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

Population genetic structure of the malaria vector Anopheles minimus A in Vietnam

Anopheles minimus A, a major malaria vector in Southeast Asia, is the main target of vector control in this area. The impact of these control measures can be influenced by the population structure of the target species. In rural areas, An. minimus breeds along the banks of small clear-water streams, yet in the suburbs of Hanoi, northern Vietnam, there is an An. minimus population whose immature stages develop in water tanks. This study uses allozyme data (1) to assess the population structure of An. minimus A and (2) to evaluate the taxonomic status of the urban An. minimus population from Hanoi. The population from the suburbs of Hanoi was identified as An. minimus A. Although significant genetic differentiation was observed between rural and urban An. minimus A populations, they have not differentiated substantially by genetic drift. Limited macrogeographical differentiation was observed between two rural populations at distances of more than 1000 km. Consequently, geographical distance is not the primary factor in differentiating An. minimus A populations having the typical breeding ecology. The estimated effective population size is consistent with the moderate macrogeographical differentiation. Furthermore, no genetic structuring was observed between adult mosquitoes having different behaviour. The macrogeographical population structure indicates that genes may spread over large areas, whereas the presence of an 'urban' An. minimus A population shows the ability of this species to adapt to anthropogenic environmental changes