Comparison of permethrin treatments for bednets in The Gambia

An experimental hut trial of bednets was undertaken in The Gambia to compare six different permethrin treatments, including a wash-resistant formulation, and to test the effect of thorough hand washing of the nets. The wash-resistant formulation on nylon and cotton nets deterred Anopheles gambiae Giles (sensu lato) from house entry. All treated bednets killed a significantly higher number of A. gambiae (sensu lato) than untreated nets. The ability to kill mosquitoes was not diminished with the wash-resistant and Wellcome formulations after washing unlike the normal ICI formulation, this result being consistent with the permethrin residues found by gas liquid chromatographic (GLC) analyses to persist on the nets after washing. The mortality on treated cotton nets was significantly less than on treated nylon nets. Of the unwashed nets only the cotton net was significantly repellent. Both washed and unwashed treated nets significantly reduced the proportion of human bloodfed mosquitoes that survived and were potentially able to transmit malaria. Although the wash-resistant formulation might prove more efficient at killing mosquitoes with time, as compared to the other treatments, in The Gambia, where women wash their nets frequently, we suggest that the cheapest and most readily available formulation is used on nylon nets where possible.