Study: Potential to reduce West Nile virus transmission in humans by mixing ivermectin into bird feed

a fancy chicken coop

Backyard chickens received ivermectin as an urban West Nile virus control strategy. Photo: Karen Holcomb/UC Davis

Story by the American Society of Tropical Medicine and Hygiene, Burness Communications. 

Researchers at the University of California, Davis and Colorado State University have found evidence that adding ivermectin to backyard bird feeders has potential to reduce local transmission of West Nile virus in the U.S. The findings were presented Nov. 18 at the virtual annual meeting of the American Society of Tropical Medicine and Hygiene.

West Nile virus is transmitted to humans by mosquitoes. But birds serve as key amplifiers of the virus in nature, keeping it circulating in local mosquito populations and maintaining its threat as the most common mosquito-borne pathogen in the U.S.

Brian Foy in the lab
CSU Professor Brian Foy has studied the use of ivermectin to treat malaria and reduce West Nile virus transmission in humans for years.

CSU Professor Brian Foy, an expert in West Nile virus and ivermectin, collaborated on the research with UC Davis scientist Karen Holcomb and Associate Professor Chris Barker, who directs California’s central diagnostic laboratory and related data management systems for surveillance of mosquito-borne viruses. Chilinh Nguyen, a postdoctoral fellow in the Foy lab, is a co-author on this study. Her dissertation focused on developing ivermectin-treated bird feed to control West Nile virus transmission by targeting mosquitos.

“We found that widespread use of ivermectin in backyard bird feeders could reduce neighborhood transmission of West Nile virus by about 60%,” said lead author Karen Holcomb, at UC Davis School of Veterinary Medicine. “That’s because a low dose of ivermectin is harmless for birds, but it gets in their blood and can be lethal for mosquitoes that feed on them.”

Holcomb said the approach could help prevent or control local outbreaks like the one that led to more than 800 confirmed and probable cases in Arizona earlier this year and killed 67 people.

The disease often causes thousands of infections each year and even people who have few symptoms from the virus can experience long-term neurological problems. There are no drugs to cure an infection or vaccines for humans to prevent the disease, which emerged in the U.S. in 1999 and is now found in every state except Alaska and Hawaii.

CSU leads international research on ivermectin to treat malaria

Foy is leading a team of international scientists studying the use of ivermectin to reduce cases of malaria in several villages in Burkina Faso. In results published in The Lancet in 2019, the team found that they were able to reduce cases of malaria in children under five years old in several villages in Burkina Faso by 20% using ivermectin.

Over the last few years, Foy has also been studying possible ways to reduce West Nile virus transmission in Fort Collins, Colorado by using ivermectin-treated bird seed.

For this new study, he helped the research team identify bird species that meet four key criteria, including that they:

  • Make frequent trips to bird feeders
  • Roost at night in areas near bird feeders
  • Are bitten frequently by West Nile virus vector mosquitoes and
  • Are present in large numbers in suburban neighborhoods across the U.S.

By tagging birds and using cameras to monitor feeders as well as locating birds at night with radio telemetry, the researchers determined that offering a wide range of common backyard bird feed blended with ivermectin could produce a sharp drop in local West Nile virus infections if efficacy and spatial coverage are high enough.

Holcomb and her colleagues initially used backyard chickens to test whether mixing ivermectin into bird feed could have a large enough impact on mosquito populations to affect West Nile virus transmission. The team found evidence of increased mortality in mosquito populations that fed on the treated chickens and a reduction in viral infections in chickens that ate the ivermectin bird feed compared with those that did not.

Based on the positive results from this work, they then moved to evaluate its potential impact with wild birds that are regular visitors to backyard bird feeders.

Foy said misguided interest in using ivermectin for COVID-19 could divert supplies needed for proven uses, like fighting river blindness, or for developing novel applications, like fighting malaria and West Nile virus. He said he has experienced difficulty finding enough ivermectin for his work — and what he is finding seems to be of lesser quality.

“I understand that at the beginning of the pandemic, when we had nothing, it was valid to at least consider a drug like ivermectin because there was some evidence of general anti-viral properties,” he said. “But we have vaccines and a growing number of proven treatments, yet the demand for ivermectin, which has no demonstrated benefit so far to treat COVID-19, seems to be growing.”