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• Posted July 10, 2024

Raw Milk Exposure a Real Bird Flu Risk for Humans, but Fast Spread Unlikely

While exposure to raw cow's milk infected with the H5N1 avian influenza virus can make you sick, a new study suggests the virus may not spread quickly to other people.

Researchers at the University of Wisconsin-Madison found that mice and ferrets got sick with influenza when H5N1 bird flu virus was dripped into their noses, but airborne transmission of the virus appeared to be limited. Airborne transmission is a common way disease spreads quickly among people.

"This relatively low risk is good news, since it means the virus is unlikely to easily infect others who aren't exposed to raw infected milk," said study leader Yoshihiro Kawaoka, a professor of pathobiological sciences at the University of Wisconsin-Madison. 

He cautioned, however, that the findings show how the virus behaves in mice and ferrets. Results in humans may not be the same.

In their experiments, researchers found that mice got bird flu after drinking even small quantities of raw milk from an infected cow. 

They then relied on ferrets to simulate how influenza viruses might spread among people, because ferrets' flu symptoms are similar to those of humans, including fever, congestion and sneezing. 

To test the ability of the bird flu virus to spread through the air, they placed infected ferrets near uninfected ones. None of the four exposed ferrets got sick, and no virus was recovered from them during the study. On further testing, however, one of the four had produced H5N1 antibodies.

"That suggests that the exposed ferret was infected, indicating some level of airborne transmissibility but not a substantial level," Kawaoka said in a university news release.

In a separate experiment, researchers mixed the bird flu virus with receptors typically recognized by human flu viruses. Receptors are molecules to which the virus binds in order to invade cells. 

They found that bird flu virus bound to both types of molecules -- an indication of its adaptability to human hosts.

Although that adaptability has so far led to only a few human H5N1 cases, researchers noted that flu viruses that caused human pandemics in 1957 and 1968 did so after developing the ability to bind to receptors bound by human flu viruses.

Finally, researchers found the virus spread to the milk glands and muscles of infected mice and that it was transmitted from mothers to their pups. 

These findings, published July 8 in the journal Nature, underscore the risks of consuming unpasterized milk and possibly undercooked beef from infected cows if the virus spreads widely among U.S. herds, they noted.

"The H5N1 virus currently circulating in cattle has limited capacity to transmit in mammals," Kawaoka said. "But we need to monitor and contain this virus to prevent its evolution to one that transmits well in humans."

More information

The U.S. Food and Drug Administration has more about the safety of raw milk.

SOURCE: University of Wisconsin-Madison, news release, July 8, 2024