VILLAGE TRIAL OF BEDNETS IMPREGNATED WITH WASH-RESISTANT PERMETHRIN COMPARED WITH OTHER PYRETHROID FORMULATIONS

A village-scale field trial of pyrethroid-impregnated mosquito nets wa s undertaken in The Gambia, West Africa, in the Mandinka village of Sa ruja (13-degrees-13'N, 14-degrees-15'W) during July-November 1989. Nea rly all the villagers possessed and used their own bednets. Anopheles gambiae is the main vector of human malaria in the area. An experiment al wash-resistant formulation of permethrin was compared with standard emulsifiable concentrate (EC) formulations of permethrin and lambda-c yhalothrin, versus placebo-treated bednets. Target concentrations of p yrethroids on bednets were permethrin 500 mg/m2 and lambda-cyhalothrin 25 mg/m2. The experimental design involved random allocation of a tre atment to one net per family. Whereas 68% of people questioned said th ey washed their nets fortnightly, observations during the 16-week tria l period showed that only 4/130 (3%) of nets involved in the trial had been washed as frequently as once per month. Early morning searches f or mosquitoes under bednets (1 day/week for 16 weeks) found significan tly more mosquitoes (60% An.gambiae) in placebo-treated nets than in p yrethroid-treated nets. The numbers found with each of the three pyret hroid treatments did not differ significantly from each other. Insecti cidal efficacy of the treatments was tested by bioassays using wild-ca ught unfed mosquitoes exposed to netting for 3 min. Linear regression analysis of bioassay mortality against number of times that a net had been washed by villagers showed that nets impregnated with the wash-re sistant permethrin retained their insecticidal properties better than nets impregnated with lambda-cyhalothrin or with the standard permethr in formulation.