the SEAR/WPR. We considered future climate and land-use data under two climate change scenarios for Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and population data from five Shared Socioeconomic Pathways (SSPs), by using three machine learning models, namely, Random Forest (RF), Boosted Regression Trees (BRT), and Maximum entropy (Maxent) to project the geographical distribution of An. Dirus and to estimate the exposed population. A pseudo-absence dataset was generated based on the relationships between model performance and the distance from the pseudo-absence point to the occurrence point in order to improve model accuracy for projection of the Environmentally Suitable Area (ESA) and exposed human population. The results show that the pseudo-absence data corresponding to the distance of 250 km are appropriate for modeling. The RF method ultimately proved to have the highest accuracy. The predicted ESA of An. dirus would mainly be distributed across Myanmar, Thailand, the southern and eastern part of India, Vietnam, the northern part of Cambodia, and the southern part of Laos. The future ESA is estimated to be reduced under the RCP 4.5 climate change scenario. In the 2070s under RCP 8.5, the reduction of ESA is even greater, especially in Thailand (loss of 35.49 10,000 square kilometers), Myanmar (26.24), Vietnam (17.52), and India (15), which may prevent around 282.6 million people from the risk of malaria under the SSP3 scenarios in the SEAR/WPR. Our predicted areas and potential impact groups for An. dirus under future climate change may provide new insights into regional malaria transmission mechanisms and deployment of malaria control measures based on local conditions in the SEAR/WPR’s.
... Before ITN rollout and mass distribution campaigns during the early 2000s in GMS countries, malaria transmission hot spots occurred within the village in southeastern Thailand, in groves of rubber and fruit trees where A. dirus sensu lato (s.l.) is the primary vector [90], and villages near the Thai-Myanmar border in Tak province [91,92]. In the southeastern region [93], it was reported that transmission probably occurred in forest and rubber plantations rather than rice fields. ...
Background:
Despite substantial reductions in malaria burden and improvement in case management, malaria remains a major public health challenge in the Asia-Pacific region. Residual malaria transmission (RMT) is the fraction of total transmission that persists after achievement of full operational coverage with effective insecticide-treated bed nets (ITNs)/long-lasting insecticidal nets (LLINs) and/or indoor residual spray interventions. There is a critical need to standardize and share best practices for entomological, anthropological, and product development investigative protocols to meet the challenges of RMT and elimination goals.
Methods:
A systematic review was conducted to describe when and where RMT is occurring, while specifically targeting ownership and usage of ITN/LLINs, indoor residual spray application, insecticide susceptibility of vectors, and human and vector biting behavior, with a focus on nighttime activities.
Results:
Sixty-six publications from 1995 to present met the inclusion criteria for closer review. Associations between local vector control coverage and use with behaviors of human and mosquito vectors varied by locality and circumstance. Consequently, the magnitude of RMT is insufficiently studied and analyzed with sparse estimates of individual exposure in communities, insufficient or incomplete observations of ITN/LLIN use, and the local human population movement into and from high-risk areas.
Conclusions:
This review identified significant gaps or deficiencies that require urgent attention, namely, developing standardized procedures and methods to estimate risk exposure beyond the peridomestic setting, analytical approaches to measure key human-vector interactions, and seasonal location-specific agricultural or forest use calendars, and establishing the collection of longitudinal human and vector data close in time and location.
... A document stated: "The danger from An. dirus is not only that it is very resistant to control within its habitat but that it is an extraordinarily efficient vector, so long-lived and anthropophilic that only a small population is necessary to maintain high malaria endemicity" (13). Generally, it is a very efficient vector for malaria (14). ...
Background: Among the blood-sucking insects, Anopheles mosquitoes have a very special position, because they transmit parasites of the genus Plasmodium, which cause malaria as one of the main vector-borne disease worldwide. The aim of this review study was to evaluate utility of complete mitochondrial genomes in phylogenetic classification of the species of Anopheles.
Methods: The complete mitochondrial genome sequences belonging to 28 species of the genus Anopheles (n=32) were downloaded from NCBI. The phylogenetic trees were constructed using the ML, NJ, ME, and Bayesian inference methods.
Results: In general, the results of the present survey revealed that the complete mitochondrial genomes act very accurately in recognition of the taxonomic and phylogenetic status of these species and provide a higher level of support than those based on individual or partial mitochondrial genes so that by using them, we can meticulously reconstruct and modify Anopheles classification.
Conclusion: Understanding the taxonomic position of Anopheles, can be a very effective step in better planning for controlling these malaria vectors in the world and will improve our knowledge of their evolutionary biology.
... These sibling species vary in behavior and distribution, as well as in their susceptibility to Plasmodium [6] . An. dirus sensu stricto (formerly An. dirus species A) is the most important malaria vector in forests of several Thai provinces, especially along the mountainous border with Cambodia [7] . An. dirus has also been proven to be an efficient laboratory vector for both Plasmodium falciparum and Plasmodium vivax [8] . ...
Malaria is a serious health problem in Southeast Asia. The malaria vectors Anopheles cracens (Stenogamous) and Anopheles dirus (Eurygamous) were used to evaluate the impact of first and second blood meals on feeding, survival and hatching rates, and fecundity. Females were divided as follows: mated with oviposition, mated without oviposition, and virgin. The blood-feeding rates of An. cracens groups were significantly lower for the second blood meal than for the first. Additionally, for An. dirus mated without oviposition and virgin groups, blood-feeding rates were significantly lower for the second blood meal compared with the first blood meal. Survivorship and the number of eggs laid increased for An. cracens after the second blood meal, but the opposite was true for An. dirus. There were no differences in hatching rates. Thus, blood meals affected survivorship and fecundity in naturally mating An. cracens and induced-mating An. dirus.
... cracens in Peninsular Malaysia [54] and An. dirus (s.l.) in Thailand [55], both of which have adapted and colonized the forest edge and plantations after deforestation. ...
Background:
We investigated the effect of five common habitat types on the diversity and abundance of Anopheles spp. and on the biting rate and time of Anopheles balabacensis (currently the only known vector for Plasmodium knowlesi in Sabah) at Paradason village, Kudat, Sabah. The habitats were forest edge, playground area, longhouse, oil palm plantation and shrub-bushes area. Sampling of Anopheles was done monthly using the human landing catch method in all habitat types for 14 months (October 2013 to December 2014, excluding June 2014). The Anopheles species were morphologically identified and subjected to PCR assay for the detection of Plasmodium parasites. Generalised linear mixed models (GLMM) were applied to test the variation in abundance and biting rates of An. balabacensis in different habitat types.
Results:
A total of 1599 Anopheles specimens were collected in the village, of which about 90% were An. balabacensis. Anopheles balabacensis was present throughout the year and was the dominant Anopheles species in all habitat types. The shrub bushes habitat had the highest Anopheles species diversity while forest edge had the greatest number of Anopheles individuals caught. GLMM analysis indicated that An. balabacensis abundance was not affected by the type of habitats, and it was more active during the early and late night compared to predawn and dawn. PCR assay showed that 1.61% of the tested An. balabacensis were positive for malaria parasites, most of which were caught in oil palm estates and infected with one to two Plasmodium species.
Conclusions:
The identification of infected vectors in a range of habitats, including agricultural and farming areas, illustrates the potential for humans to be exposed to P. knowlesi outside forested areas. This finding contributes to a growing body of evidence implicating environmental changes due to deforestation, expansion of agricultural and farming areas, and development of human settlements near to forest fringes in the emergence of P. knowlesi in Sabah.
... Whilst groups of homesteads consisting of asymptomatic carriers can act as stable clusters over several years [7], it is likely that the light range of 800 m for An. dirus may account for increased probability of repeated mosquito feeding in the same house and clustering of cases over the dry season in Southeastern Thailand [58]. Recent clusters of malaria infection among the parasite reservoir responsible for preserving malaria over the dry season in Ratanakiri Province (northeastern Cambodia) may also explain recurrent transmission at the onset of the rainy season when the vector populations expand [59]. ...
