Seasonal density, sporozoite rates and entomological inoculation rates of Anopheles gambiae and Anopheles funestus in a high-altitude sugarcane growing zone in Western Kenya.

An entomological study was conducted on vectors of malaria and their relative contribution to Plasmodium falciparum transmission in Mumias, a high-altitude site and large-scale sugarcane growing zone in Kakamega district, western Kenya. Anopheles gambiae s.l., the predominant vector species, represented 84% (n=2667) of the total Anopheles mosquitoes collected with An. funestus comprising only 16%. Polymerase chain reaction (PCR) identified all 600 specimens of the An. gambiae complex tested as An. gambiae sensu stricto, an indication that it is the only sibling species represented in the high-altitude sites in western Kenya. Plasmodium falciparum sporozoite rates of 6.3% (133/2118) for An. gambiae s.l. and 9.5% (38/402) for An. funestus by ELISA were obtained in Mumias. None of 1600 mosquitoes tested for P. malariae sporozoites was positive. ELISA tests of mosquito blood meals indicated a high tendency of anthropophagy, a behaviour contributing significantly to malaria transmission by the vector species, with 95.9%, 4.86% and 0.2% having taken at least one blood meal on human, bovine and avian hosts, respectively Malaria transmission intensity was low as revealed by the low entomological inoculation rates (EIR) recorded. The EIR values for An. gambiae s.l. were 29.2 infective bites per person per year (ib/p/year) and 17.5 ib/p/year for An. funestus in Mumias. The highest inoculation rate for both vector species was 7.0 ib/p/month in July. Plasmodium falciparum parasite rate among asymptomatic children was 55.4% and 44% in the wet (July-September) and dry (December-February) seasons, respectively. These results indicate that malaria transmission intensity in the high-altitude site is low but perennial, with transmission being maintained by An. gambiae s.s. and An. funestus.