Highly efficient dry season transmission of malaria in Thailand.
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.
Department of Entomology, Walter Reed Army Institute of Research, Washington, DC.
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D000818:Animals; D000852:Anopheles / Q000469:parasitology; D006801:Humans; D007303:Insect Vectors / Q000469:parasitology; D008288:Malaria / Q000469:parasitology / Q000635:transmission*; D010963:Plasmodium falciparum / Q000302:isolation & purification; D010966:Plasmodium vivax / Q000302:isolation & purification; D012621:Seasons; D013785:Thailand