... jeyporiensis were found. All species, except An. jeyporiensis, have been recorded in rubber plantations before (Singh and Tham 1988, Rosenberg et al. 1990, Sallum et al. 2005, Sinka et al. 2011, Bhumiratana et al. 2013a. Interestingly, hardly any specimens of An. dirus s.l. were collected in the secondary forests, even though these primary malaria vectors are often found in SEA forests (Obsomer et al. 2007, Tananchai et al. 2012). ...
The impact of the rapid expansion of rubber plantations in South-East Asia on mosquito populations is uncertain. We compared the abundance and diversity of adult mosquitoes using human-baited traps in four typical rural habitats in northern Lao PDR: secondary forests, immature rubber plantations, mature rubber plantations, and villages. Generalized estimating equations were used to explore differences in mosquito abundance between habitats, and Simpson's diversity index was used to measure species diversity. Over nine months, 24,927 female mosquitoes were collected, including 51 species newly recorded in Lao PDR. A list of the 114 mosquito species identified is included. More mosquitoes, including vector species, were collected in the secondary forest than immature rubber plantations (rainy season, odds ratio [OR] 0.33, 95% confidence interval [CI] 0.31-0.36; dry season, 0.46, 95% CI 0.41-0.51), mature rubber plantations (rainy season, OR 0.25, 95% CI 0.23-0.27; dry season, OR 0.25, 95% CI 0.22-0.28), and villages (rainy season, OR 0.13, 95% CI 0.12-0.14; dry season, 0.20, 95% CI 0.18-0.23). All habitats showed high species diversity (Simpson's indexes between 0.82-0.86) with vectors of dengue, Japanese encephalitis (JE), lymphatic filariasis, and malaria. In the secondary forests and rubber plantations, Aedes albopictus (Skuse), a dengue vector, was the dominant mosquito species, while in the villages, Culex vishnui (Theobald), a JE vector, was most common. This study has increased the overall knowledge of mosquito fauna in Lao PDR. The high abundance of Ae. albopictus in natural and man-made forests warrants concern, with vector control measures currently only implemented in cities and villages.
... Walton, personal observation). Anopheles baimaii has been found breeding in rock-lined wells shaded by mature trees in southern Myanmar (Tun-Lin et al., 1987;Oo et al., 2002) and both species have occasionally been found in mature plantations and orchards (Rosenberg et al., 1990;Obsomer et al., 2007). These are the limits of the use of man-altered environments by these species and both of these settings imitate their natural habitat. ...
Anopheles dirus and Anopheles baimaii are closely related species which feed on primates, particularly humans, and transmit malaria in the tropical forests of mainland Southeast Asia. Here, we report an in-depth phylogeographic picture based on 269 individuals from 21 populations from mainland Southeast Asia. Analysis of 1537 bp of mtDNA sequence revealed that the population history of A. baimaii is far more complex than previously thought. An old expansion (pre-300 kyr BP) was inferred in northern India⁄Bangladesh with a wave of south-eastwards expansion arriving at the Thai border (ca 135– 173 kyr BP) followed by leptokurtic dispersal very recently (ca 16 kyr BP) into peninsular Thailand. The long and complex population history of these anthropophilic species suggests their expansions are not in response to the relatively recent (ca 40 kyr BP) human expansions in mainland Southeast Asia but, rather, fit well with our understanding of Pleistocene climatic change there.
... In Thailand, the primary vectors are night-biting Anopheles mosquitoes which belong to several species complexes. Seven anopheline species are considered primary ) malaria vectors in the country, including Anopheles dirus Peyton & Harrison (Rosenberg et al. 1990, Green et al. 1991, followed by several species that have been incriminated as secondary or suspected human malaria vectors in Thailand, including Anopheles epiroticus Linton & Harbach (Rattanarithikul et al. 2006, Thongsahuan et al. 2011. Still other species, like Anopheles harrisoni Harbach & Manguin, a member of the Minimus Complex, are regarded as very poor or non-vector species in most circumstances . ...
Indoor Residual Spray (IRS) with insecticides has been a procedure used for decades to protect humans from biting mosquitoes and potential vectors of pathogens. The objective of this study was to determine the responses of three wild-caught species of malaria vectors exposed to pyrethroids of three different surface coverage percents using an excito-repellency test box. Each species was exposed to three insecticide-treated surfaces at varying exposure levels (full coverage, 50%, and 25% of the maximum allowable by the test system) to a single standard field dose of either lambda-cyhalothrin or alpha-cypermethrin. Larger numbers of mosquitoes escaped the treated chambers in the direct contact test compared to the spatial repellent chambers in all three different treated surface exposures. No significant differences in the percent of escaped mosquitoes were detected in the 50% and full coverage surface coverage exposures, whereas the 25% coverage produced significantly lower avoidance responses for both compounds. This study found that varying levels of surface exposure with synthetic pyrethroids can impact the behavioral avoidance responses of Anopheles; however, it may also be possible to reduce the amount of coverage to achieve similar avoidance actions. This information may assist policy makers in designing more cost effective strategies involving residual insecticides to control mosquito vectors.
... Aedes aegypti (L) is the primary vector of dengue, yellow fever, and chikungunya and is widely distributed in tropical and subtropical zones (Hales et al. 2002). Anopheles dirus (Peyton and Harrison) is a major vector of malaria in Thailand and other countries in Southeast Asia (Rosenberg et al. 1990). All mosquitoes have aquatic immature stages, and larviciding is a common method to reduce mosquito populations and prevent disease. ...
Cymbopogon citratus (lemongrass) and Syzygium aromaticum (clove) oils were evaluated to determine mortality rates, morphological aberrations, and persistence when used against third and fourth larval instars of Aedes aegypti and Anopheles dirus. The oils were evaluated at 1, 5, and 10 % concentrations in mixtures with soybean oil. Persistence of higher concentrations was measured over a period of 10 days. For Ae. aegypti, both plant oils caused various morphological aberrations to include deformed larvae, incomplete eclosion, white pupae, deformed pupae, dead normal pupae, and incomplete pupal eclosion. All of these aberrations led to larval mortality. In Ae. aegypti larvae, there were no significant differences in mortality at days 1, 5, and 10 or between third and fourth larval instar exposure. In An. dirus, morphological aberrations were rare and S. aromaticum oil was more effective in causing mortality among all larval stages. Both oils were equally effective at producing mortality on days 1, 5, and 10. Both oils had slightly increased LT50 rates from day 1 to day 10. In conclusion, both lemongrass and clove oils have significant effects on the immature stages of Ae. aegypti and An. dirus and could potentially be developed for use as larvicides.
... The GMS hosts a great diversity of anopheline vectors capable of transmitting malaria [17][18][19] . The roles of these vectors in transmitting parasites from several ART-resistant founder populations and the ART-sensitive core population of Cambodia have not been investigated. ...
Artemisinin-resistant Plasmodium falciparum parasites are rapidly spreading in Southeast Asia, yet nothing is known about their transmission. This knowledge gap and the possibility that these parasites will spread to Africa endanger global efforts to eliminate malaria. Here we produce gametocytes from parasite clinical isolates that displayed artemisinin resistance in patients and in vitro, and use them to infect native and non-native mosquito vectors. We show that contemporary artemisinin-resistant isolates from Cambodia develop and produce sporozoites in two Southeast Asian vectors, Anopheles dirus and Anopheles minimus, and the major African vector, Anopheles coluzzii (formerly Anopheles gambiae M). The ability of artemisinin-resistant parasites to infect such highly diverse Anopheles species, combined with their higher gametocyte prevalence in patients, may explain the rapid expansion of these parasites in Cambodia and neighbouring countries, and further compromise efforts to prevent their global spread.
