The H5N1 avian influenza virus, obtained from infected cow’s milk, can cause illness in mice and ferrets when introduced into their nasal passages, but the ability for airborne transmission of the virus between ferrets, commonly used as a model for human transmission, seems restricted.
New research findings regarding the strain of H5N1 present in North American dairy cattle this year stem from laboratory experiments conducted by University of Wisconsin-Madison researchers, as detailed in the journal Nature. These results indicate that exposure to raw milk contaminated with the current virus strain poses a genuine risk of infecting humans, though the virus may not easily spread to others.
“The relatively low risk is encouraging news, as it suggests that the virus may not readily infect individuals who have not been exposed to raw infected milk,” mentions Yoshihiro Kawaoka, a professor of pathobiological sciences at UW-Madison leading the study with Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, and collaborating with teams from Texas A&M University, the University of Shizuoka in Japan, and elsewhere.
Kawaoka notes, however, that the findings pertain to the virus’s behavior in mice and ferrets and may not fully reflect the process of infection and evolution in humans.
In the experiments, the UW-Madison researchers observed that mice could fall ill with the flu after consuming even small amounts of raw milk from an infected cow in New Mexico.
The team also investigated the potential airborne spread of the bovine H5N1 virus by placing infected ferrets near uninfected ones without physical contact. Ferrets share similarities with humans in terms of respiratory symptoms when sick with the flu, such as congestion, sneezing, and fever, making them a valuable model for understanding influenza transmission among humans. Efficient airborne transmission would indicate a significant escalation in the virus’s potential to cause a human pandemic.
None of the four exposed ferrets exhibited symptoms, and no virus was detected in them during the study. Nevertheless, one exposed ferret was found to have produced antibodies to the H5N1 virus upon further examination.
“This indicates that the exposed ferret was infected, suggesting some degree of airborne transmissibility, although not at a significant level,” explains Kawaoka.
Additionally, the researchers combined the bovine H5N1 virus with receptors that the virus typically binds to in order to enter cells, which are usually associated with avian or human influenza viruses. They observed that the bovine H5N1 could bind to both types of receptors, demonstrating its adaptability to human hosts.
While this adaptability has resulted in a limited number of human H5N1 cases thus far, past influenza viruses that triggered human pandemics in 1957 and 1968 did so after developing the ability to bind to receptors targeted by human influenza viruses.
Finally, the UW-Madison team discovered that the virus could spread to the mammary glands and muscles of mice infected with H5N1 and could be transmitted from mothers to their offspring, likely through infected milk. These findings highlight the potential risks associated with consuming unpasteurized milk and potentially undercooked beef from infected cattle if the virus becomes more widespread among beef cattle, according to Kawaoka.
“The current H5N1 virus in cattle has limited mammalian transmission capabilities,” he states. “However, we must actively monitor and contain this virus to prevent its evolution into a strain that can efficiently transmit among humans.”
