A second version of bird flu is infecting cows. What does that mean?

The same version of the bird flu that has caused the most serious human illnesses in North America has now turned up in dairy cows. Cattle in Nevada tested positive for the H5N1 viral variant D1.1, which has been circulating in poultry and wild birds. It’s the first time this version has been detected in dairy cattle, the U.S. Department of Agriculture announced February 5.

“What [experts] are concerned about is that it could indicate another independent spillover event into dairy cows,” says veterinarian and environmental epidemiologist Meghan Davis of the Johns Hopkins Bloomberg School of Public Health. The risk of H5N1 still remains low for most people, she notes.

For about a year, a different H5N1 variant called B3.13 has been wreaking havoc among dairy cattle in the United States. Nearly 1,000 herds across 16 states have tested positive for H5N1. Cow infections led to 40 of the 67 confirmed human cases of highly pathogenic avian influenza in the United States since early 2024, according to the U.S. Centers for Disease Control and Prevention. None were caused by human-to-human transmission.

Most human infections have been in farm workers who presented mild symptoms, such as pink eye, fever and a cough. Two serious cases arose late last year, in a Louisiana man over the age of 65 and a Canadian 13-year-old girl. The Louisiana man became the first person in the U.S. to die with H5N1, while the Canadian teen is now in recovery. Both patients caught the D1.1 version of the virus, the one newly identified in cows.

Science News spoke with Davis to learn more about the implications of finding a second H5N1 variant in dairy cows. The conversation has been edited for length and clarity.

SN: What does finding the H5N1 variant of bird flu called D1.1 in dairy cattle mean?

Davis: For the past year, the hypothesis has been that there was a single spillover event in late 2023 and subsequent cow-to-cow spread. If we now have a new spillover event, there are two things that we immediately need to think about.

The first is it’s happening more often than we think. Are we going to see these sporadic cases on dairy farms? These would be on farms that might not have risk factors that we’ve been working to identify to date. We may need to be thinking far more seriously about improving the level of biosecurity on dairy farms.

Another concern I have is that when we are looking at disease related to [B3.13], this has been by and large mild. But we have seen more severe human illness with the D1.1. I do worry that the clinical presentation in [dairy] workers and other people exposed to the dairy cows or their products could be different and potentially more severe.

SN: What does this finding mean for H5N1 spread in humans?

Davis: We’re talking about a strain that we have seen in poultry, so we have had exposures in poultry workers.

We don’t know yet what kind of presentation we’re going to see in the cows. If [many copies of the D1.1 virus are] in the milk, then we could have those same splash exposures in the milking parlor [as seen with the B3.13 version]. There could be other ways in which dairy worker exposure could be different than poultry worker exposure. That could influence not only whether they’re exposed, but the route of exposure that could influence disease.

SN: What are viral variants, and how can different versions of a virus act differently?

Davis: When we think about genetic relatedness, [D1.1 and B3.13] are very close, like siblings. There are some ways in which they’re going to behave the same, and some ways they may behave differently. Even within a strain, if you have mutations, that could set up new characteristics.

These changes [can] set up a selective advantage for the virus — so it’s good for the virus, in terms of transmission, or is bad for us in terms of antiviral resistance or the ability to cause more severe disease. If there is a competitive advantage, these strains could become more dominant, and that’s a problem.

SN: What do we need to do to stay on top of the H5N1 bird flu outbreak in dairy cattle?

Davis: The identification of [D1.1 in cattle] was through an enhanced surveillance program that’s just come online relatively recently. Through [the USDA’s National Milk Testing Strategy], there is testing by states of bulk milk tanks and sometimes of individual farm bulk milk. This is a way to do high-level screening of your herds.

I think what this shows us is that if we can get new spillover events, then we need to maintain much more rigorous active surveillance and passive surveillance programs for this so that we are able to identify new spillover events as they occur.