Raw milk is risky, but airborne transmission of H5N1 through cow’s milk is inefficient in mammals

While the H5N1 bird flu virus, which comes from infected cow’s milk, sickens mice and ferrets when it drips into their noses, airborne transmission of the virus between ferrets — a common model for human transmission — appears to be limited.

These and other new findings about the H5N1 strain circulating this year among North American dairy cattle come from a series of laboratory experiments led by researchers at the University of Wisconsin-Madison, reported in the news Nature. Together, they suggest that exposure to raw milk contaminated with the currently circulating virus poses a real risk of infecting people, but that the virus may not spread very far or quickly to others.

“This relatively low risk is good news because the virus is unlikely to easily infect others who are not exposed to raw infected milk,” said Yoshihiro Kawaoka, professor of pathobiological sciences at UW-Madison who led the study with Keith Poulsen . , director of the Wisconsin Veterinary Diagnostic Laboratory, and with collaborators at Texas A&M University, Japan’s Shizuoka University, and elsewhere.

However, Kawaoka cautioned that the findings reflect the behavior of the virus in mice and ferrets and may not explain the infection and evolutionary process in humans.

In their experiments, the UW-Madison team found that mice can get the flu after drinking even relatively small amounts of raw milk from an infected cow in New Mexico.

Kawaoka and his colleagues also tested the ability of the bovine H5N1 virus to spread through the air by placing ferrets infected with the virus close to but out of physical contact with uninfected ferrets. Ferrets are a common model for understanding how influenza viruses can spread among humans because the small mammals exhibit respiratory symptoms similar to those of people sick with the flu, including congestion, sneezing and fever. Efficient airborne transmission would signal a serious escalation in the virus’s potential to cause a human pandemic.

None of the four exposed ferrets became ill and no virus was recovered from them during the study. However, upon further testing, the researchers found that one exposed ferret had produced antibodies against the H5N1 virus.

“That suggests the exposed ferret was infected, indicating some level of airborne transmissibility, but not a substantial level,” Kawaoka said.

In addition, the team mixed the bovine H5N1 virus with receptors – molecules that the virus binds to to enter cells – that are typically recognized by bird flu and human flu viruses. They found that bovine H5N1 bound to both types of molecules, providing further evidence of its adaptability to human hosts.

While that adaptability has resulted in a limited number of human H5N1 cases so far, previous 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, the UW-Madison team found that the virus spread to the mammary glands and muscles of mice infected with the H5N1 virus and that the virus spread from mothers to their pups, likely through infected milk. These findings underscore the potential risks of consuming unpasteurized milk and possibly undercooked beef from infected cattle if the virus spreads widely among beef cattle, Kawaoka said.

“The H5N1 virus currently circulating in cattle has limited transmission capacity to mammals,” he says. “But we need to monitor and contain this virus to prevent it from developing into a virus that is highly transmissible to humans.”