Theeraphap Chareonviriyaphap

The dominant Anopheles vectors of human malaria in the Asia-Pacific region: occurrence data, distribution maps and bionomic précis

BackgroundThe final article in a series of three publications examining the global distribution of 41 dominant vector species (DVS) of malaria is presented here. The first publication examined the DVS from the Americas, with the second covering those species present in Africa, Europe and the Middle East. Here we discuss the 19 DVS of the Asian-Pacific region. This region experiences a high diversity of vector species, many occurring sympatrically, which, combined with the occurrence of a high number of species complexes and suspected species complexes, and behavioural plasticity of many of these major vectors, adds a level of entomological complexity not comparable elsewhere globally. To try and untangle the intricacy of the vectors of this region and to increase the effectiveness of vector control interventions, an understanding of the contemporary distribution of each species, combined with a synthesis of the current knowledge of their behaviour and ecology is needed.ResultsExpert opinion (EO) range maps, created with the most up-to-date expert knowledge of each DVS distribution, were combined with a contemporary database of occurrence data and a suite of open access, environmental and climatic variables. Using the Boosted Regression Tree (BRT) modelling method, distribution maps of each DVS were produced. The occurrence data were abstracted from the formal, published literature, plus other relevant sources, resulting in the collation of DVS occurrence at 10116 locations across 31 countries, of which 8853 were successfully geo-referenced and 7430 were resolved to spatial areas that could be included in the BRT model. A detailed summary of the information on the bionomics of each species and species complex is also presented.ConclusionsThis article concludes a project aimed to establish the contemporary global distribution of the DVS of malaria. The three articles produced are intended as a detailed reference for scientists continuing research into the aspects of taxonomy, biology and ecology relevant to species-specific vector control. This research is particularly relevant to help unravel the complicated taxonomic status, ecology and epidemiology of the vectors of the Asia-Pacific region. All the occurrence data, predictive maps and EO-shape files generated during the production of these publications will be made available in the public domain. We hope that this will encourage data sharing to improve future iterations of the distribution maps.

A global map of dominant malaria vectors

BackgroundGlobal maps, in particular those based on vector distributions, have long been used to help visualise the global extent of malaria. Few, however, have been created with the support of a comprehensive and extensive evidence-based approach.MethodsHere we describe the generation of a global map of the dominant vector species (DVS) of malaria that makes use of predicted distribution maps for individual species or species complexes.ResultsOur global map highlights the spatial variability in the complexity of the vector situation. In Africa, An. gambiae, An. arabiensis and An. funestus are co-dominant across much of the continent, whereas in the Asian-Pacific region there is a highly complex situation with multi-species coexistence and variable species dominance.ConclusionsThe competence of the mapping methodology to accurately portray DVS distributions is discussed. The comprehensive and contemporary database of species-specific spatial occurrence (currently available on request) will be made directly available via the Malaria Atlas Project (MAP) website from early 2012.

A new classification system for the actions of IRS chemicals traditionally used for malaria control.

Knowledge of how mosquitoes respond to insecticides is of paramount importance in understanding how an insecticide functions to prevent disease transmission. A suite of laboratory assays was used to quantitatively characterize mosquito responses to toxic, contact irritant, and non-contact spatial repellent actions of standard insecticides. Highly replicated tests of these compounds over a range of concentrations proved that all were toxic, some were contact irritants, and even fewer were non-contact repellents. Of many chemicals tested, three were selected for testing in experimental huts to confirm that chemical actions documented in laboratory tests are also expressed in the field. The laboratory tests showed the primary action of DDT is repellent, alphacypermethrin is irritant, and dieldrin is only toxic. These tests were followed with hut studies in Thailand against marked-released populations. DDT exhibited a highly protective level of repellency that kept mosquitoes outside of huts. Alphacypermethrin did not keep mosquitoes out, but its strong irritant action caused them to prematurely exit the treated house. Dieldrin was highly toxic but showed no irritant or repellent action. Based on the combination of laboratory and confirmatory field data, we propose a new paradigm for classifying chemicals used for vector control according to how the chemicals actually function to prevent disease transmission inside houses. The new classification scheme will characterize chemicals on the basis of spatial repellent, contact irritant and toxic actions.

Developing global maps of the dominant anopheles vectors of human malaria.

