LONG-TERM USE OF PERMETHRIN-IMPREGNATED NETS DOES NOT INCREASE ANOPHELES-GAMBIAE PERMETHRIN TOLERANCE

Previous use of permethrin-impregnated bednets (mosquito nets) and cur tains in four Kenyan villages for one year, 1990-91, raised the permet hrin LT(50) of Anopheles gambiae to 2.4-fold above its baseline value, designated permethrin tolerance (PT), as measured by exposure to 0.25 % permethrin-impregnated papers in W.H.O. test-kits, During 1992-93, w ith ongoing use of permethrin-impregnated nets and curtains, PT regres sed slightly compared with the contemporary susceptibility level of An ,gambiae from non-intervention villages, to 1.8-fold in 1992 and only 1.6-fold in 1993. Thus the selection pressure of impregnated nets for PT in An.gambiae appears to be minimal in our study villages, although the impact of permethrin was demonstrated by a significantly lower pa rous-rate of An. gambiae females in the intervention (63-66%) than in non-intervention (79%) villages, and by reduced malaria transmission ( reported elsewhere), In a selected stock of An.gambiae from the study area, PT did not affect the susceptibility to deltamethrin, fenitrothi on, propoxur or DDT. Bioassays described herein provide easy procedure s for field-monitoring of mosquito susceptibility/tolerance/resistance to insecticides used for net impregnation in operational programmes.