... 7 Dry season transmission of malaria has been reported in areas with permanent bodies of water where anophelines develop. 10 Two El Niño Southern Oscillation events (1987 and 1992) occurred during this study, which for Venezuela meant less rain than expected. There was also 1 La Niña event in 1988 in which more rain fell than expected ( Figure 2). ...
We stratified the risk of malaria transmission (Plasmodium vivax) in 35 villages along a coastal range
in northeastern Venezuela (51 km2) where the main vector is the mosquito Anopheles aquasalis. After 20 years without local malaria transmission, reinfection of the entire area occurred from May to December 1985 by local (continuous) and jump (discontinuous) dispersal. Epidemiologic, environmental, and vector variables were investigated with the aid of a Geographic Information System. Risk factors for malaria transmission were human population density, proximity to pre-adult mosquito habitats (, 500 m), and the number of pre-adult habitats nearby. Most inhabitants, immature mosquito habitats, and malaria cases were located at low elevations and on gentle slopes. High prevalence of malaria during the dry seasons was associated with the presence of permanent bodies of water containing An. aquasalis. Occurrence of a La Nin˜a event in 1988 (wet and cool phase of the El Nin˜o Southern Oscillation) triggered malaria transmission to unusually high levels, consolidating infection in the area, and rendering traditional
control efforts useless. We recommend tracking malaria persistence per village and associated risk factors as methods to reduce the cost of malaria control programs.
... 7 Dry season transmission of malaria has been reported in areas with permanent bodies of water where anophelines develop. 10 Two El Niño Southern Oscillation events (1987 and 1992) occurred during this study, which for Venezuela meant less rain than expected. There was also 1 La Niña event in 1988 in which more rain fell than expected ( Figure 2). ...
... Environmental factors can have a pronounced impact on the distribution and behavior of malaria vectors [71]. Anopheles dirus occurs in forest areas but has an ability to adapt to changing environmental conditions from natural forest habitats to cultivated forests, such as rubber and tea plantations and various types fruit orchards [72,73,27]. Deforestation is one of the most potent factors either promoting or reducing infectious diseases, in particular malaria in SEA [74,75,57]. ...
... Aedes aegypti, the primary carrier for viruses that cause dengue fever, dengue haemorrhagic fever, chikungunya fever, and yellow fever is widespread over large areas of the tropics and sub-tropics (Yang et al., 2009). Additionally, Anopheles dirus is the major vector of malaria in border of Thailand and other countries (Rosenberg et al., 1990). Culex quinquefasciatus, although not regarded as a vector of any diseases in Thailand, is a potential vector of Lymphatic filariasis caused by Wuchereria bancrofti (Jitpakdi et al., 1998). ...
Repellency activity of Thai essential oils derived from ylang ylang (Cananga odorata (Lamk.) Hook.f. & Thomson: Annonaceae) and lemongrass (Cymbopogon citratus (DC.) Stapf: Poaceae) were tested against two mosquito vectors, Aedes aegypti (L.) and Culex quinquefasciatus (Say). There were compared with two chemical repellents (DEET 20% w/w; Sketolene Shield(®) and IR3535, Ethyl butylacetylaminopropionate 12.5% w/w; Johnson's Baby Clear Lotion Anti-Mosquito(®)). Each herbal repellent was applied in 3 diluents; coconut oil, soybean oil and olive oil at 0.33μl/cm(2) on the forearm of volunteers. All herbal repellent exhibited higher repellent activity than IR3535 12.5% w/w, but lower repellent activity than DEET 20% w/w. The C. odorata oil in coconut oil exhibited excellent activity with 98.9% protection from bites of A. aegypti for 88.7±10.4min. In addition, C. citratus in olive oil showed excellent activity with 98.8% protection from bites of C. quinquefasciatus for 170.0±9.0min. While, DEET 20% w/w gave protection for 155.0±7.1min to 182.0±12.2min and 98.5% protection from bites of two mosquito species. However, all herbal repellent provided lower repellency activity (97.4% to 98.9% protection for 10.5min to 88.7min) against A. aegypti than C. quinquefasciatus (98.3% to 99.2% protection for 60min to 170min). Our data exhibited that C. odorata oil and C. citratus oil are suitable to be used as green repellents for mosquito control, which are safe for humans, domestic animals and environmental friendly.
Copyright © 2014. Published by Elsevier B.V.
... Although studies demonstrating a direct association between biting rate and disease incidence are few, Gerry et al. (2001) showed that an increase in bluetongue virus infection in cattle was associated with an increase in the biting rate of the primary midge vector Culicoides sonorensis Wirth and Jones. Similarly, an increase in the human biting rate of Anopheles mosquitoes has been associated with an increase in the incidence of malaria (e.g., Rosenberg et al. 1990, Moreno et al. 2007). Monitoring changes in biting rate therefore should provide "early warning" of increased pathogen transmission or disease outbreaks. ...
... Deforestation process in Amazon forest, Brazil for construction of hydro-electric power station increased the malaria incidence pattern of the area 26 . In Chantaburi, Thailand, deforestation done for rubber plantation and other fruit tree cultivations, favoured An. dirus, due to which malaria transmission was established at higher levels 27 . Deforestation followed by development of coffee plantations in southeast Thailand favoured the breeding of An. minimus s.l. and made the previously malaria-free region to hyperendemic 28 . ...
Background & objectives:
An alarming rate of deforestation has been reported from Sonitpur district of Assam, India therefore, a study was initiated during 2009 using remote sensing (RS) to assess deforested areas in the district and to study the impact on malaria vectors in order to formulate appropriate control strategy.
Methods:
RS imageries of 2000 and 2009 were used to assess deforested areas in the selected district. Entomological data were collected in four surveys during 2009-2011. The data were analyzed statistically using test of single proportions (χ 2 ) and pair-wise comparison. Vector incrimination was done using enzyme-linked immunosorbent assay (ELISA) and entomological inoculation rate (EIR) was calculated to estimate transmission intensity.
Results:
The deforested areas were identified in north-western parts of Sonitpur district falling in Dhekiajuli Primary Health Centre (PHC). The forest cover of the PHC decreased >50% during 2000-2009. Five species of anopheline vectors were collected. Anopheles minimus sensu lato (s.l.) was collected least abundantly while An. culicifacies s.l. prevailed most abundantly and significant difference was observed between proportions of the collected vector species. Pair-wise comparison between An. culicifacies s.l. and An. minimus s.l. was also found statistically significant indicating that An. culicifacies s.l. is establishing its population in deforested areas. An. culicifacies s.l. was found ELISA positive and EIR was measured as 4.8 during transmission season.
Conclusion:
An. culicifacies s.l. replaced An. minimus s.l., the vector of malaria in northeast India and was found ELISA positive, therefore could have possible role in malaria transmission in the deforested areas of the district.
... Of the approximately 73 Anopheles species found in Thailand, select members of the Leucosphyrus Group (Neomyzomyia Series), Maculatus Group (Neocellia Series), and Minimus Subgroup (Myzomyia Series), as the most important malaria vectors in the country [38]. Five species within these 3 assemblages are incriminated as primary malaria vectors in Thailand, including Anopheles baimaii (previously An. dirus D) [61], Anopheles dirus [61,62], Anopheles minimus (previously species A) [63], Anopheles pseudowillmori [61] and Anopheles aconitus [61,64,65]. Manguin et al. [38] provides a current review of the vectorial capacity and bionomics of malaria vectors in the SEA region. ...
