Madison Reporter

While H5N1 avian influenza virus taken from infected cow’s milk makes mice and ferrets sick when dripped into their noses, airborne transmission of the virus between ferrets — a common model for human transmission — appears to be limited.

These findings about the strain of H5N1 circulating among North American dairy cattle this year come from laboratory experiments led by University of Wisconsin–Madison researchers, reported in the journal Nature. They suggest that exposure to raw milk infected with the currently circulating virus poses a real risk of infecting humans, but that the virus may not spread very far or quickly to others.

“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,” says Yoshihiro Kawaoka, a UW–Madison professor of pathobiological sciences who led the study alongside Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, and collaborators at Texas A&M University, Japan’s University of Shizuoka and elsewhere.

Kawaoka cautioned that the findings represent the behavior of the virus in mice and ferrets and may not account for the infection and evolution process in humans.

In their experiments, the UW–Madison team found that mice can become ill with influenza after drinking even relatively small quantities of raw milk taken from an infected cow in New Mexico.

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

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

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

Separately, researchers mixed bovine H5N1 with receptors — molecules typically recognized by avian or human influenza viruses. They found that bovine H5N1 bound to both types of molecules, representing evidence of its adaptability to human hosts.

While this adaptability has resulted in a limited number of human H5N1 cases so far, previous influenza viruses causing pandemics in 1957 and 1968 did so after developing binding abilities similar to those recognized by human influenza viruses.

Finally, researchers found that H5N1 spread to mammary glands and muscles in mice infected with it and transmitted from mothers to pups via likely infected milk. These findings underscore potential risks associated with consuming unpasteurized milk or undercooked beef derived from infected cattle if it spreads widely among beef cattle.

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