Impact of the resistance to pyrethroids on the efficacy of impregnated bednets used as a means of prevention against malaria: results of the evaluation carried out with deltamethrin SC in experimental huts.

The effects of impregnated bednets treated with deltamethrin at a dosage of 25 Mg a.i./m(2) Were evaluated at two testing stations in Cote d'Ivoire. T he first one was located in Yaokoffikro where, Anopheles gambiae s.s. are r esistant to pyrethroids (including deltamethrin), and the second in M'be, c lose to a large rice-growing area where An. gambiae s.s. are susceptible py rethroids. In both situations, treating bednets with deltamethrin was very effective in limiting contact between man and vector. 72% fewer female An. gambiae entered the huts in the susceptible area, whereas a decrease of 43% was recorded in the resistant area, indicating that deltamethrin still has a certain repellent effect on resistant populations of An. gambiae s.s. ov erall mortality induced by bednets treated with insecticide was significant ly higher in the resistant area (56.4% as versus 44.3%). An explanation for this apparent paradox is that the mosquitoes being less repelled by the in secticide remain on the treated material for longer periods of time and mos t of them eventually die. The results of this study indicate that bednets t reated with deltamethrin are an effective prophylactic measure even in area s where An. gambiae s.s. are resistant to pyrethroids and should still be c onsidered as a practical means of personal protection against malaria even in pyrethroid resistance areas with high frequency of kdr resistance genes.