Updated February 06, 2025 at 12:55 PM ET
A variant of H5N1 bird flu that has circulated widely in wild birds — and in several instances led to severe illness in humans — has turned up in dairy cattle for the first time.
The findings were relayed in a short update from the U.S. Department of Agriculture on Wednesday which traced the new variant back to dairy herds in Nevada.
While the implications aren't entirely clear, scientists say it's a concerning development that could mark a new chapter in the outbreak. The news comes at a time when a pause in routine communication from federal health agencies has made it hard to discern exactly what's going on.
"We really haven't heard a word about how the CDC is continuing to respond, or if they're responding at all, which makes it very, very difficult for academics and people outside the government to contribute," says Angela Rasmussen, a virologist at the University of Saskatchewan in Canada.
The variant, known as D1.1 genotype, belongs to a different genetic lineage than what's fueled the infections in dairy cattle over the past year.
Scientists believe a single spillover event, from birds to cattle, in the Texas Panhandle in late 2023 seeded the current nationwide outbreak. But this new finding points to at least one additional instance of the virus hopping into dairy cattle.
"It shows that even if you get rid of one virus that's established itself, another one can pop up like whack-a-mole," says Michael Worobey, an evolutionary biologist at the University of Arizona.
It's not yet clear just how long the virus has been circulating in cattle and whether it's widespread. For now, Worobey says it opens up a whole new set of questions about the trajectory of the outbreak in cattle and what that could mean for humans who are exposed.
One possibility: The immunity built up in the dairy population against bird flu may not hold up well against this D1.1 genotype, and vaccines in the pipeline may need to be retooled. That could make eliminating the virus from dairy cattle much more "difficult, complicated and less likely," Worobey says.
Cattle infections with D1.1 aren't expected to be much different in terms of "virulence and transmission," Dr. Jürgen Richt, a veterinary microbiologist at Kansas State University, told NPR in an email. But its track record in humans could be a "major change from the earlier scenario."
So far, bird flu infections in dairy workers have largely led to mild illness. In contrast, two cases of severe illness in North America were associated with this D1.1. genotype — in a Louisiana resident who caught the virus from backyard flocks and died, and a teenager in British Columbia who ended up in critical condition. (The source of her infection was never identified).
Scientists have speculated this variant could be intrinsically more deadly for humans, although it's possible other factors like the route of exposure could be a factor, too.
"We don't yet know if these viruses are better at infecting people. But if they are, then introduction into dairy cattle could enhance human exposure risk," said Louise Moncla, a virologist at the University of Pennsylvania.
However, in both of those cases, the patients developed concerning mutations while they were sick, which could indicate the virus was evolving to be better at infecting human cells, says Worobey. "We don't know if maybe more [people] will land in the hospital, maybe you'll see more upper respiratory infections."
Because so much of the work over the past year focused on the other variant, Worobey says "it's going to be a long time before the experiments are done to get a better handle" on what this could mean for humans.
Questions surround federal response
The detection of the variant in Nevada marks one of the few new pieces of information on bird flu to filter down from federal agencies since the Trump administration took over.
On Wednesday, a CDC spokesperson told NPR that some external communications are no longer subject to the pause and that HHS has "approved numerous communications related to critical health and safety needs and will continue to do so." Website data on testing and surveillance is being updated, they added.
Lori Freeman, who leads the National Association of County and City Health Officials, says her members have not been receiving regular briefings and updates on the bird flu situation as had been the case under the previous administration.
"We're definitely feeling a gap there and hoping those will come back online relatively quickly," she told NPR earlier this week.
Federal officials involved in the response held at least one briefing with state-level health officials last week, according to Dr. Manisha Juthani, who is commissioner of the Connecticut Department of Public Health.
She says it's her understanding that the pause is temporary and they're "looking forward to the resumption of routine communications."
Testing data lacking or slow to be shared
Worries about bird flu go beyond dairy cattle.
Last month saw an uptick in outbreaks among poultry facilities, with more than 80 commercial flocks affected. Some states are warning the public to take extra precautions because detections in wild birds and backyard flocks are so prevalent.
"Overall for me, it does feel like the tempo of what we're seeing has gone up," says Juthani, adding that there's still no evidence of human-to-human spread.
Dr. Meghan Davis says the poultry outbreaks are very likely tied to bird migration patterns, along with gaps in biosecurity at poultry farms.
"A lot of times we think about the migratory waterfowl as being major drivers of some of these outbreaks," says Davis, an epidemiologist and veterinarian at Johns Hopkins Bloomberg School of Public Health.
Davis says tracking the scope of the outbreak in dairy cattle remains challenging — not only can it be hard to suss out symptoms in cattle, but also testing on farms is lacking.
"It's really difficult for a producer just to know if that cow is positive, which could really help in terms of on-farm control measures," she says.
A major frustration throughout the past year has been a lack of timely or complete data on the samples that are collected and uploaded by the federal government.
Without this information, Worobey says he and his colleagues who are tracking the trajectory of the outbreak and looking for dangerous mutations are seriously hampered.
And finding the D1.1 variant in dairy cattle only underscores the urgency here.
"We are kind of tying one arm behind our backs," he says. "We do have the ability to understand things in virtually real-time, day to day, but that's not being fully deployed."
Edited by Jane Greenhalgh and Carmel Wroth.
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