Resistance of malaria vectors to pyrethroids used for impregnated bednets,Benin, west Africa.

Impregnated bednets can be considered a major tool for reducing Anopheles b ites, malaria morbidity and overall mortality. The resistance of Anopheles gambiae to pyrethroids used to impregnate bednets and curtains has already been noted in the urban area of Cotonou in Benin (18, 21). In this study we wished to find out if the resistance observed in Cotonou is localized only in this town or is already extensive throughout Benin, in this case, such resistance would be a handicap to the promotion of impregnated bednets in B enin. The study was carried out in 15 localities throughout the different ecologi cal zones of Benin. The study has also taken into account environmental fac tors favouring the emergence of resistance. We did susceptibility tests wit h WHO test kits for adult mosquitoes using impregnated papers. The papers w ere impregnated with permethrin 0,25%, deltamethrin 0,025% and lambdacyhalo thrin 0,1 %. We also tested DDT 4% to find out if there was a cross resista nce between DDT and the pyrethroids. Two mosquito species were tested: An. gambiae and An. melas. In northern Benin, where farmers use insecticides against cotton pests vect ors are susceptible to deltamethrin and lambdacyhalothrin and resistant to permethrin. In the south, An. gambiae is resistant to deltamethrin and permethrin. This resistance is high in the urban zone of Cotonou, in the coastal and lagoon areas and at Krake, a frontier village with Nigeria. The resistance observ ed in southern Benin is confirmed by the lengthening of the knock-down time of mosquitoes which were exposed for 1 hour to insecticide in impregnated WHO test tubes, and by a reduction of permethrin and deltamethrin remanence effect.