Madison Reporter

A dairy worker in Texas contracted the H5N1 avian influenza virus, likely during cow milking. The virus sample from this worker proved 100% lethal to ferrets in laboratory studies conducted by researchers at the University of Wisconsin–Madison. However, it spread inefficiently and does not seem to be circulating widely.

The infected worker experienced only mild symptoms and has since recovered. The strain responsible for the infection appears not to have continued spreading in nature.

Yoshihiro Kawaoka, a professor of pathobiological sciences at UW–Madison, led the study and expressed surprise at how deadly this strain was to ferrets. "This is one of the most pathogenic viruses I’ve ever seen in ferrets," he stated.

Ferrets are often used as models for studying influenza viruses that primarily affect birds but can adapt to mammals. This research is part of ongoing efforts by Kawaoka and his team at UW–Madison’s Influenza Research Institute to understand such adaptations due to their potential pandemic implications.

H5N1 viruses mutate rapidly when infecting new hosts, sometimes gaining the ability to infect different species more easily. This process allowed these viruses, which have been affecting birds globally, to begin spreading among mammals like North American dairy cattle in 2024.

The virus isolated from the Texas worker contained a mutation first identified by Kawaoka's team in 2001 as crucial for causing severe disease. Fortunately, this particular strain seems to have died out. "This isolate is unique among the H5N1 viruses circulating in cows," Kawaoka noted.

Kawaoka hypothesizes that H5N1 took two paths after jumping from birds to cows, aided by mutations enhancing mammalian adaptation. One path resulted in a concerning mutation found in the Texas case; another led to a less dangerous mutation.

"Both mutations give the virus the ability to adapt to mammals," said Kawaoka. "But the good thing is the one containing this more pathogenic mutation has not been detected again."

The potential danger of such a mutated virus for humans remains uncertain. Kawaoka speculated on why humans might experience milder infections: exposure to seasonal flu might offer some protection through antibodies or perhaps infection routes matter—since conjunctivitis was noted as a symptom suggesting eye entry rather than respiratory.

Alternatively, increased surveillance could mean more cases are being identified now than before—including mild ones—or this strain might inherently be less severe for humans compared with other mammals like ferrets.

"We’re now trying to understand why this virus is so pathogenic in ferrets and what that could mean for human infections," concluded Kawaoka.