A strain of H5N1 avian influenza virus that infected a Texas dairy worker earlier this year has demonstrated the ability to spread through the air between ferrets, though inefficiently, according to new research from the University of Wisconsin-Madison. While the infected worker experienced only mild symptoms and made a full recovery, laboratory tests showed the virus was lethal in ferrets, killing 100% of those infected.
The findings, published on October 28, 2024, in Nature, highlight ongoing concerns about the virus’s potential to evolve as it spreads among dairy cattle and occasionally to farm workers. Lead researcher Yoshihiro Kawaoka, a professor of pathobiological sciences at UW-Madison, described the strain’s impact on ferrets as unusually severe.
Airborne Transmission Raises Red Flags
Ferrets are widely used in influenza research because their respiratory systems closely resemble those of humans. Kawaoka and his team have long studied how bird flu viruses adapt to mammals, as such changes could increase the risk of a future pandemic.
The fact that the virus could spread through the air—even inefficiently—suggests that it has taken key steps toward mammalian adaptation. However, scientists stress that there is no evidence that this strain is transmitting efficiently between humans.
A Dangerous Mutation, But No Longer Circulating
Genetic analysis of the virus found in the Texas dairy worker revealed a mutation that Kawaoka’s team first identified in 2001 as being associated with severe disease. Fortunately, researchers believe this specific strain has since disappeared from circulation.
Kawaoka hypothesizes that when H5N1 jumped from birds to cows, it evolved along two distinct paths. One led to the highly pathogenic strain found in the Texas worker, while the other resulted in a less severe version currently circulating in cattle. The concern is that if a cow-adapted strain were to acquire the more dangerous mutation, it could become a more significant threat.
H5N1 Still Unclear How Dangerous It Could Be to Humans
One of the biggest mysteries surrounding this case is why the Texas dairy worker had only mild symptoms despite the virus’s lethality in ferrets. Kawaoka suggests several possibilities: prior exposure to seasonal flu viruses may have provided some immunity, the infection may have entered through the eyes rather than the respiratory tract, or the strain may be less harmful to humans than to ferrets.
Enhanced surveillance of dairy workers since the virus began spreading in cattle may also lead to more detections of mild cases. However, scientists caution that ongoing mutations could still pose a risk.
While this strain is not a current public health threat, researchers continue monitoring H5N1’s evolution. The study reminds us that flu viruses can change rapidly, and any adaptation that makes them more transmissible among mammals warrants close attention.
Reference: Chunyang Gu, Tadashi Maemura, Lizheng Guan, Amie J. Eisfeld, Asim Biswas, Maki Kiso, Ryuta Uraki, Mutsumi Ito, Sanja Trifkovic, Tong Wang, Lavanya Babujee, Robert Presler, Randall Dahn, Yasuo Suzuki, Peter J. Halfmann, Seiya Yamayoshi, Gabriele Neumann, Yoshihiro Kawaoka. A human isolate of bovine H5N1 is transmissible and lethal in animal models. Nature, 2024.
