The insecticide resistance status of malaria vectors in the Mekong region

<p>Abstract</p> <p>Background</p> <p>Knowledge on insecticide resistance in target species is a basic requirement to guide insecticide use in malaria control programmes. Malaria transmission in the Mekong region is mainly concentrated in forested areas along the country borders, so that decisions on insecticide use should ideally be made at regional level. Consequently, cross-country monitoring of insecticide resistance is indispensable to acquire comparable baseline data on insecticide resistance.</p> <p>Methods</p> <p>A network for the monitoring of insecticide resistance, MALVECASIA, was set up in the Mekong region in order to assess the insecticide resistance status of the major malaria vectors in Cambodia, Laos, Thailand, and Vietnam. From 2003 till 2005, bioassays were performed on adult mosquitoes using the standard WHO susceptibility test with diagnostic concentrations of permethrin 0.75% and DDT 4%. Additional tests were done with pyrethroid insecticides applied by the different national malaria control programmes.</p> <p>Results</p> <p><it>Anopheles dirus s.s</it>., the main vector in forested malaria foci, was susceptible to permethrin. However, in central Vietnam, it showed possible resistance to type II pyrethroids. In the Mekong delta, <it>Anopheles epiroticus </it>was highly resistant to all pyrethroid insecticides tested. It was susceptible to DDT, except near Ho Chi Minh City where it showed possible DDT resistance. In Vietnam, pyrethroid susceptible and tolerant <it>Anopheles minimus s.l</it>. populations were found, whereas <it>An. minimus s.l</it>. from Cambodia, Laos and Thailand were susceptible. Only two <it>An. minimus s.l</it>. populations showed DDT tolerance. <it>Anopheles vagus </it>was found resistant to DDT and to several pyrethroids in Vietnam and Cambodia.</p> <p>Conclusion</p> <p>This is the first large scale, cross-country survey of insecticide resistance in <it>Anopheles </it>species in the Mekong Region. A unique baseline data on insecticide resistance for the Mekong region is now available, which enables the follow-up of trends in susceptibility status in the region and which will serve as the basis for further resistance management. Large differences in insecticide resistance status were observed among species and countries. In Vietnam, insecticide resistance was mainly observed in low or transmission-free areas, hence an immediate change of malaria vector control strategy is not required. Though, resistance management is important because the risk of migration of mosquitoes carrying resistance genes from non-endemic to endemic areas. Moreover, trends in resistance status should be carefully monitored and the impact of existing vector control tools on resistant populations should be assessed.</p

High heterogeneity in Plasmodium falciparum risk illustrates the need for detailed mapping to guide resource allocation: a new malaria risk map of the Lao People's Democratic Republic

<p>Abstract</p> <p>Background</p> <p>Accurate information on the geographical distribution of malaria is important for efficient resource allocation. The Lao People's Democratic Republic has experienced a major decline in malaria morbidity and mortality in the past decade. However, efforts to respond effectively to these changes have been impeded by lack of detailed data on malaria distribution. In 2008, a countrywide survey on <it>Plasmodium falciparum </it>diagnosed in health centres and villages was initiated to develop a detailed <it>P. falciparum </it>risk map with the aim to identify priority areas for malaria control, estimate population at risk, and guide resource allocation in the Lao People's Democratic Republic.</p> <p>Methods</p> <p><it>P. falciparum </it>incidence data were collected from point-referenced villages and health centres for the period 2006-2008 during a country-wide survey between December 2008 and January 2009. Using the highest recorded annual rate, continuous surfaces of <it>P. falciparum </it>incidence were produced by the inverse distance weighted interpolation technique.</p> <p>Results</p> <p>Incidence rates were obtained from 3,876 villages and 685 health centres. The risk map shows that <it>P. falciparum </it>is highly heterogeneous in the northern and central regions of the country with large areas of no transmission. In the southern part, transmission is pervasive and the risk of <it>P. falciparum </it>is high. It was estimated that 3.4 million people (60% of the population) live at risk of malaria.</p> <p>Conclusions</p> <p>This paper presents the first comprehensive malaria risk map of the Lao People's Democratic Republic based entirely on empirical data. The estimated population at risk is substantially lower than previous estimates, reflecting the presence of vast areas with focal or no malaria transmission as identified in this study. These findings provide important guidance for malaria control interventions in the Lao People's Democratic Republic, and underline the need for detailed data on malaria to accurately predict risk in countries with heterogeneous transmission.</p

