Optimizing endectocide and ectocide compound evaluation in Anopheles malaria vectors

Mass endectocide or ectocide treatment of humans or livestock has been suggested as a possible malaria vector control tool. This work provides guidance for in vitro endectocide and ectocide experiments and raises biological points for further evaluation.

Three experiments with ivermectin were performed with Anopheles dirus and Anopheles minimus. The first experiment assessed the impact of a sugar diet (10% sucrose, "Sucrose"; multivitamin syrup, "Multivitamin"; or multivitamin syrup followed by 10% sucrose, "Mix") on mosquito mortality following ingestion of a range of ivermectin-spiked blood meal concentrations. The lethal concentrations that kill 50% (LC₅₀) and 90% (LC₉₀) of mosquitoes were estimated using a normalized concentration-response analysis (IC₅₀ and Hill slope). The second experiment assessed the impact on mosquito mortality after ingesting ivermectin spiked into a plasma meal or a blood meal that was either fresh or previously frozen. Log-rank survival curve analysis (Mantel-Cox method) was used to compare mosquito survival between groups. The third experiment sought to quantify the concentration of ivermectin in a blood meal compared to the amount imbibed into the mosquito midgut as measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

The Multivitamin diet was found to substantially increase An. dirus LC₅₀ compared to Sucrose and Mix diets, while the Sucrose diet had reduced control survival post-blood meal for both An. dirus and An. minimus. Ivermectin mortality response was substantially increased when ingested in a blood meal compared to a plasma meal for An. dirus, while the inverse was observed for An. minimus. For both An. dirus and An. minimus, an approximately 20% loss in ivermectin concentration was observed in the midgut compared to the blood meal.

The Mix diet appears to be best for minimizing control mosquito mortality, without altering the mosquito survival response following ivermectin ingestion. An unexplained biological phenomenon occurred when ivermectin was ingested in either a blood meal or a plasma meal. The concentration of ivermectin imbibed by the mosquito was lower than that observed in the blood meal, suggesting that some of the ivermectin may be excreted by the mosquito during the blood meal.