A significant decline in malaria prevalence has been observed recently due to the successful deployment of vector control strategies. However, residual transmission persists because of partially zoophagic vectors that feed on domestic animals when human hosts are unavailable. Treating these animals with endectocides such as ivermectin or moxidectin can help reduce the mosquito population, hence, reduce malaria transmission.
The study aimed to investigate the efficacy of these two drugs with known activity against arthropod ectoparasites against the mosquito vector, Anopheles farauti.
Initially, activity of these drugs was tested against colony mosquitoes by membrane feeding assays (MFA) where mosquitoes fed on drug-seeded blood. Subsequently, in-vivo efficacy was investigated by conducting a clinical trial where pigs were treated by subcutaneous injection of ivermectin and moxidectin at 0.6 mg/kg. Venous blood and skin biopsy were collected from experimental pigs to monitor drug levels. Anopheles farauti mosquitoes were allowed to feed directly on the pigs and mortality was monitored for 12 days.
Non-linear regression curve fit analysis of MFA data showed an LC50 of 17.78ng/ml for ivermectin and 6651ng/ml for moxidectin. Pharmacokinetic assays showed detectable levels of ivermectin in the blood and skin of pigs up to 2 weeks post treatment. This is in agreement with lethal effects of ivermectin to mosquitoes that fed on the pigs for two weeks- with a remarkable kill effect (100% mosquito mortality after 48 hours post feed on the first week post treatment of pigs) and a waning kill effect on the second week (100% mosquito mortality on Day 12 post feeding). In contrast, moxidectin did not demonstrate any lethal effect on mosquitoes.
Results of this study will assist in evaluating the potential of utilising domestic animals as a complimentary vector control tool applicable in areas where residual transmission persists impeding progress towards malaria elimination.