The U.S. Department of Agriculture announced Wednesday that four dairy herds in Nevada recently found to be infected with H5N1 bird flu were in fact infected with a different strain of the virus than has been circulating in cows for the past year.
The discovery, experts said, makes it clear driving this virus out of cows will be harder than the USDA has estimated. There is so much H5N1 virus in the environment, in wild birds, domestic poultry, and a variety of mammalian species, that the opportunity for further spillovers remains a real risk, they said.
The version in the Nevada herds is one that has been circulating in wild birds. It is also the version behind the severe infection of a teenager in British Columbia, Canada, last year, and a fatal infection in Louisiana last month. To date none of the human infections involving the version of the virus responsible for the main outbreak in cows have resulted in serious illness.
Scott Hensley, a professor of microbiology at the University of Pennsylvania’s Perelman School of Medicine, called the finding “a huge development.” For starters, he said, there is reason to think the wild-bird version of the virus might have an easier time acquiring the mutations needed to adapt to spread in people than the version that has been spreading in cows.
Research done by scientists in Canada and at the Centers for Disease Control and Prevention has shown that in the two known severe human cases, the viruses were starting to acquire changes during the infections that would have allowed them to more easily infect human respiratory tract cells.
And the fact that H5N1, which before last year was not known to infect cows, has spilled over into them at least twice suggests the risk of future spillovers may be higher than previously believed — which would substantially complicate the effort to get the virus out of cows, Hensley said.
“It is much easier to control the spread of virus between 1,500 pound cows rather than from birds with wings that are flying around!” he wrote in an email. “And we haven’t been great at controlling the virus spreading between cows.”
Both the cow version of the virus and the one circulating in wild birds belong to a large family — a clade, in flu parlance — known as 2.3.4.4b. The subset of viruses responsible for the outbreak in dairy cows is genotype B3.13. The version of the virus found in the Nevada cows was genotype D1.1.
Early in the cow outbreak, which was first confirmed in late March, scientists from outside the USDA analyzed the genetic sequences of the viruses available up to that point and determined that the infected herds were all part of a single outbreak, which was moving from herd to herd. That group has been monitoring the sequence data from the outbreak posted by USDA to see if that remained the case, Michael Worobey, an evolutionary biologist at the University of Arizona who convened that group of outside experts, told STAT.
The new Nevada herds are the first confirmed new spillover of virus into cows. Because testing has not been rigorous in many states — many farmers have refused to let their animals be tested — it is not possible to rule out the possibility that there have been other such events that went undetected.
But the fact that two different versions of the virus could make the leap suggests more such events could happen, Worobey said.
“It was an open question whether the first jump had exactly the right pre-adaptations in birds that made it the rare one that could successfully establish itself in cattle,” he said. “If you’re seeing successful jumps from fairly distant branches on the evolutionary tree, there may be lots and lots of viruses out there that, given the chance to interact with cattle, might also become established.”
That raises concerns about how effective vaccines produced to protect cows might be, Worobey said. Antibodies generated by a vaccine targeting one version of the virus — or indeed by infection — “might not protect, or protect as well,” he warned.
There are currently no licensed H5N1 vaccines for cows, but some are in development.
Since Worobey and his colleagues reported that the early herd infections were linked, the USDA has maintained that all the infections — in over 950 herds in 16 states to date — trace back to a single introduction of the virus into cows in late 2023 or early 2024, possibly in Texas. The agency suggested control measures like the requirement that cows moving interstate must be tested before transport would lead to a situation where the virus would run out of new animals to infect.
This new finding, which was made as a result of a national milk testing strategy the USDA instituted in late 2024, makes prospects for containment appear dim, said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy.
“I’m not surprised that we’re seeing additional introductions, particularly given the very widespread nature right now of H5N1 in migratory waterfowl throughout North America,” Osterholm told STAT. “From a containment standpoint, it just points out that this is not going to burn itself out.”
Virologist Angela Rasmussen agreed. “I think it pretty much shows that that idea was completely wrong,” said Rasmussen, who studies emerging infectious diseases at the University of Saskatchewan’s Vaccine and Infectious Disease Organization in Saskatoon, Canada. The fact that the new infections were detected through milk testing shows that relying on finding where the virus is spreading in cows by having farmers report symptomatic animals did not work, she said. “They need to do more testing.”
The USDA said in a statement posted to its website that the finding has not made the department rethink its approach.
“The detection does not change USDA’s [H5N1] eradication strategy and is a testament to the strength of our National Milk Testing Strategy,” it said.
This story has been updated with additional reporting.