REDUCED SUSCEPTIBILITY OF ANOPHELES-GAMBIAE TO PERMETHRIN ASSOCIATED WITH THE USE OF PERMETHRIN-IMPREGNATED BEDNETS AND CURTAINS IN KENYA

Susceptibility of the malaria vector Anopheles gambiae to permethrin decreased following the installation of mosquito nets impregnated with 0.5 g permethrin per square metre in four villages near Kisumu, Kenya. During the first year that permethrin-impregnated bednets and curtains were in place, the exposure time to 50% mortality (LT(50)) increased 2.5-fold from 13 to 33 min, while the LT(50) for An. gambiae was unchanged in two other villages where no intervention measures were used. Two years after permethrin-impregnated mosquito nets were distributed the LT(50)s for An. gambiae were 28, 28 and 16 min, respectively, in the villages with bednets, curtains and with no such intervention. Using a colony of An. gambiae derived from females collected in the villages using permethrin-impregnated moquito nets, we lengthened the LT(50) from 28 to 41 min in two generations by exposing all females to permethrin-treated papers for 60 min and rearing offspring of the survivors. Permethrin-impregnated bednets and curtains are intended to reduce vectorial capacity. Reduced susceptibility to permethrin could counter this beneficial effect.