Hyperendemic malaria in a Thai village: dependence of year-round transmission on focal and seasonally circumscribed mosquito (Diptera: Culicidae) habitats.

In a longitudinal study of hyperendemic malaria in a village in eastern Thailand (from October 1985 to November 1987), man-biting anopheline mosquitoes were collected for 16 man-nights per month in 22 of 26 mo. Mosquitoes were separated according to collection sites (inner, central, more populated; outer, peripheral, more forested), biting period, and parity, and then they were tested for sporozoite antigen using an enzyme-linked immunosorbent assay (ELISA). Abundance of Anopheles dirus Peyton & Harrison was greater in outer than inner village sites, with bimodal peaks in the postmonsoon (October-November) and early rainy (April or May) seasons. Parity rates at both sites were high in postmonsoon seasons (means about 67%), low in cool dry (December-January, less than 56%) and monsoon (June-September, less than 60%) seasons, and variable in other seasons. Of 1,861 An. dirus collected, 16 (0.9%) were positive for Plasmodium falciparum (PF) and nine (0.4%) for P. vivax (PV), whereas of 386 An. minimus, one (0.3%) and three (0.8%) were PF- and PV-positive, respectively. Entomological inoculation rates (EIR) were higher in outer (means PF = 0.91, PV = 0.34) than inner village sites (means = 0.01 for PF and PV). The EIR of PF appeared bimodal, high in postmonsoon (October-November) and early rainy (April or May) seasons, low in monsoon seasons, and variable in other seasons. The vectorial capacity of An. dirus was higher than that of An. minimus, indicating that the two species were primary and secondary vectors, respectively. Human malaria prevalence data indicated that transmission depended greatly on the higher year-round vector abundance in outer than in inner village sites.