Millions of wild birds, chickens and turkeys in the U.S. have been affected by the highly pathogenic avian influenza H5N1 virus since 2022. Last year, it spilled over into dairy cows for the first time and has since spread to over 1,000 herds.
Todd Bell, professor of veterinary pathology at Iowa State University, was part of the research team that discovered the “lock-and-key” mechanism allowing H5N1 to infect cells in cow udders.
“Viruses are parasites, and by their nature, they need to attach to a cell, get inside, start to use the cell’s machinery in order to reproduce,” Bell said. “Then they pop back out, go to the next cell, infect that cell or the next animal and continue on their life cycle.”
Proteins topped with sugar molecules dot the outer surface of dairy cow mammary glands, Bell explained. The H5N1 virus fits into some of these “locks” called sialic acid receptors.
Bell and other scientists from ISU's College of Veterinary Medicine, George Mason University and the U.S. Department of Agriculture's National Animal Disease Center in Ames recently found the same receptors in the mammary glands of pigs, sheep, goats, beef cattle, alpacas — and humans.
The authors said their results suggest a potential health risk for farm workers and consumers of raw, unpasteurized dairy products beyond cow milk.
But Rahul Nelli, a flu virus receptor expert at Iowa State and lead author of the study, emphasized more research is needed to understand the risks and infection pathways of H5N1 for people and other species.
For example, migrating birds can shed the virus through feces. But Nelli said it’s unclear if goats and sheep could get sick while grazing or laying in a contaminated area. Additional research is also needed to know whether H5N1 can efficiently replicate in the mammary glands of small ruminants, pigs and people, he said.
“In general, we ingest and inhale lots of viruses, and our immunity kicks in, and then it actually knocks out those viruses and pathogens,” Nelli said. “The issue is how much dose is necessary to cause a systemic infection.”
At least 71 people in the U.S. have been infected with H5N1, including two who died. Most were in close contact with infected poultry or dairy cows, and there’s “no known person-to-person spread at this time,” according to the Centers for Disease Control and Prevention.
But public health experts are concerned H5N1 could evolve into a more dangerous form that more easily spreads between people.
The health of people, animals and the environment are inextricably linked, said Bell.
“We need to make sure that we're vigilant and we keep monitoring what's happening at this human-animal-interface, so we can determine whether or not there's a change in the flu virus,” Bell said. “We just need to be proactive and hopefully be ready in the event that this turns into something different.”
