On Oct. 28, 2024, University of Wisconsin–Madison researchers reported that a strain of H5N1 avian influenza virus found in a Texas dairy worker who was infected this spring was able to spread among ferrets through the air, although inefficiently, and killed 100% of infected animals in studies performed with the strain earlier this year.

The infected dairy worker experienced mild symptoms and fully recovered, and the H5N1 strain that infected the worker does not appear to have continued spreading in the wild. The researchers found that the H5N1 virus that infected the Texas dairy worker included a mutation that the team first identified in 2001 as important for causing severe disease.

Like other influenza viruses, H5N1 viruses mutate at a relatively rapid clip as they infect new hosts. Sometimes these mutations allow the viruses to more easily infect and spread among new species. That’s how the current viruses, which have been infecting birds around the world in recent years, began to spread among mammals, most notably North American dairy cattle in 2024. The study was published in the journal Nature.