Malathion resistance and prevalence of the malathion carboxylesterase mechanism in populations of mosquito vectors of disease in Sri Lanka

Abstract

Objective: To determine the levels of malathion resistance and prevalence of the malathion carboxylesterase mechanism among mosquitoes in Sri Lanka. Methods: Bioassays were carried out using WHO-recommended methods on samples of the following Sri Lankan mosquito vectors: Culex quinquefasciatus, C. tritaeniorhynchus, C. gelidus, Anopheles culicifacies B, A. subpictus Aedes aegypti and A. albopictus. Findings: Malathion-specific carboxylesterase mechanisms were found in A. culicifacies and A. subpictus, both giving high rates of insecticide metabolism. In contrast, malathion resistance in C. quinquefasciatus and C. tritaeniorhynchus is linked to broad-spectrum resistance to organophosphorus compounds due to elevated levels of esterases that sequester malaoxon, but are unable to metabolize malathion. Conclusions: Resistance among the Anopheles spp. must have occurred as a direct result of antimalarial activities, since malathion use in Sri Lanka is limited to public health treatments. In contrast, resistance among Culex spp. has resulted from large-scale use of the organophosphorus insecticide group as larvicides for filariasis control and on rice paddy, where C tritaeniorhynchus predominantly breeds, for agricultural purposes.