Abstract
BackgroundWidespread and large scale use of ivermectin in humans and domestic animals can have unexpected effects on non-target organisms. As a search for a possible explanation for an observed longitudinal decline in density of anopheline vector mosquitoes, but not in Culex quinquefasciatus, in an area of north-eastern Tanzania which has been exposed to ivermectin mass drug administration, this study assessed and compared the effect of human ivermectin treatment on blood feeding Anopheles gambiae and Cx. quinquefasciatus.MethodsConsenting adult volunteers were randomized into two groups to receive either ivermectin or placebo. Twenty four hours after treatment, one volunteer from each group was concurrently exposed to 50 laboratory reared An. gambiae on one arm and 50 laboratory reared Cx. quinquefasciatus on the other arm for 15–30 minutes. Engorged mosquitoes were maintained on 10% glucose solution for 12 days and observed for survival and fecundity. The experiment was repeated 15 times.ResultsTwo days after the blood meals, nearly half (average 47.7% for the 15 experiments) of the blood fed An. gambiae in the ivermectin group had died while almost all in the placebo group were alive (97.2%), and the difference in survival between these two groups continued to widen on the following days. There was no clear effect of ivermectin on Cx. quinquefasciatus, which had high survival in both ivermectin and placebo group on day 2 (95.7% and 98.4%, respectively) as well as on the following days. Ivermectin completely inhibited egg laying in An. gambiae, while egg laying and subsequent development of immature stages appeared normal in the other three groups.ConclusionBlood meals taken on ivermectin treated volunteers significantly reduced survival and halted fecundity of An. gambiae but had only limited or no effect on Cx. quinquefasciatus. The result suggests that widespread use of ivermectin may have contributed to the observed decline in density of An. gambiae, without similar decrease in Cx. quinquefasciatus, in north-eastern Tanzania.
Highlights
Widespread and large scale use of ivermectin in humans and domestic animals can have unexpected effects on non-target organisms
It has been documented that female Anopheles gambiae mosquitoes have a much higher mortality if they feed on humans shortly after treatment with ivermectin than if they feed on untreated humans [4]
This has led to the suggestion that human mass drug administration (MDA) with ivermectin could be used as a supplement measure for malaria transmission control [9,10,11,12]
Summary
Widespread and large scale use of ivermectin in humans and domestic animals can have unexpected effects on non-target organisms. Ivermectin, a macrocyclic lactone derived as a fermentation product from the soil bacteria Streptomyces avermilitis [1], is widely used globally in human and veterinary medicine as a drug for treatment and control of nematode and ecto-parasitic infections It is currently used in large scale programs to eliminate human. A longitudinal study conducted in north-eastern Tanzania has documented a dramatic decline in anopheline mosquito densities during the past decade, whereas no decline has been seen in densities of Cx. quinquefasciatus [13,14] The cause of these changes, which have important implications for the transmission and control of malaria and lymphatic filariasis in the area, remains somewhat unclear, and could not immediately be related to change in rainfall pattern or major application of mosquito control interventions [14]. As MDA with ivermectin has been widely applied in the area for control of onchocerciasis and lymphatic filariasis [13,15], the present study assessed and compared the survival and fecundity of An. gambiae and Cx. quinquefasciatus mosquitoes after taking a blood meal on humans treated with ivermectin
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