Malaria transmission and vector behaviour in a forested malaria focus in central Vietnam and the implications for vector control

BACKGROUND: In Vietnam, malaria is becoming progressively restricted to specific foci where human and vector characteristics alter the known malaria epidemiology, urging for alternative or adapted control strategies. Long-lasting insecticidal hammocks (LLIH) were designed and introduced in Ninh Thuan province, south-central Vietnam, to control malaria in the specific context of forest malaria. An entomological study in this specific forested environment was conducted to assess the behavioural patterns of forest and village vectors and to assess the spatio-temporal risk factors of malaria transmission in the province. METHODS: Five entomological surveys were conducted in three villages in Ma Noi commune and in five villages in Phuoc Binh commune in Ninh Thuan Province, south-central Vietnam. Collections were made inside the village, at the plot near the slash-and-burn fields in the forest and on the way to the forest. All collected mosquito species were subjected to enzyme-linked immunosorbent assay (ELISA) to detect Plasmodium in the head-thoracic portion of individual mosquitoes after morphological identification. Collection data were analysed by use of correspondence and multivariate analyses. RESULTS: The mosquito density in the study area was low with on average 3.7 anopheline bites per man-night and 17.4 culicine bites per man-night. Plasmodium-infected mosquitoes were only found in the forest and on the way to the forest. Malaria transmission in the forested malaria foci was spread over the entire night, from dusk to dawn, but was most intense in the early evening as nine of the 13 Plasmodium positive bites occurred before 21H. The annual entomological inoculation rate of Plasmodium falciparum was 2.2 infective bites per person-year to which Anopheles dirus s.s. and Anopheles minimus s.s. contributed. The Plasmodium vivax annual entomological inoculation rate was 2.5 infective bites per person-year with Anopheles sawadwongporni, Anopheles dirus s.s. and Anopheles pampanai as vectors. CONCLUSION: The vector behaviour and spatio-temporal patterns of malaria transmission in Southeast Asia impose new challenges when changing objectives from control to elimination of malaria and make it necessary to focus not only on the known main vector species. Moreover, effective tools to prevent malaria transmission in the early evening and in the early morning, when the treated bed net cannot be used, need to be developed

Mapping of mosquito breeding sites in malaria endemic areas in Pos Lenjang, Kuala Lipis, Pahang, Malaysia

<p>Abstract</p> <p>Background</p> <p>The application of the Geographic Information Systems (GIS) to the study of vector transmitted diseases considerably improves the management of the information obtained from the field survey and facilitates the study of the distribution patterns of the vector species.</p> <p>Methods</p> <p>As part of a study to assess remote sensing data as a tool for vector mapping, geographical features like rivers, small streams, forest, roads and residential area were digitized from the satellite images and overlaid with entomological data. Map of larval breeding habitats distribution and map of malaria transmission risk area were developed using a combination of field data, satellite image analysis and GIS technique. All digital data in the GIS were displayed in the WGS 1984 coordinate system. Six occasions of larval surveillance were also conducted to determine the species of mosquitoes, their characteristics and the abundance of habitats.</p> <p>Results</p> <p>Larval survey studies showed that anopheline and culicine larvae were collected and mapped from 79 and 67 breeding sites respectively. Breeding habitats were located at 100-400 m from human settlement. Map of villages with 400 m buffer zone visualizes that more than 80% of <it>Anopheles maculatus s.s</it>. immature habitats were found within the buffer zone.</p> <p>Conclusions</p> <p>This study amplifies the need for a broadening of the GIS approach which is emphasized with the aim of rejuvenating the dynamic aspect of entomological studies in Malaysia. In fact, the use of such basic GIS platforms promote a more rational basis for strategic planning and management in the control of endemic diseases at the national level.</p