Physiological resistance and behavioral responses of mosquito vectors to insecticides are critical aspects of the chemical-based disease control equation. The complex interaction between lethal, sub-lethal and excitation/repellent ('excito-repellent') properties of chemicals is typically overlooked in vector management and control programs. The development of "physiological" resistance, metabolic and/or target site modifications, to insecticides has been well documented in many insect groups and disease vectors around the world. In Thailand, resistance in many mosquito populations has developed to all three classes of insecticidal active ingredients currently used for vector control with a majority being synthetic-derived pyrethroids. Evidence of low-grade insecticide resistance requires immediate countermeasures to mitigate further intensification and spread of the genetic mechanisms responsible for resistance. This can take the form of rotation of a different class of chemical, addition of a synergist, mixtures of chemicals or concurrent mosaic application of different classes of chemicals. From the gathered evidence, the distribution and degree of physiological resistance has been restricted in specific areas of Thailand in spite of long-term use of chemicals to control insect pests and disease vectors throughout the country. Most surprisingly, there have been no reported cases of pyrethroid resistance in anopheline populations in the country from 2000 to 2011. The precise reasons for this are unclear but we assume that behavioral avoidance to insecticides may play a significant role in reducing the selection pressure and thus occurrence and spread of insecticide resistance. The review herein provides information regarding the status of physiological resistance and behavioral avoidance of the primary mosquito vectors of human diseases to insecticides in Thailand from 2000 to 2011.
... The Ae. aegypti is the principal transmitter of dengue fever and dengue haemorrhagic fever (DHF) in Thailand but it also transmits Chikungunya fever [1] . In addition, An. dirus is the major vector of malaria in border of Thailand and other countries [2] . Cx. quinquefasciatus is a vector of Japanese encephalitis (JE) and it also causes annoyance and dermatitis [3] . ...
Objective
To assess the repellency to female Aedes aegypti (Ae. aegypti), Anopheles dirus (An. dirus) and Culex quinquefasciatus (Cx. quinquefasciatus) of seven essential oils using two treatment methods.Methods
Topical applications of three dose concentrations (0.02, 0.10 and 0.21 mg/cm2) were made on the forearms of volunteers. Dose-response study and protection time study were employed in the experiment.ResultsIn the dose-response test, Cymbopogon citratus (C. citratus), Cymbopogon nardus (C. nardus), Syzygium aromaticum (S. aromaticum) and Ocimum basilicum (O. basilicum) exhibited a high repellency against Ae. aegypti with ED50 at < 0.045 mg/cm2, whereas C. citratus, C. nardus and S. aromaticum showed repellency against An. dirus with ED50 at <0.068 mg/cm2. Furthermore, the essential oils of C. citratus, C. nardus, S. aromaticum, O. basilicum and Cananga odorata gave strong effective dose (ED50) values at <0.003 mg/cm2 when tested against Cx. quinquefasciatus. For testing by arm in cage method, at 0.21 mg/cm2, protection time of C. citratus gave the longest lasting period against three mosquito species, 72 min for Ae. aegypti, 132 min for An. dirus and 84 min for Cx. quinquefasciatus. In addition, the two essential oils exhibited moderate repellency against Ae. aegypti, An. dirus and Cx. quinquefasciatus, at 60, 90 and 78 min with C. nardus, and 54, 96 and 72 min with S. aromaticum, respectively.Conclusions
The percentage repellency increased when the concentration of essential oils increased. In contrast, biting rates decreased when the concentration of essential oils increased. C. citratus exhibited high efficiency for the protection time and the percentage of biting deterrent against all of 3 mosquito species.
... Vectorial capacity, a parasite-independent measure of malaria transmission, has been widely used (REISEN, 1989;ROSENBERG et al., 1990;SAUL et al., 1990). However, DAY (1992) pointed out some difficulties and inadequacies in its application. ...
Pooling sampled mosquitoes for sporozoite detection by immunoassays is an efficient and economic approach in situations of very low vector infectivity. In the present study a strategy was proposed to estimate sporozoite rates using this approach. For this purpose there should be only one infected mosquito in any positive pool, so that the number of positive pools examined is an approximation of the number of infected mosquitoes in sample collections. The rationale underlying the strategy is to specify the maximal pool sizes for the given sporozoite rates so that it is reasonable to assert there is no more than one infected mosquito in any positive pool. A statistical model was developed to calculate the maximal pool sizes for various guessed sporozoite rates. The results showed that maximal pool sizes declined non-linearly with increases in the given sporozoite rates. With a guessed sporozoite rate <1% as many as 35 mosquitoes could be pooled for sporozoite determination.
... Theobald, while Anopheles aconitus Doenitz, Anopheles pseudowillmori Theobald and Anopheles epiroticus Linton and Harbach are considered secondary vectors (Gould et al. 1967, Scanlon et al. 1968, Harrison 1980, Rosenberg et al. 1990, Green et al. 1991, Rattanarithikul et al. 1996, Subbarao 1998, Linton et al. 2005, Sallum et al. 2005a) Anopheles vagus Doenitz, are suspected vectors based on positive enzyme linked immunosorbent assay results for oocysts in the midgut and/or circumsporozoite antigens (Baker et al. 1987, Harbach et al. 1987, Gingrich et al. 1990, Frances et al. 1996, Rattanarithikul et al. 1996). As early as 1953, the An. ...
The establishment and spread of antimalarial drug resistance vary drastically across different biogeographic regions. Though most infections occur in sub-Saharan Africa, resistant strains often emerge in low-transmission regions. Existing models on resistance evolution lack consensus on the relationship between transmission intensity and drug resistance, possibly due to overlooking the feedback between antigenic diversity, host immunity, and selection for resistance. To address this, we developed a novel compartmental model that tracks sensitive and resistant parasite strains, as well as the host dynamics of generalized and antigen-specific immunity. Our results show a negative correlation between parasite prevalence and resistance frequency, regardless of resistance cost or efficacy. Validation using chloroquine-resistant marker data supports this trend. Post discontinuation of drugs, resistance remains high in low-diversity, low-transmission regions, while it steadily decreases in high-diversity, high-transmission regions. Our study underscores the critical role of malaria strain diversity in the biogeographic patterns of resistance evolution.
Background
The Anopheles dirus complex plays a significant role as malaria vectors in the Greater Mekong Subregion (GMS), with varying degrees of vector competence among species. Accurate identification of sibling species in this complex is essential for understanding malaria transmission dynamics and deployment of effective vector control measures. This becomes increasingly crucial as the GMS advances towards malaria elimination while facing with the emergence of zoonotic simian malaria transmission. However, the original molecular identification assay, Dirus Allele Specific PCR (AS-PCR) targeting the ITS2 region have pronounced non-specific amplifications leading to ambiguous results and misidentification of the sibling species. This study investigates the underlying causes of these inconsistencies and develops new primers for accurate identification of species within the Anopheles dirus complex.
Methodology/Principal findings
Despite several optimizations by reducing primer concentration, decreasing thermal cycling time, and increasing annealing temperature, the Dirus AS-PCR continued to produce inaccurate identifications, particularly for Anopheles dirus , Anopheles scanloni , and Anopheles nemophilous . Subsequently, in silico analyses pinpointed problematic primers with high GC content and multiple off-target binding sites. Through a series of in silico analyses and laboratory validation, a new set of primers for Dirus Complex Species Identification PCR (DiCSIP) has been developed. DiCSIP primers improve specificity, operational range, and sensitivity for accurate identification of five complex member species found in the GMS. Validation with laboratory and field An. dirus complex specimens demonstrated that DiCSIP could correctly identify all samples while the original Dirus AS-PCR misidentify An. dirus as other species when used with different thermocyclers.
Conclusion/Significance
The DiCSIP assay offers a significant improvement in An. dirus complex identification, addressing challenges in specificity and efficiency of the previous ITS2-based assay. This new primer set provides a valuable tool for accurate entomological surveys, supporting effective vector control strategies to reduce transmission, and prevent the re-introduction of malaria in the GMS.
