A MALARIA CONTROL TRIAL USING INSECTICIDE-TREATED BED NETS AND TARGETED CHEMOPROPHYLAXIS IN A RURAL AREA OF THE GAMBIA, WEST-AFRICA .7. IMPACT OF PERMETHRIN-IMPREGNATED BED NETS ON MALARIA VECTORS

The impact of permethrin-impregnated bed nets on malaria vectors was s tudied in 6 pairs of villages during the rainy season in 1989. In each pair, the residents of one village had their nets treated whilst thos e of the other remained untreated. Routine collections of mosquitoes w ere made outdoors in the early evening using human-biting collections, and indoors with insecticide sprays, light traps and by searches unde r bed nets. Mosquitoes of the Anopheles gambiae complex, An. gambiae s ensu stricto, An. arabiensis and An. melas, were present in large numb ers for 5 months of the study period. These mosquitoes were susceptibl e to permethrin as judged by bioassay results. Outdoor human-biting ra tes in the early evening in communities with treated bed nets were sim ilar to those in communities with untreated nets. In villages with tre ated bed nets most biting occurred outdoors in the early evening with little taking place under impregnated nets. The insecticidal activity of permethrin-impregnated bed nets, dipped by the local population, pr ovided good individual protection against mosquitoes throughout the ra iny season and bed nets remained effective even when washed up to 3 ti mes. There was little to suggest that the use of insecticide-treated n ets reduced the survival of mosquito populations in villages with impr egnated nets. The absence of the expected village-wide effects of net impregnation may have resulted from the circulation of mosquitoes betw een villages with treated and untreated nets. The proportion of mosqui toes which fed on humans did not differ significantly between villages with treated and untreated nets. Permethrin-impregnated bed nets prov ed an effective barrier against vectors when people were under their n ets, but had no apparent effect on biting outdoors before individuals retired to bed.