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PubMed1999

[Resistance of malaria vectors to pyrethrins used for impregnating mosquito nets in Benin, West Africa].

Impregnated bednets can be considered a major tool for reducing Anopheles bites, malaria morbidity and overall mortality. The resistance of Anopheles gambiae to pyrethroids used to impregnate bednets and curtains has already been noted in the urban area of Cotonou in Benin (18, 21). In this study, we wished to find out if the resistance observed in Cotonou is localized only in this town or is already extensive throughout Benin. In this case, such resistance would be a handicap to the promotion of impregnated bednets in Benin. The study was carried out in 15 localities throughout the different ecological zones of Benin. The study has also taken into account environmental factors favouring the emergence of resistance. We did susceptibility tests with WHO test kits for adult mosquitoes using impregnated papers. The papers were impregnated with permethrin 0.25%, deltamethrin 0.025% and lambdacyhalothrin 0.1%. We also tested DDT 4% to find out if there was a cross resistance between DDT and the pyrethroids. Two mosquito species were tested: An. gambiae and An melas. In northern Benin, where farmers use insecticides against cotton pests, vectors are susceptible to deltamethrin and lambdacyhalothrin and resistant to permethrin. In the south, An. gambiae is resistant to deltamethrin and permethrin. This resistance is high in the urban zone of Cotonou, in the coastal and lagoon areas and at Kraké, a frontier viliage with Nigeria. The resistance observed in southern Benin is confirmed by the lengthening of the knock-down time of mosquitoes which were exposed for 1 hour to insecticide in impregnated WHO test tubes, and by a reduction of permethrin and deltamethrin remanence effect.

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PubMed2000

[Impact of pyrethrin resistance on the efficacity of impregnated mosquito nets in the prevention of malaria: results of tests in experimental cases with deltamethrin SC].

The effects of impregnated bednets treated with deltamethrin at a dosage of 25 mg a.i./m2 were evaluated at two testing stations in Côte d'Ivoire. The first one was located in Yaokoffikro, where Anopheles gambiae s.s. are resistant to pyrethroids (including deltamethrin), and the second in M'be, close to a large rice-growing area where An. gambiae s.s. are susceptible pyrethroids. 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. Overall mortality induced by bednets treated with insecticide was significantly 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 insecticide remain on the treated material for longer periods of time and most of them eventually die. The results of this study indicate that bednets treated with deltamethrin are an effective prophylactic measure even in areas where An. gambiae s.s. are resistant to pyrethroids and should still be considered as a practical means of personal protection against malaria even in pyrethroid resistance areas with high frequency of kdr resistance genes.

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