Insecticide susceptibility status in individual species of the Anopheles gambiae complex (Diptera: Culicidae) in an area of The Gambia where pyrethroid impregnated bednets are used extensively for malaria control

Pyrethroid-impregnated bednets are being used nationwide in The Gambia. The future success of this malaria control programme depends partly on the vectors remaining susceptible to those insecticides used for treating the nets. The present study was carried out on the south bank of the river Gambia, during the first large scale trial of nets in this country. Thus this area represents a sentinel site for detecting insecticide resistance in local vectors. This study gives an example of how a system of early detection for resistance problems can be set up in a relatively complex situation where multiple vectors and non-vectors are present. Samples of the Anopheles gambiae complex were caught indoors using light traps in twelve villages used in the bednet study. In all villages A. gambiae sensu stricto Giles was the predominant member of the complex as determined using the rDNA-PCR diagnostic assay. Limited bioassays with DDT and permethrin, and biochemical assays for a range of insecticide resistance mechanisms suggest that the A. gambiae complex remains completely susceptible to all major classes of commonly used insecticides including pyrethroids. Biochemical assays suggest that a low frequency of DDT resistance may occur in A. melas Theobald. This is based on elevated glutathione S-transferase levels coupled with increased levels of DDT metabolism and does not involve cross-resistance to pyrethroids. Therefore we do not envisage a decline in the efficacy of treated nets against malaria vectors in the study area in the immediate future, although monitoring should be continued whilst wide-scale use of impregnated bednets is operational.