Author Summary
Several species of Anopheles mosquitoes belong to species complexes due to their indistinguishable morphology from closely related sibling species. However, members of the same species complex may exhibit varied vectorial capability, i.e. the ability to spread human pathogens, in this case malaria parasites, ranging from dominant vector to non-vector. The Dirus AS-PCR molecular assay to identify species within the An. dirus complex, a significant malaria vector species in the GMS, can produce inconsistent PCR results leading to misidentification. Furthermore, our analysis of An. dirus ITS2 sequences in the NCBI database indicated misidentification between these sibling species suggesting the need for a new set of primers to improve reproducibility and sensitivity in identifying members of the An. dirus complex. This study presents thorough analyses of the existing primers that cause difficulties in correct amplification and a development of a new set of primers for Dirus Complex Species Identification PCR (DiCSIP) assay. The DiCSIP assay offers several advantages over the original Dirus AS-PCR. It provides a higher specificity, sensitivity and wider operational range allowing for the use of the DNAzol direct reagent to process mosquito samples without the need for DNA extraction, saving both time and cost in sample processing. Our study provides a valuable molecular tool for entomological surveys, which is crucial for effective vector control measures in the GMS.
During sporogony, malaria-causing parasites infect a mosquito, reproduce and migrate to the mosquito salivary glands where they can be transmitted the next time blood-feeding occurs. The time required for sporogony, or extrinsic incubation period (EIP), is a crucial determinant of malaria transmission intensity. The EIP is typically estimated as the time for a given percentile of infected mosquitoes to have salivary gland sporozoites (the infectious parasite life stage). Many mechanisms, however, affect the observed sporozoite prevalence including the human-to-mosquito transmission probability and possibly differences in mosquito mortality according to infection status. To account for these various mechanisms, we present a mechanistic mathematical model (“mSOS”), which explicitly models key processes at the parasite, mosquito and observational scales. Fitting this model to experimental data, we find greater variation in EIP than previously thought: we estimated the range between two percentiles of the distribution, EIP 10 –EIP 90 (at 27°C), as 4.5 days, compared to 0.9 days using existing methods. This pattern holds over the range of study temperatures included in the dataset. Increasing temperature from 21°C to 34°C decreased the EIP 50 from 16.1 to 8.8 days and the human-to-mosquito transmission probability from 84% to 42%. Our work highlights the importance of mechanistic modelling of sporogony to (1) improve estimates of malaria transmission under different environmental conditions or disease control programs and (2) evaluate novel interventions that target the mosquito life stages of the parasite.
Author summary
Anopheles mosquitoes become infected with malaria-causing parasites when blood feeding on an infectious human host. The parasites then process through a number of life stages, which begin in the mosquito gut and end in the salivary glands, where the newly formed infectious parasites can be transmitted to another host the next time a mosquito blood-feeds. The large variability in parasite numbers and development times that exists between mosquitoes, environments and parasites, mean that understanding parasite population dynamics from individual mosquito dissections is difficult. Here, we introduce a mathematical model of the mosquito life stages of parasites that mimics key characteristics of the biology. We show that the model’s parameters can be chosen so that its predictions correspond with experimental observations. In doing so, we estimate key system characteristics that are crucial determinants of malaria transmission intensity. Our work is a step towards a realistic model of within-mosquito parasite dynamics, which is increasingly important given that many recently proposed disease interventions specifically target mosquito life stages of the parasite.
Background: clinical and epidemiological aspects of mixed plasmodial infection and mixed malaria are poorly understood; in America this information is almost absent. Objective: review information about mixed plasmodial infection and mixed malaria and collect and organize Colombian data. Methodology: different expressions were used to find information in PubMed and Lilacs. Results and conclusions: information retrieved was little, dispersed and difficult to find. Mixed plasmodial infection and mixed malaria frequency is variable and relatively high but is underestimated by microscopy. New plasmodial species often is detected after successful treatment of other species. Plasmodial coinfections are mutually suppressive; P. falciparum dominating on P. vivax and P. vivax attenuating severity of P. falciparum. Clinical and parasitemia of each species in mixed plasmodial infection and mixed malaria are not sufficiently studied; its epidemiological behavior is very variable and depends on transmission intensity and stability, rain cycles, age of patients and anopheline fauna. There is insufficient basis to guide the mixed plasmodial infection and mixed malaria treatment, but it is suggested a simultaneous treatment of the different species. (MÉD.UIS. 2011;25(1):45-54)
The Cambodian National Strategic Plan for Elimination of Malaria aims to move step by step toward elimination of malaria across Cambodia with an initial focus on Plasmodium falciparum malaria before achieving elimination of all forms of malaria, including Plasmodium vivax in 2025. The emergence of artemisinin-resistant P. falciparum in western Cambodia over the last decade has drawn global attention to support the ultimate goal of P. falciparum elimination, whereas the control of P. vivax lags much behind, making the 2025 target gradually less achievable unless greater attention is given to P. vivax elimination in the country. The following review presents in detail the past and current situation regarding P. vivax malaria, activities of the National Malaria Control Program, and interventional measures applied. Constraints and obstacles that can jeopardize our efforts to eliminate this parasite species are discussed.
The maps presented (e.g. Figure 1) provide species location information and highlight the existence of a greater number of vector species than is often considered, many in sympatry, across the malarial zones. Amongst these DVS, there are often important behavioural characteristics that must be considered if successful vector control is to be applied. For example, some species do not always enter houses to bite, are most active in the early evening, and prefer to rest outdoors after feeding, such as many of the species common in South America (e.g. An. albopictus, An. marajoara, An. nuneztovari), An. dirus in South-East Asia and An. farauti in the Australian-Pacific region. Others are highly opportunistic in their feeding habits (including An. darlingi, the most ‘dominant’ south American species amongst the South American DVS); biting readily indoors or out. As such, a large investment in insecticide treated bednets (ITNs) or insecticide residual spraying (IRS) will not reduce malaria transmission where such species occur. Thus, to appreciate where different malaria control methods are best applied, one must know what Anopheles species exists in an area and understand their behaviour. A map clearly and simply addresses the first of these needs, and if accompanied by a behavioural summary, than informed decisions about how to combat malaria transmission can be made.
Maps clearly illustrate the spatial extent of a species’ distribution. Often, even within a single Anopheles species range, behaviour can vary depending on location. The best known example is the An. gambiae complex. The An. gambiae complex was initially considered as a single species. Clear differences in behaviour reported across its distribution caused it to be examined more closely and now this complex is considered to include eight species [1, 6] including the DVS: An. arabiensis, An. gambiae, An melas and An. merus. Anopheles arabiensis, is considered mostly zoophilic, when compared to the highly anthropophilic An. gambiae, but still plays a very important role in malaria transmission – indeed, its presence and propensity to rest outdoors is attributed (amongst other factors) to the ‘failure’ of the mass indoor residual spraying program intended to control malaria in Nigeria during the Garki project [7]. Variability in behaviour within the An. gambiae species (rather than the complex) is also commonly reported [5]. Such spatially dependant variability amongst the DVS will be discussed further within this chapter.
Beside the apparent inability to exist in the forested west of Africa, An. arabiensis appears to tolerate a much greater range of larval sites than An. gambiae. Similar to its sibling, it makes use of sunlit, temporary, shallow fresh-water habitats and the larger more permanent sites as characterised by An. gambiae M form, but it is also able to survive in flowing water, turbid or polluted sites and even, on occasion, brackish habitats [5, 63-66]. It readily makes use of rice fields, although its propensity for sunlit water means it is primarily found when the rice plants are small and larval numbers reduce substantially as the plants mature [67-70]. The adaptability, plasticity and general tendencies for An. arabiensis to feed outdoors on animals (Table 2) means that this species does not readily succumb to traditional methods of control such as IRS or ITNs [59, 71, 72].
Unprecedented economic growth in Southeast Asia (SEA) has encouraged the expansion of rubber plantations. This land-use transformation is changing the risk of mosquito-borne diseases. Mature plantations provide ideal habitats for the mosquito vectors of malaria, dengue, and chikungunya. Migrant workers may introduce pathogens into plantation areas, most worryingly artemisinin-resistant malaria parasites. The close proximity of rubber plantations to natural forest also increases the threat from zoonoses, where new vector-borne pathogens spill over from wild animals into humans. There is therefore an urgent need to scale up vector control and access to health care for rubber workers. This requires an intersectoral approach with strong collaboration between the health sector, rubber industry, and local communities.
