Elevated oxidase and esterase levels associated with permethrin tolerance in Anopheles gambiae from Kenyan villages using permethrin-impregnated nets

The permethrin tolerance (PT) of a population of the mosquito Anopheles gam biae (Diptera: Culicidae) increased following the introduction of permethri n-impregnated nets for malaria control in certain villages near Kisumu, wes tern Kenya. Using a biochemical test that indirectly measures oxidases asso ciated with permethrin resistance, we found that this population had higher oxidase levels than a comparison population from villages without impregna ted nets. Mosquitoes from a colony of An. gambiae selected for PT, the RSP (reduced susceptibility to permethrin) strain, were exposed to permethrin w ith or without the oxidase inhibitor piperonyl butoxide (PB). Significantly higher mortality rates occurred when permethrin was synergized by PB, pres umably by suppression of oxidases responsible for PT. An unselected (UNS) c olony of An. gambiae that was more susceptible than RSP in a permethrin-sus ceptibility bioassay (i.e. LT50 22 min for UNS, vs. 42 min for RSP) was com pared with the RSP colony for levels of oxidases and esterases. The levels of both enzymes were very significantly higher in the RSP strain (P < 0.000 1). We speculate that use of impregnated nets selected for higher oxidase a nd esterase levels in An. gambiae to metabolize permethrin acquired from th e nets. Both oxidase and esterase mechanisms could confer cross-resistance to other pyrethroids.