Mosquito Nets Treated with a Mixture of Chlorfenapyr and Alphacypermethrin Control Pyrethroid Resistant Anopheles gambiae and Culex quinquefasciatus Mosquitoes in West Africa

BACKGROUND: The effectiveness of insecticide treated nets is under threat across Africa south of the Sahara from the selection of pyrethroid resistance in Anopheles gambiae mosquitoes. To maintain progress against malaria it is necessary to identify alternative residual insecticides for mosquito nets. Mixtures of pyrethroid and insecticides with novel mode of action provide scope for both improved control and management of resistance through concurrent exposure to unrelated insecticides. METHODS: The pyrrole chlorfenapyr and the pyrethroid alphacypermethrin were tested individually and as a mixture on mosquito nets in an experimental hut trial in southern Benin against pyrethroid resistant An gambiae and Culex quinquefasciatus mosquitoes. The nets were deliberately holed to simulate the effect of wear and tear. RESULTS: The nets treated with the mixture of chlorfenapyr 200 mg/m(2) and alphacypermethrin 25 mg/m(2) killed a proportion of An gambiae (77%, 95%CI: 66–86%) significantly greater than nets treated with alphacypermethrin 25 mg/m(2) (30%, 95%CI: 21–41%) but not significantly different from nets treated with chlorfenapyr 200 mg/m(2) (69%, 95%CI: 57–78%). The nets treated with the mixtures procured personal protection against An gambiae biting(58–62%) by a greater margin than the alphacypermethrin treated net (39%), whereas the chlorfenapyr treated net was not protective. A similar trend in mortality and blood feeding inhibition between treatments was observed in Cx quinquefasciatus to that seen in An. gambiae, although the effects were lower. A mixture of alphacypermethrin with chlorfenapyr applied at 100 mg/m(2) had an effect similar to the mixture with chlorfenapyr at 200 mg/m(2). CONCLUSION: The effectiveness of ITNs against pyrethroid resistant mosquitoes was restored by the mixture: the alphacypermethrin component reduced human-vector contact while the chlorfenapyr controlled pyrethroid-resistant mosquitoes. The complementary action of these unrelated insecticides demonstrates that the combination on nets has potential for preventing malaria transmission in areas compromised by the spread of pyrethroid resistance.