High seasonal malaria transmission rates in the intermediate rainfall zone of Sri Lanka.

Malaria transmission was studied at Nikawehera, a long-established farming village, located in the intermediate rainfall zone of Sri Lanka. Observations were made over a 12-month period (October-September) that included the main rainy season which occurred during the north-east monsoon in November-January. Anolpheles culicifacies, the recognized vector of malaria in Sri Lanka, was the predominant anopheline mosquito collected by human night baiting at Nikawehera. High entomological inoculation rates with An. culcifacies (0.12/hour for Plasmodium vivax) were observed during the height of the transmission season which occurs during, and immediately after, the north-east monsoon. Anolpheles subpictus was identified as a possible additional vector at Nikawehera. Anopheles annularis, a major vector at Weheragala, a site in a new irrigation development (the Mahaweli Scheme) located 70 km away in the dry zone, was not collected by human baiting at Nikawehera. Clinical, entomological and parasitological data suggest that malaria is hyperendemic at Nikawehera, with high seasonal transmission rates.