Absence of knockdown resistance suggests metabolic resistance in the main malaria vectors of the Mekong region

<p>Abstract</p> <p>Background</p> <p>As insecticide resistance may jeopardize the successful malaria control programmes in the Mekong region, a large investigation was previously conducted in the Mekong countries to assess the susceptibility of the main malaria vectors against DDT and pyrethroid insecticides. It showed that the main vector, <it>Anopheles epiroticus</it>, was highly pyrethroid-resistant in the Mekong delta, whereas <it>Anopheles minimus sensu lato </it>was pyrethroid-resistant in northern Vietnam. <it>Anopheles dirus sensu stricto </it>showed possible resistance to type II pyrethroids in central Vietnam. <it>Anopheles subpictus </it>was DDT- and pyrethroid-resistant in the Mekong Delta. The present study intends to explore the resistance mechanisms involved.</p> <p>Methods</p> <p>By use of molecular assays and biochemical assays the presence of the two major insecticide resistance mechanisms, knockdown and metabolic resistance, were assessed in the main malaria vectors of the Mekong region.</p> <p>Results</p> <p>Two FRET/MCA assays and one PCR-RFLP were developed to screen a large number of <it>Anopheles </it>populations from the Mekong region for the presence of knockdown resistance (<it>kdr</it>), but no <it>kdr </it>mutation was observed in any of the study species. Biochemical assays suggest an esterase mediated pyrethroid detoxification in <it>An. epiroticus </it>and <it>An. subpictus </it>of the Mekong delta. The DDT resistance in <it>An. subpictus </it>might be conferred to a high GST activity. The pyrethroid resistance in <it>An. minimus s.l</it>. is possibly associated with increased detoxification by esterases and P450 monooxygenases.</p> <p>Conclusion</p> <p>As different metabolic enzyme systems might be responsible for the pyrethroid and DDT resistance in the main vectors, each species may have a different response to alternative insecticides, which might complicate the malaria vector control in the Mekong region.</p

The Role of Climate Variability in the Spread of Malaria in Bangladeshi Highlands

Malaria is a major public health problem in Bangladesh, frequently occurring as epidemics since the 1990s. Many factors affect increases in malaria cases, including changes in land use, drug resistance, malaria control programs, socioeconomic issues, and climatic factors. No study has examined the relationship between malaria epidemics and climatic factors in Bangladesh. Here, we investigate the relationship between climatic parameters [rainfall, temperature, humidity, sea surface temperature (SST), El Niño-Southern Oscillation (ENSO), the normalized difference vegetation index (NDVI)], and malaria cases over the last 20 years in the malaria endemic district of Chittagong Hill Tracts (CHT)

Examining Landscape Factors Influencing Relative Distribution of Mosquito Genera and Frequency of Virus Infection

Mosquito-borne infections cause some of the most debilitating human diseases, including yellow fever and malaria, yet we lack an understanding of how disease risk scales with human-driven habitat changes. We present an approach to study variation in mosquito distribution and concomitant viral infections on the landscape level. In a pilot study we analyzed mosquito distribution along a 10-km transect of a West African rainforest area, which included primary forest, secondary forest, plantations, and human settlements. Variation was observed in the abundance of Anopheles, Aedes,Culex, and Uranotaenia mosquitoes between the different habitat types. Screening of trapped mosquitoes from the different habitats led to the isolation of five uncharacterized viruses of the families Bunyaviridae, Coronaviridae, Flaviviridae, and Rhabdoviridae, as well as an unclassified virus. Polymerase chain reaction screening for these five viruses in individual mosquitoes indicated a trend toward infection with specific viruses in specific mosquito genera that differed by habitat. Based on these initial analyses, we believe that further work is indicated to investigate the impact of anthropogenic landscape changes on mosquito distribution and accompanying arbovirus infection

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

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

<p>Abstract</p> <p>Background</p> <p>The 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.</p> <p>Results</p> <p>Expert 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.</p> <p>Conclusions</p> <p>This 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.</p