Effects of larval exposure to sublethal doses of ivermectin on adult fitness and susceptibility to ivermectin in Anopheles gambiae s.s.

Abstract

Background The effects of ivermectin (endectocide) on mosquito survival has prompted it to be considered as a potential new malaria vector control tool. The drug can be administered to mosquito disease vectors through blood hosts that include humans and livestock. It’s increased use may cause contamination of larval habitats, either directly through livestock excreta, or indirectly, through leaching or run-off from contaminated soil albeit in sublethal doses. However, the effects of such exposure on immature stages, and the subsequent adults that emerge is poorly understood. This current study was undertaken to evaluate the impact of ivermectin exposure on An. gambiae s.s. larvae and its effects on fitness, and susceptibility to ivermectin in the emerging adults. Methods Laboratory-reared Anopheles gambiae s.s. (Kilifi strain) larvae were exposed to five different ivermectin concentrations; 0, 0.00001, 0.0001, 0.001, and 0.01 ppm, and larval survival monitored to determine the appropriate sub-lethal dose. Concentrations with survival greater than 50% (0.00001 and 0.0001 ppm) were selected and used as the sub-lethal doses. The fecundity, fertility, and susceptibility to ivermectin of adults emerging after larval exposure to the sub-lethal doses was examined. Results Overall, exposure of An. gambiae s.s. aquatic stages to ivermectin reduced larval survival irrespective of the stage at which the larvae were exposed. Exposure to ivermectin in the larval stage reduced the time taken to oviposition and did not have an effect on either the number of eggs laid nor the hatch rate. Additionally, exposure to ivermectin in the larval stage did not affect susceptibility of the emerging adults to the drug. Conclusions This study shows that contamination of larval habitats with ivermectin affects An. gambiae s.s. larval survival and could potentially have an impact on public health. However, there are no carry-over effects on the fecundity, fertility and susceptibility of the emerging adults to ivermectin. In addition, this study shows that environmental exposure to ivermectin in the larval habitats is unlikely to compromise the efficacy of ivermectin in the emerging adults.