Simon Hay and colleagues describe how the Malaria Atlas Project has collated anopheline occurrence data to map the geographic distributions of the dominant mosquito vectors of human malaria.

Insecticide Susceptible/Resistance Status in Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) in Thailand During 2003–2005

Abstract Susceptibility baselines and diagnostic doses of the technical grade insecticides deltamethrin, permethrin, fenitrothion, and propoxur were established based on Aedes aegypti (L.), Bora (French Polynesia), a reference susceptible strain. Field-collected Aedes mosquitoes from each part of Thailand were subjected to bioassay for their susceptibility to the diagnostic doses of each insecticide. Almost all Ae. aegypti collected were incipient resistant or resistant to deltamethrin and permethrin, except those from some areas of Songkhla (southern) and Phan district of Chiang Rai (northern) province. Susceptibility to fenitrothion was found in mosquitoes from Bangkok (central), Chonburi (eastern), Chiang Rai, Kanchanaburi (western), and Songkhla, whereas they were resistant in almost all areas of Nakhon Sawan (north central) and Nakhon Ratchasima (northeastern) provinces. Most of Ae. aegypti were susceptible to propoxur except those from Mae Wong, Nakhon Sawan province. Various levels of insecticide resistance and susceptibility in adjacent areas revealed a focal susceptible/resistance profile in the country. It could be noted that almost all of Ae. albopictus were susceptible to the insecticides tested at the same diagnostic doses. In conclusion, resistance to pyrethroids (permethrin and deltamethrin) has developed in Ae. aegypti in most of the collected areas, suggesting that an alternative choice of insecticide or other control measures should be applied.

Review on global co-transmission of human Plasmodium species and Wuchereria bancrofti by Anopheles mosquitoes

Malaria and lymphatic filariasis are two of the most common mosquito-borne parasitic diseases worldwide which can occur as concomitant human infections while also sharing common mosquito vectors. This review presents the most recent available information on the co-transmission of human Plasmodium species and Wuchereria bancrofti by Anopheles mosquitoes. Important biological and epidemiological aspects are also described including the lifecycle of each parasite species and their specificities, the geographical biodiversity of each pathogen and their vectors where the parasites are co-endemic, and biological, environmental and climatic determinants influencing transmission. The co-transmission of each disease is illustrated from both a global perspective and a country level using Thailand as a study case. Different diagnostic methods are provided for the detection of the parasites in biological samples ranging from traditional to more recent molecular methods, including methodologies employing concomitant detection assays of W. bancrofti and Plasmodium spp. parasites. The relevant issues of combined malaria and Bancroftian filariasis control strategies are reviewed and discussed.

Behavioral Responses of Aedes aegypti (Diptera: Culicidae) Exposed to Deltamethrin and Possible Implications for Disease Control

Abstract Behavioral responses of nine Aedes aegypti (L.) strains, six from recent field collections and three from the long-established laboratory colonies, were tested under laboratory-controlled conditions by using an excito-repellency test system. All nine strains showed significant behavioral escape responses when exposed to deltamethrin at the standard field dose (0.02 g/m2), regardless of background insecticide susceptibility status (susceptible or tolerant/resistant). Insecticide contact irritancy played a predominate role in overall female mosquito escape responses, whereas noncontact repellency was not observed at levels significantly different from paired noncontact control tests (P > 0.01). Among the six field populations, the Jakarta (Indonesia) Toba (north Sumatra), and Bangkok female mosquitoes showed rapid exit (>78%) during 30 min of direct contact with insecticide-treated surfaces, whereas the other three strains demonstrated only moderate escape responses (32–56%) from the chambers. Moderate escape responses during direct insecticidal contact also were observed in the three laboratory test populations (44–60%). Higher percentage of mortality was observed from laboratory strains (8–33%) that failed to escape compared with nonescape females of field strains (2–16%), possibly a reflection of background deltamethrin susceptibility status. We conclude that contact irritancy is a major behavioral response of Ae. aegypti when exposed directly to deltamethrin and that rapid flight escape from areas exposed to space sprays or surfaces treated with residual pyrethroids could have a significant impact on the effectiveness of adult mosquito control and disease transmission reduction measures.

