Dosage-dependent effects of permethrin-treated nets on the behaviour of Anopheles gambiae and the selection of pyrethroid resistance

BACKGROUND: The evolution and spread of pyrethroid resistance in Anopheles gambiae s.s, the major malaria vector in sub-Saharan Africa, is of great concern owing to the importance of pyrethroid-treated nets in the WHO global strategy for malaria control. The impact of kdr (the main pyrethroid-resistance mechanism) on the behaviour of An. gambiae is not well understood. The objective of this study was to determine whether high or low doses of permethrin differ in their resistance-selection effects. METHODS: The effect of permethrin treatment was assessed under laboratory conditions using the tunnel test technique against susceptible, heterozygous and homozygous genotypes. Experimental huts trials were then carried out in Benin to assess the level of personal protection conferred by nets treated with a variety of permethrin concentrations and their impact on the selection for kdr allele. RESULTS: Tunnel tests showed that nets treated with permethrin at 250 and 500 mg/m(2 )induced higher mortality and blood feeding reduction among susceptible and heterozygous (RS) females as compared to the lower concentration (100 mg/m(2)). The experimental hut trials showed that the best personal protection was achieved with the highest permethrin concentration (1,000 mg/m(2)). Mosquito genotyping revealed a non-linear relationship in the survival of kdr susceptible and resistant genotypes with permethrin dosage. Higher dosages (≥250 mg/m(2)) killed more efficiently the RS genotypes than did lower dosages (50 and 100 mg/m(2)). CONCLUSION: This study showed that nets treated with high permethrin concentrations provided better blood feeding prevention against pyrethroid-resistant An. gambiae than did lower concentrations. Permethrin-treated nets seem unlikely to select for pyrethroid resistance in areas where the kdr mutation is rare and present mainly in heterozygous form.