Mosquito survival probabilities. Data with regard to the daily survival probability of adult mosquitoes (pd) as derived from entomological field studies.
Sexual parasite ratios. Data with regard to the sexual parasite ratio (sPR), that is the percentage of humans with gametocytes in their blood as well as the ratio between sexual and asexual parasite ratio (SAR), which is the proportion of malaria parasite positive humans that are gametocytaemic.
Complexes of closely related cryptic species, that are indistinguishable morphologically yet have different ecology and host preferences, are widespread in mosquitoes. The advantages of DNA-based methods of identification mean that they have now largely replaced other methods of species determination for such complexes. Here we discuss the relative merits of three different approaches to species identification, all of which use amplification of the ribosomal RNA genes by the polymerase chain reaction. They include: restriction fragment length polymorphism in the Anopheles maculipennis complex from the UK; allele-specific amplification in the An. dirus complex from Thailand; and single strand conformational polymorphism in the An. minimus complex from Thailand. The application of these methods is considered in the context of recent data on intraspecific genetic variation, geographic population structure and genetic introgression.
A study of species diversity of Anopheles mosquitoes, biting patterns, and seasonal abundance of important
mosquito vectors was conducted in two villages of Chang Island, Trat Province, in eastern Thailand, one located along the coast and the other in the low hills of the central interior of the island. Of 5,399 captured female anophelines, 70.25% belong to the subgenus Cellia and remaining specimens to the subgenus Anopheles. Five important putative malaria vectors were molecularly identified, including Anopheles epiroticus, Anopheles dirus, Anopheles sawadwongporni, Anopheles maculatus, and Anopheles minimus. Anopheles epiroticus was the most commonly collected species in the coastal site, whereas An. dirus was found to be
most abundant in the forest-hill site. From both locations, a greater number of mosquitoes was collected during the dry season compared to the wet. Anopheles epiroticus showed greater exophagic and zoophilic behavior with the highest blood feeding densities occurring between 18:00 and 19:00. In contrast, An. dirus demonstrated an activity peak between midnight and 01:00. We conclude that An. epiroticus and An. dirus, in coastal and inland areas, respectively, appear to be the most epidemiologically important malaria vectors on Chang Island. As no studies of vector competency specific to Chang Island have been conducted, our conclusions that these two species play a primary role in malaria transmission are based on evidence from other localities in Thailand and mainland Southeast Asia. This information serves as a basis for designing improved vector control programs that target specific species, and if integrated with other interventions could result in the elimination of malaria transmission on the island.
Malaria and filariasis are two mosquito borne diseases that are in the pipeline for elimination in most countries in Southeast Asia. In this review, the bionomics of the important vectors are discussed in relation to the changing environment and landscape. Due to good control programmes, the cases of malaria and filariasis have diminished in numbers and so have the vectors. However, behavior of vec-tors have changed. The challenges faced in maintaining the diseases at low levels are discussed.
Revision of the Leucosphyrus Group of Anopheles (Cellia) (Diptera, Culicidae). This is a comprehensive revision of 20 species of the Leucosphyrus Group of the Neomyzomyia Series of Anopheles (Cellia). Morphological description of the adults, male and female, male genitalia, pupa and fourth-instar larva are provided for each taxon in addition to bionomics, distribution data and systematic discussion for each species, including diagnostic characters. Identification keys for females and fourth-instar larvae are provided. When possible medical importance of each species is included. Illustrations of of the adults, fourth-instar larvae and pupae are provided. Distribution maps for each species are mainly based on the material examined; however, when possible published data were also used. Tables on adult character variations, fourth-instar larval and pupal setal branching are included as appendices. A neotype for An. takasagoensis Morishita and An. sulawesi Koesoemawinangoen, and a lectotype for An. balabacensis Baisas, are designated. The authorship of An. sulawesi previously cited as Waktoedi is corrected to Koesoemawinangoen.
Malaria is largely neglected in the South-East Asia Region (SEAR), although it has the highest number of people susceptible to the disease. Malaria in the SEAR exhibits special epidemiological characteristics such as "forest malaria" and malaria due to migration across international borders. The Greater Mekong Subregion (GMS) has been a focal-point for the emergence of drug resistant malaria. With the recent emergence of artemisinin resistance, coupled with the limited availability of insecticides, malaria control efforts in the SEAR face a steep challenge. Indirect man-made factors such as climate change, as well as direct man-made factors such as the circulation of counterfeit drugs have added to the problem. Increased monitoring, surveillance, pharmacovigilance as well as cross-border collaboration are required to address these problems. Regional networking and data-sharing will keep all stakeholders updated about the status of various malaria control programmes in the SEAR. Cutting-edge technologies such as GIS/GPS (geographical information system/global positioning system) systems and mobile phones can provide information in "real-time". A holistic and sustained approach to malaria control by integrated vector management (IVM) is suggested, in which all the stakeholder countries work collaboratively as a consortium. This approach will address the malaria problem in a collective manner so that malaria control can be sustained over time.
Reported are malaria sporozoite and inoculation rates over a 1-year period in eight epidemiologically defined villages of different endemicity in Madang Province, Papua New Guinea. In the study, more than 41 000 wild-caught mosquitos were analysed for Plasmodium falciparum and P. vivax sporozoites by ELISA. In a given village the entomological inoculation rates correlated strongly with the prevalences of both these malarial parasites in children. However, the prevalence of P. falciparum infections in children was much higher than that of P. vivax, despite similar inoculation rates for the two species. These data suggest that in Papua New Guinea P. falciparum is more efficiently transmitted than P. vivax from mosquito to man. The increased efficiency of transmission of P. falciparum may be due to the heavier sporozoite densities in wild-caught mosquitos naturally infected with P. falciparum sporozoites that were tenfold greater than the sporozoite densities in mosquitos infected with P. vivax.
The relationship of ANOPHELES BALABACENSIS Baisas to the transmission of human malaria was studied in village and forest settings in Thailand. Data were accumulated on seasonal abundance, biting habits and infection rates of mosquitoes. In the dry season, An. balabacensis could only be found in the forest. While indoor transmission probably occurred in the villages during the rainy season, the risk of acquiring malaria was highest in the forest. Emphasis was also placed on locating and describing oviposition sites throughout the year. Oviposition occurred all year in wet areas of the forest, but larvae could be collected near villages only during the rainy season. Eggs of An. balabacensis, when deposited on moist soil at margins of ground pools, remained viable long enough to survive intervals between rains in the monsoon season, but there was no evidence that they could survive the dry season.
THE daily survival-rates of Anopheles gambiae and A. funestus in two areas of tropical Africa have recently been estimated from the ratio of their sporozoite-rates immediately on capture and those recorded after keeping wild-caught specimens alive for the duration of the extrinsic cycle of the predominant malaria parasite1,2. This method naturally involves considerable time for the dissection and keeping alive of significant numbers of mosquitoes.
Studies on malaria and on A. b. balabacensis and A. minimus responses to DDT spraying were conducted in a forested hilly area in northern Thailand. In a first phase, base-line data were collected from July 1970 to March 1972. In a second phase, the study area received five round of DDT spraying over a period of two years and at the same time all malaria infections received radical treatment. During this two-year period of field operations, entomological and epidemiological observations were continued. The studies carried out in the second phase, showed that malaria transmission decreased under the applied optimum anti-malarial measures but was not interrupted. Human ecology and population movement inside the forest, especially during the dry season, contributed to a great extent to this result. The transmission occurring in the early part of the monsoon season clearly indicates the importance of the timing of DDT spraying. A. b. balabacensis appeared to be transmitting malaria all the year round in the deep forest but only in the monsoon season in the forest fringe. The vectorial capacity of both vectors was estimated separately for indoor and outdoor populations. The pre-spraying values obtained for A. b. balabacensis were much higher thaan for A. minimus. After DDT spraying A. b. balabacensis showed a decrease in vectorial capacity estimated at 31.5 times for the indoor population and 18 times for the outdoor population. A. minimus, on the other hand, showed a much smaller decrease, estimated at 6.8 and 1.9 times for the indoor and outdoor populations respectively.
