Bird flu has infected a person after spreading to cows. Here’s what to know

The risk of H5N1 to people remains low, the CDC says

Multiple cows on a dairy farm eat hay while some black birds eat from the same hay piles

Dairy farms in at least four states have reported cases of bird flu in cattle, and one person exposed to an infected cow has fallen ill. The risk to people remains low, but researchers are keeping a close eye on the outbreak.

Dusty Pixel photography/Getty Images

A strain of avian influenza that has likely sickened and killed millions of birds around the globe has popped up in an unexpected species: Cows.

On April 1, the U.S. Department of Agriculture confirmed that cattle on a farm in New Mexico had tested positive for a strain of bird flu called H5N1. The news followed a March 25 announcement that officials had detected the virus on two dairy farms in Kansas and two Texas farms. The virus was also detected in a Michigan herd that had recently received cows from Texas, the USDA reported on March 29. Five additional herds in Texas have tested positive, and an outbreak in Idaho is presumed to have been caused by H5N1.

A person exposed to cattle on one of those Texas farms has tested positive too, becoming only the second person in the United States ever documented to be infected with H5N1, the U.S. Centers for Disease Control and Prevention reported April 1. The individual is being treated with an antiviral drug for a single symptom, eye redness.

It’s unclear how the cows were infected, although consuming wild bird excrement is a possibility. The person was probably infected while in close contact with the cows. Most people fall ill with H5N1 only after close contact with infected animals, typically poultry, and these infections can be mild to deadly (SN: 3/6/23).

Right now, the CDC considers H5N1’s risk to people to be low. But milk from infected cows should be thrown out before reaching our food supply, the USDA says. Pasteurization would also kill the virus, so it should not spread through the country’s milk, the agency said.

While less concerning for humans at this moment, the virus is particularly lethal for poultry — resulting in the culling of millions of farmed birds — and it has also proved dangerous for myriad other animals since it began sweeping around the globe in 2021. In addition to killing wild birds, the virus has occasionally popped up in mammals, including seals, sea lions, foxes, raccoons and bears (SN: 1/25/24). Now, cows have joined the list, showing symptoms such as reduced milk supply and decreased appetite.    

“Cows were not high on the list of animals that [could] get infected,” says virologist Andrew Pekosz. “But polar bears were probably not high on the list a few years ago, and we’ve seen a number of polar bear infections.”

To understand what it means that bird flu has jumped to yet another new animal species, Science News spoke with Pekosz, of Johns Hopkins Bloomberg School of Public Health. This conversation has been edited for length and clarity.

SN: What have you been keeping an eye on during the bird flu outbreak?

Pekosz: Since around 2022, H5N1 outbreaks have really increased in terms of their size and in the number of bird species that are being infected. That was always worrying scientists because those kind of massive increases in numbers of infected waterfowl eventually means that there’s going to be a greater chance of that virus spilling over into other animal species.

In fact, we’ve been seeing that consistently across the globe. A number of different animals have been infected with H5N1. We don’t have real strong evidence that H5N1 has been transmitted between mammals, but that’s really the critical thing that scientists have been looking for. That would be a real warning bell if we saw [mammal-to-mammal transmission].

SN: What does it mean that the virus appears to be infecting cows, and that one person got sick?

Pekosz: The [most recent] human case of H5N1, while rare, is probably not surprising to scientists. It sounds like this individual was in close contact with an infected cow and ended up contracting the virus as conjunctivitis, an infection of the eye, which is something that’s fairly common with avian influenza viruses when they infect humans or other mammals.

The real issue right now is whether there is a common source that all these cows are getting infected with or whether there is transmission among the cows at the farms. There’s been some reports of movement of some of these cows to other farms and detection of infected cows in those other farms. It’s not clear to me yet whether an infected cow that moved was [subsequently diagnosed with bird flu] or whether there was transmission at the new farms. If there has been transmission from cow-to-cow, that’s something that would be quite worrisome to us.

SN: What kind of threat might the virus pose for people?

Pekosz: This virus changes over time but it’s never really gone away as a threat. It keeps infecting birds very effectively. It keeps spilling over into mammals occasionally. Those are warning signs for a virus that might potentially be able to make the jump into humans and be able to transmit. This virus continues to be on the high priority list in terms of viruses to watch because it keeps doing the things that we know to expect from a virus that’s making the jump into humans.

Any time a virus like this gets into a farm setting, you have conditions that might make it right for a virus to spread. Cows are usually kept fairly close together in indoor spaces for large periods of time. Those kinds of conditions are ripe for spread of viruses. Every time you get an infection in a mammal, there’s a slightly greater chance that the virus will [mutate in ways that help it better] replicate in mammals.… It’s really one of those times when farmers could probably take stock of some of their biosecurity measures to try and understand if their animals are getting exposed to migratory, wild waterfowl, and if there are ways that they can minimize that chance of exposure.

Erin I. Garcia de Jesus is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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