The variant of H5N1 bird flu has been circulating in wild birds, but this is the first time it’s been detected in cattle.

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Dusty Pixel photography/Getty Images/Moment RF

A variant of H5N1 bird flu that has circulated widely in wild birds — and in several instances led to severe illness in several 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, which traced the new variant back to dairy herds in Nevada.

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 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.

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.

Edited by Jane Greenhalgh