Ivermectin to Curb West Nile Virus? Not Such a Bird-Brained Idea

Putting ivermectin in bird-feeders in places where Culex mosquitoes congregate showed promise in reducing the number of infectious mosquitoes that could transmit West Nile virus to humans, a researcher said.

A pilot trial found that when ivermectin-treated bird-feeders were placed in contiguous lots, there was up to a 16% additional reduction in infectious mosquito days compared to when the bird-feeders were randomly placed, where there was only a 5% additional decline, reported Karen Holcomb, PhD, of the University of California Davis.

Furthermore, with just 33% coverage of ivermectin-treated bird-feeders in a neighborhood, there was a 15% to 45% reduction in infectious mosquito days, she stated in a presentation at the American Society of Tropical Medicine & Hygiene (ASTMH) virtual meeting.

Holcomb explained how there have been over 25,000 neuroinvasive cases of West Nile virus in the U.S. since 1999, and 2,400 deaths. As there is no human vaccine, the primary strategy has been vector control, she added. However, insecticide has a low specificity in targeting mosquitoes involved in West Nile transmission, and Holcomb also discussed some "non-targeted effects" of spraying in the human population.

Her group hypothesized that because ivermectin had mosquitocidal properties and low toxicity in mammals and birds, it might be possible to treat bird-feeders with ivermectin, where the mosquitoes would bite the birds, ingest the ivermectin, and die before the virus could be transmitted to humans.

Two earlier studies laid the groundwork: one that treated chickens with ivermectin and found a decrease in seroconversion and fewer older mosquitoes near the treated flocks, and an increase in mortality of mosquitoes following a blood meal on treated chickens.

Nevertheless, "the link between ivermectin and West Nile virus transmission was not fully elucidated," Holcomb said, adding there was no significant difference in mosquito abundance or infection prevalence, as well as variable serum concentrations in chickens.

Next steps of the study included determining what type of set-up worked best for ivermectin-treated bird-feeders in neighborhoods: either contiguous (with all treated bird-feeders in a row) or random. Holcomb's group found similar reductions in infections in mosquitoes and birds, but the greatest reductions in treated lots were from contiguous, not random placement, she said.

Holcomb noted there is still uncertainty in ivermectin-induced mortality in wild mosquitoes, as well as the proper dose of ivermectin to induce mortality.

Given the higher profile of ivermectin during the COVID pandemic, Holcomb said she has encountered several logistical challenges in moving the research forward.

"During the past year, our collaborators noted it's become harder to obtain ivermectin, and the ivermectin they were getting was lower quality than prior to COVID," she told MedPage Today.

Still, Holcomb noted that this strategy won't be used in field control trials for at least a couple more years, and "during that time, there should be a reduction in demand for ivermectin," she said.

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