Two numerically minor components of Plasmodium falciparum prevalence-gametocytaemia and trophozoite densities 99/500 white blood cells-displayed an annual cycle that reflected the
seasonal abundance of infective Anopheles dirus at a hyperendemic focus in Thailand, even though the gross monthly prevalence for combined ages remained stable. Gametocyte
prevalence rose more than 300% within 30 d after the capture of the dry season's first infective mosquito, remained at about
8% until the beginning of the monsoon 7 months later, then fell within 60 d to about 2%. The number of cases with a high density
of trophozoites behaved similarly. These periodic fluctuations represented changes in incidence, at least half of which appeared
to be due to superinfection. Almost 49% of all gametocyte carriers were older than 14 years, but nearly all gametocyte densities
20/500 white blood cells were in children. These observations, as well as the calculated efficiency of human infectivity,
imply that superinfection of adults may contribute significantly to transmission in semiimmune populations.
Blood from most of the 250 residents of a non-migratory farming village in south-eastern Thailand was visually examined for malaria parasites monthly for 2 years. Nearly 97% of the population had at least one (median = 5) patent Plasmodium falciparum infection per year; 72% had one due to P. vivax (median = 1). This contrasted with a slide positivity rate of 17% calculated from 12 months of passive case detection before the study began. Children 1-9 years old had the highest mean monthly prevalence (51%) and highest geometric mean density (10/500 white blood cells) of P. falciparum. Fewer than half the expected number of mixed infections were found but these were more common at high densities of P. falciparum. Individuals over 19 years old comprised 52% of the population but accounted for only 18% of P. vivax and 32% of P. falciparum gametocytaemias. Fever rates were marginally higher in those below 10 years old (8%) but occurred with equal frequency in those with patent infections or negative. The spleen rate (89% stage 1) was 24% in those under 15 years old and 7% in those older. No malaria mortality was seen P. falciparum cases treated for 10 d with quinine+tetracycline (QT) cleared the infection as often as those given one dose of mefloquine+sulfadoxine+pyrimethamine (MSP); both treatments reduced densities in cases not cured. Apparently unsupervised compliance was no better with MSP than with QT. The role played by hyperendemic, cryptic foci in Asian epidemics of malaria may have been underestimated.
Restrained Anopheles stephensi mosquitoes infected with Plasmodium falciparum were made to produce time-dependent series of saliva droplets in mineral oil. The relative volume of each droplet and the number of sporozoites each contained were determined microscopically; gland sporozoites were estimated with an enzyme-linked immunosorbent assay. Median gland infection was 8170 and median number of sporozoites ejected was 15 (range, 0-978). Inoculum size was positively correlated to the number of sporozoites in the salivary glands. Most mosquitoes ejected sporozoites only at the beginning of salivation; this suggests that only those parasites in the common and secondary salivary ducts at the time of feeding can be ejected. The small size of inocula may explain some aspects of malaria transmission, including the often observed discrepancy between inoculation and incidence rates.
Enzyme-linked immunosorbent assays (ELISA's) were compared with dissection results on mosquitoes of the Anopheles punctulatus Doenitz complex collected in Papua New Guinea. Assays were based on monoclonal antibodies developed against the immunodominant peptide repeat portion of the circumsporozoite (CS) proteins of Plasmodium falciparum (Welch) and Plasmodium vivax (Grassi & Feletti) sporozoites. Excellent correlation was observed between the two methods, with positivity rates of 4.0% for salivary-gland midgut infections determined by dissection and 3.4% for the combined ELISA's. These species-specific assays, with a 10-fold increase in sensitivity over previously used ELISA's, gave geometric means of 920 P. falciparum and 460 P. vivax sporozoites per infected mosquito. Use of genetically engineered P. falciparum and synthetic P. vivax CS antigens eliminated the requirement for salivary-gland sporozoites as quantitative positive controls. Aproximately 30 pg of recombinant P. falciparum and 60 pg of synthesis P. vivax CS antigen corresponded to 100 salivary-gland sporozoites in the respective ELISA.
In order to assess the factors of malaria vectorial capacity and the daily reproduction rate, separate consideration is given to data from Kankiya, Northern Nigeria, concerning the incidence of vector-man contact (the man-biting rate), the vector's expectation of infective life, as reflected by the proportion of parous mosquitos under certain conditions, and the vector's man-biting habit, comprising the frequency of feeding and the human blood index. The main difficulty in the assessment of each of these factors was shown to be that of representative and adequate sampling, especially in a sprayed area. In order to compensate for deficiencies in the Kankiya data, especially with regard to the daily and cyclic survival-rates, the gonotrophic cycle and the effective sporogonic period, more complete published data on an Anopheles gambiae population in East Africa were examined, and extrapolations were made from these data in spite of the consequential risks involved.The results of the analysis show that the spraying of an area with DDT reduced the malaria vectorial capacity of Anopheles gambiae sp. B (the main vector of Plasmodium falciparum in the area) by an over-all factor of about 23 times. Nevertheless the basic reproduction rate of the disease is estimated to have averaged slightly over 20 in the sprayed area during the 6 months of the main transmission season. This is consistent with an observed recovery in the parasite rate, which had been reduced to a very low level by regular mass drug administration through the preceding dry season.The analysis was a tentative exercise in "epidemiological entomology" and it is suggested that in the postgraduate teaching of tropical hygiene, the epidemiological approach to entomology should be preferred to the classical morphological-bionomical approach.
The entomological inoculation rate is presented as a comprehensive indicator of malaria transmission level, its relative changes reflecting the risk of potential epidemic development. This rate is a known function of several epidemiological direct factors and is particularly sensitive to the survival rate and the sporogonic cycle of the vector. Although not yet fully quantifiable, relationships exist between direct factors responsible for the transmission of infection and certain meteorological and environmental indirect factors like air temperature, relative humidity, or importation of malaria parasites.The establishment of a two-stage monitoring system is suggested: the first stage would involve setting up a warning system based on the surveillance of the relevant indirect factors; at the appropriate time, this would trigger the second stage monitoring of the epidemiological direct factors having a definite bearing on the development of malaria outbreaks. It is recognized that the gain in reliability of the proposed approach depends largely on the progress still to be achieved in the quantification of the complex system of relations connecting the main direct factors with single or combined indirect factors. It is also noted that the proposed monitoring system should, in due course, provide the decision-makers with the epidemiological information required for the selection and implementation of intervention measures designed to prevent epidemic resurgences.
A thorough, repetitive, 21-month search for breeding sites of Anopheles dirus (=An. balabacensis s.l.) was conducted within a 3-km radius of an isolated, holoendemic malaria focus. During the annual 6-month monsoon, all breeding within 1.5 km of the focus took place at eight sites in 1975 and at nine sites in 1976. These sites were of two topographic types--puddles on footpaths and turbulence pits at the heads of drainage gullies--that shared the characteristic of being able to hold water for at least 5 days without supplemental rainfall. The only dry season breeding was in a stream 3 km from the focus. The effective flight range in forest of An. dirus was deduced to be about 1.5 km. Immatures were found to be peculiarly adapted to, and possibly dependent on, small, temporary pools. Eggs were deposited above the water line where they could remain viable for 2 weeks. Heavy rain produced a synchronous hatch that completed immature development in 5-6 days and could eventually produce waves of parous females biting man. Larvae were observed to leave a draining pool before it completedly emptied and to crawl overland as far as 53 cm, sometimes to another pool. Larvae stranded by receding water often escaped predation that eliminated cohabiting species. They could be recovered alive from drained pool bottoms up to 94 hours after being stranded. Fourth instar An. dirus are passively predatory of first and second instar mosquito larvae. The limited number and topographic distinctness of the habitat of immature An. dirus in Bangladesh may make larviciding an effective control measure.