Frequency of Pyrethroid Resistance in Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in Thailand

ABSTRACT: Thirty-two Aedes aegypti populations collected throughout Thailand and five populations of Aedes albopictus from southern Thailand were subjected to standard WHO contact bioassays to assess susceptibility to three commonly used synthetic pyrethroids: permethrin, deltamethrin, and lambda-cyhalothrin. A wide degree of physiological response to permethrin was detected in Ae. aegypti, ranging from 56.5% survival (Lampang, northern Thailand) to only 4% (Kalasin in northeastern and Phuket in southern Thailand). All 32 populations of Ae. aegypti were found to have evidence of incipient resistance (62.5%) or levels of survival deemed resistant (37.5%) to permethrin. Four populations of Ae. albopictus were found with incipient resistance (97 – 80% mortality) and one with resistance (< 80%) to permethrin. The majority of Ae. aegypti populations (68.7%) was susceptible (> 98% mortality) to deltamethrin, with incipient resistance (observed 97–82% mortality) in other localities. In contrast, all populations of Ae. aegypti were completely susceptible (100% mortality) to the recommended operational dosage of lambda-cyhalothrin. All five populations of Ae. albopictus were found completely susceptible to both deltamethrin and lambda-cyhalothrin. Evidence of defined incipient or resistance to synthetic pyrethroids mandates appropriate response and countermeasures to mitigate further development and spread of resistance. In light of these findings, we conclude that routine and comprehensive susceptibility monitoring of dengue mosquito vectors to synthetic pyrethroids should be a required component of resistance management policies and disease control activities.

Identifying genomic changes associated with insecticide resistance in the dengue mosquito Aedes aegypti by deep targeted sequencing

The capacity of mosquitoes to resist insecticides threatens the control of diseases such as dengue and malaria. Until alternative control tools are implemented, characterizing resistance mechanisms is crucial for managing resistance in natural populations. Insecticide biodegradation by detoxification enzymes is a common resistance mechanism; however, the genomic changes underlying this mechanism have rarely been identified, precluding individual resistance genotyping. In particular, the role of copy number variations (CNVs) and polymorphisms of detoxification enzymes have never been investigated at the genome level, although they can represent robust markers of metabolic resistance. In this context, we combined target enrichment with high-throughput sequencing for conducting the first comprehensive screening of gene amplifications and polymorphisms associated with insecticide resistance in mosquitoes. More than 760 candidate genes were captured and deep sequenced in several populations of the dengue mosquito Ae. aegypti displaying distinct genetic backgrounds and contrasted resistance levels to the insecticide deltamethrin. CNV analysis identified 41 gene amplifications associated with resistance, most affecting cytochrome P450s overtranscribed in resistant populations. Polymorphism analysis detected more than 30,000 variants and strong selection footprints in specific genomic regions. Combining Bayesian and allele frequency filtering approaches identified 55 nonsynonymous variants strongly associated with resistance. Both CNVs and polymorphisms were conserved within regions but differed across continents, confirming that genomic changes underlying metabolic resistance to insecticides are not universal. By identifying novel DNA markers of insecticide resistance, this study opens the way for tracking down metabolic changes developed by mosquitoes to resist insecticides within and among populations.

Seasonal Abundance and Blood Feeding Activity of Anopheles minimus Theobald (Diptera: Culicidae) in Thailand

Abstract Anopheline mosquito larvae and adults were sampled at Ban Pu Teuy, Tri-Yok District, Kanchanaburi Province, western Thailand, from January 2000 to December 2001. Over the period of 2 yr, Anopheles minimus sensu lato was the most commonly collected species, followed by Anopheles swadiwongporni and Anopheles dirus sensu lato; all three species are important vectors of malaria in Thailand. Attempted blood feeding by An. minimus occurred throughout the night, with two distinct feeding peaks: strong activity immediately after sunset (1800–2100 hours), followed by a second, less pronounced, rise before sunrise (0300–0600 hours). Anopheles minimus were more abundant during the wet season compared with the dry and hot seasons, although nocturnal adult feeding patterns were similar. Anopheles minimus fed readily on humans inside and outside of houses, showing a slight preference for exophagy. The human-biting peak of An. minimus in our study area differed from other localities sampled in Thailand, indicating the possible existence of site-specific populations of An. minimus exhibiting different host-seeking behavior. These results underscore the importance of conducting site-specific studies to accurately determine vector larval habitats and adult activity patterns and linking their importance in malaria transmission in a given area.