Abstract
BackgroundPyrethroid resistant mosquitoes are becoming increasingly common in parts of Africa. It is important to identify alternative insecticides which, if necessary, could be used to replace or supplement the pyrethroids for use on treated nets. Certain compounds of an earlier generation of insecticides, the organophosphates may have potential as net treatments.MethodsComparative studies of chlorpyrifos-methyl (CM), an organophosphate with low mammalian toxicity, and lambdacyhalothrin (L), a pyrethroid, were conducted in experimental huts in Côte d'Ivoire, West Africa. Anopheles gambiae and Culex quinquefasciatus mosquitoes from the area are resistant to pyrethroids and organophosphates (kdr and insensitive acetylcholinesterase Ace.1R). Several treatments and application rates on intact or holed nets were evaluated, including single treatments, mixtures, and differential wall/ceiling treatments.Results and ConclusionAll of the treatments were effective in reducing blood feeding from sleepers under the nets and in killing both species of mosquito, despite the presence of the kdr and Ace.1R genes at high frequency. In most cases, the effects of the various treatments did not differ significantly. Five washes of the nets in soap solution did not reduce the impact of the insecticides on A. gambiae mortality, but did lead to an increase in blood feeding. The three combinations performed no differently from the single insecticide treatments, but the low dose mixture performed encouragingly well indicating that such combinations might be used for controlling insecticide resistant mosquitoes. Mortality of mosquitoes that carried both Ace.1R and Ace.1S genes did not differ significantly from mosquitoes that carried only Ace.1S genes on any of the treated nets, indicating that the Ace.1R allele does not confer effective resistance to chlorpyrifos-methyl under the realistic conditions of an experimental hut.
Highlights
Pyrethroid resistant mosquitoes are becoming increasingly common in parts of Africa
A total of 1328 mosquitoes were recorded of which 41.6% were A. gambiae, 19.4% Culex spp., 31.6% Mansonia spp., 4.3% Aedes spp., 2% A. funestus, 0.9% Anopheles pharaonsis, 0.5% Anopheles coustani, 0.2% Coquillettidia crystata and 0.1% Eretmapodites
Efficacy of treatment From 363 hut-nights collections (33 days × 11 huts) a total of 5,639 mosquitoes were recorded of which 10% were A. gambiae and 45% Culex spp. (C. quinquefasciatus predominated)
Summary
Pyrethroid resistant mosquitoes are becoming increasingly common in parts of Africa. It is important to identify alternative insecticides which, if necessary, could be used to replace or supplement the pyrethroids for use on treated nets. In Kenya, a different form of the kdr gene was found in Anopheles gambiae by Ranson et al [5], in an area where one of the most successful large-scale ITN trials was subsequently carried out [16,17] These findings may allay initial fears but it would be complacent to assume that pyrethroid-treated nets will remain effective indefinitely. The danger of pyrethroid resistance is apparent in Tanzanian C. quinquefasciatus It effectively prevents mortality with pyrethroid treated nets in experimental huts [20] and prevents Culex population suppression when ITNs are used by whole communities, in contrast to high mortality and population suppression of susceptible A. gambiae populations under these conditions [21]
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