Seasonal, holoendemic malaria transmission in a small, isolated forest community was studied by doing outdoor and indoor all-night man-biting catches over 21 consecutive months. More than 3.8% of Anopheles dirus (=An. balabacensis s.l.), the most frequently caught anopheline, were infective. One An. annularis was also infective. Transmission occurred only during the 7-month monsoon. In the absence of DDT, An. dirus bit with equal frequency indoors and outdoors. When DDT was present in dwellings, fewer females fed indoors and they fed earlier. Feeding pattern was influenced by the phase of the moon: peak outdoors feeding was sharpest and earliest at first quarter and came later as the moon rose later. An average 31% of biting An. dirus lived long enough to reach infectivity of P. falciparum. Although fewer than 10 females fed per man per night, a resident could have received more than 100 infective bites in 2 years. Correlation between actual and calculated rates of gametocytemia were poorest in months when calculated survival rates of mosquitoes were most suspect.
Insecticide susceptibility tests with diagnostic dosages of 4% DDT and 5% malathion concentrations conducted at field sites on the two important principal malaria vectors namely, Anopheles dirus and An. philippinensis have been reported. They are found to be susceptible to DDT and malathion. The implication of the use of insecticides in controlling malaria is also discussed.
The malarial infectivity of an African village community with particular reference to Plasmodium falciparum has been tested by selecting subjects at random, irrespective of their blood picture, and feeding batches of laboratory-bred Anopheles gambiae on them. The degree of infectiousness has been judged by the number and identity of oocysts found in the stomach of the engorged mosquitoes after varying periods of incubation.
Random tests on 347 subjects of all ages indicate that for every 100 individuals in the African village population of average age composition there are at any one time about 10 or 11 individuals capable of infecting mosquitoes with malaria parasites, mainly P. falciparum. All age groups contribute to this infective reservoir, the actual numbers observed being 4.2 infants and toddlers (under 5 years), 3.0 school children (5 to 14 years) and 3.3 adolescents and adults (15 years and over). Most of the infections picked up in this random survey had a low infectivity to mosquitoes. There is no indication that unusually high infectivity is necessarily associated with the lower age groups. It is considered that under these conditions of “hyperendemic” malaria, adolescents and adults form at least 30 per cent of the total reservoir of malaria infection in the human population.
In malaria eradication the residual insecticide exerts upon the mosquito's vectorial capacity a direct insecticidal impact, the order of which may be measured by observing the decrease in the proportion of parous females. The impact is expressed as the product of the degree of reduction of the expectation of infective life (termed the longevity factor of impact) and that of the expectation of life (the density factor). To compute the factors from the proportion parous it is necessary to know also the mean difference in age between the nulliparous and the youngest parous females in the sample, and the sporogonic period of the parasite.Graphs are presented to enable the field worker, who has observed these parameters, to read off from his data the proportion surviving one day, the expectation of infective life and the expectation of life. Examples from the field are used to illustrate the manner of computing the direct insecticidal impact with the aid of the graphs. It is emphasized that this method can only measure the relative impact on vectorial capacity, and will not show whether the actual level of vectorial capacity is such that a malaria reproduction rate of <1 is indicated.
Field studies were conducted in southeastern Thailand on the biology of ANOPHELES BALABACENSIS Baisas, 1936, and its relationship to malarial transmission in a hyperendemic area. In recent years A. balabacensis has been found to be an important vector of human malaria in the forested hills of Thailand and adjacent countries of SE Asia. It also appears to be the most important vector of malaria in regions where drug-resistant falciparum strains have been detected. Emphasis was placed on determination of feeding habits, and the time and season of maximum abundance. A. balabacensis was found most abundantly in the latter part of the monsoon season (Sept.–Nov.); it congregated near humans in early evening, but did not feed in large numbers until after midnight. The species was found to be exophilic and exophagic, and none could be found away from the jungle during daylight hours. Gland infection rate was over 8% during the period of peak abundance, and several of our team contracted falciparum malaria which was refractory to treatment by synthetic drugs such as chloroquine. Data on larval breeding sites were accumulated, and comments made on methods for control of malaria in the study area.
The progress of knowledge regarding the importance of A. leucosphyrus as a vector of malaria is discussed.
A. leucosphyrus is distributed across Asia from India to Celebes, and from Java to Formosa.
It was first described in 1901, but there appear to be at least six or seven forms of the insect.
It was first recorded as suspected on indirect evidence in 1914 in North Borneo, and has since been recorded as suspected on epidemiological evidence in Sarawak, Indo-China, Burma, Assam and elsewhere.
A. leucosphyrus was first recorded (in 1920) as having been found “positive” on dissection in 1918 in Sumatra.
It was first recorded as having a high preference for human blood in 1932 in the Netherlands Indies, and the same observation has since been made in Borneo, Assam and Bengal, Indo-China and Burma.
The first positive gland dissection appears to have been carried out in 1938 and recorded in 1941 in Assam.
Infections have since been recorded of A. leucosphyrus in Sumatra, Dutch Borneo, Indo-China, Assam, North Borneo, Labuan, Sarawak and Burma, and of vat. hackeri in Celebes.Of the different forms of A. leucosphyrus-apart from vat. hackeri in Celebes—the evidence is all in favour of the type form, the most widespread of all, being the only one of importance as a vector of malaria.
Sixty series of dissections are collected from all over Asia from India to Celebes.Of this series, more than half are positive, and every series of more than 40 dissections is positive.
A. leucosphyrus is frequently described as being harmless or of little importance.
From a consideration, however, of its widespread distribution, its sometimes very elusive nature, its preference for human blood, the epidemiological evidence, the paucity of dissections, and the fact that it has been found infected wherever it has been dissected in adequate numbers, it is considered that A. leucosphyrus is more important as a vector of malaria throughout Asia than has hitherto been appreciated.
Following attack on an insect vector-perhaps with insecticide or by removing breeding sites-we need to know what reduction in disease can be expected. In general, reliable predictions of the epidemiological consequences of any entomological change are highly desirable. Such predictions require a measure of the capacity of an insect population to transmit disease. A popular approach is to try to quantify all the entomological components of transmission, in isolation from the parasitological components, and then calculate the maximum daily reproductive rate of the disease-known as the Vectorial Capacity. But to measure accurately all the entomological components is technically demanding. In this article, Chris Dye argues that epidemiological predictions could be made by including only the dominant entomological variables in an index-a streamlined Vertical Capacity-which can then be fine-tuned by closely matching entomological and parasitological observations.
Observations on the periodicity of natural infections in the anopheline mosquitoes of Borneo
- Colless
Colless, D. H. (1952). Observations on the periodicity of natural infections in the anopheline mosquitoes of Borneo. Medical Journal of Malaya, 6, 234-240.
Studies on malaria and Anopheles balabacensis balabacensis and Anopheles minimus in Burma. Geneva: World Health Organization, Mimeographed document no
- Khin
- S M Winn
Khin-Maung-Kyi & Winn, S. M. (1976). Studies on malaria and Anopheles balabacensis balabacensis and Anopheles minimus in Burma. Geneva: World Health Organization, Mimeographed document no. WHO/MAL/76.875.
Susceotibilitv of Thai malaria vectors to insecticides. In: Se&f Conference on Malaria Re-search
- S Patiuonase
Patiuonase, S. (1986). Susceotibilitv of Thai malaria vectors to insecticides. In: Se&f Conference on Malaria Re-search. Bangkok: Ministry of Public Health, p. 170.
The Anopheles leucosphyrus group
- Colless
Studies on malaria and Anopheles balabacensis balabacensis and Anopheles minimus in Burma
- Khin-Maung-Kyi
Observations on the flight range of A. balabacensis Baisas (= A. leucosphyrus, auct)
- Colless