What is the risk to humans from cows infected with H5N1 avian flu?

The discovery of H5N1 bird flu in U.S. cattle — and the news that at least one person in Texas has been infected, apparently through contact with infected cows — has taken scientists who study influenza by surprise.

But after absorbing their shock, several admitted there was evidence to suggest bovine infection could happen — it just hadn’t been reported with this particular strain of influenza virus until now.

The situation is evolving. But here are some questions we can answer now about the curious case of highly pathogenic H5N1 avian influenza in dairy cattle.

How widespread is the virus in cows?

Cows from 11 dairy herds in four states have tested positive for H5N1. Those states are Texas (seven herds), Kansas (two), Michigan (one), and New Mexico (one). Confirmatory testing is still being conducted on samples from a herd in Idaho.

Is that the full extent of the problem?

Some experts believe it’s unlikely — if only because people haven’t been looking for bird flu infections in cattle before now. “It could have been infecting dairy cattle a year ago. We just never thought about looking … for it,” said David Swayne, an avian influenza expert who is now a private consultant after having worked for the U.S. Department of Agriculture for nearly 30 years.

Should this news have been such a shock?

Probably not, Swayne admitted.

In the 1990s, there were reports from the United Kingdom of cows that were infected with human flu viruses. And there is a type of influenza known as influenza D that cows are susceptible to. (Humans mainly contract influenza A and B viruses.)

As to infection with H5N1 specifically, there has been evidence for a while that it could occur — at least in a laboratory setting. Scientists from the Friedrich-Loeffler-Institut, in Greifswald-Insel Riems, Germany, reported in 2008 that they had experimentally infected six calves with an H5 virus retrieved from an infected cat. All remained healthy but all sero-converted — they developed antibodies that indicated that an infection had taken hold.

How did U.S. cows become infected?

The list of species that this virus has been shown to be able to infect has grown substantially in the past couple of years, as widespread infection in wild birds has brought the virus to new parts of the globe.

As early as 2003 the virus was seen to be able to infect big cats, having infected tigers and snow leopards in a zoo in Thailand. Later it was seen to infect domestic cats and small carnivorous mammals like martens and mink. More recently, infected black bears, brown bears, polar bears, raccoons, skunks, and coyotes have been found; aquatic mammals like sea lions and seals have also been infected. USDA keeps a list of animal species in which infections have been detected here.

For some of these mammals, the route of infection is pretty obvious. Scavengers that feast on dead bird carcasses can become infected that way.

But cows? Experts believe the more likely route of infection here is via water or grasslands contaminated with virus by infected birds.

“There is massive (unprecedented) exposure of wild carnivores (feasting on sick/dead birds) and potentially also other mammals (through contaminated water and surface areas, including grasslands),” Ron Fouchier, an influenza virologist at Erasmus Medical Center in Rotterdam, the Netherlands, told STAT via email. “The infected mammals get infected upon feeding/drinking, which is an alternative route of infection of mammals.”

Richard Webby, an influenza virologist who heads the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals at St. Jude Children’s Research Hospital in Memphis, Tenn., said infected birds — and mammals — shed large amounts of H5N1 virus, increasing the risk to other species. “There is this elevated exposure,” he said.

Are beef cattle also infected?

There have been no reports to date of infected herds, but people should be on the lookout, Swayne suggested.

Is there a risk to people from milk produced by infected cows?

Yes, in theory. In practice, though, it shouldn’t be an issue.

Infected cows shed virus in their milk. Farms that have detected H5N1 in their cows are supposed to destroy the milk produced by infected cows, so it should not make its way into the food chain. Even if some did, inadvertently, pasteurization would kill the viruses, the USDA says.

Unpasteurized milk from affected cows would pose a risk to people. But unpasteurized milk poses multiple risks to people and public health authorities recommend against its consumption.

Are dairy cows spreading the virus to other cows?

That’s suspected, but it is uncertain at this point. The infected herd in Michigan had received cows recently from Texas.

Why does that matter?

Bird flu viruses are genetically structured to infect birds. They can occasionally spill over into mammals, but typically those infections should dead-end in those new hosts. The viruses would need to adapt to be able to transmit efficiently in the new species.

Viruses that mutate to be able to transmit in one species of mammals could find it easier to infect people. That’s why particular attention is paid when bird flu viruses are seen to infect people, or mammal species with which humans interact. An animal of particular concern is pigs.

Why are pigs a big worry?

Pigs are traditionally referred to as “mixing bowls” or “mixing vessels” for flu species, because they can be infected with bird flu viruses and with the flu viruses that infect people. If pigs become co-infected with various types of flu, the viruses can swap genes and form hybrids that can then spread from pigs to people. The 2009 H1N1 flu pandemic was caused by a virus that jumped from pigs to people.

Are pigs susceptible to H5N1?

Swayne said back in the early 2000s there were occasional reports from Asia of pigs being infected with the virus, but those events appeared to die out. Pigs are not among the list of mammals that have more recently been seen to be susceptible to the virus.

However, scientists have tried infecting pigs in laboratories, with differing results. German scientists reported in 2023 that only one of eight pigs they infected with a strain of H5N1 from a chicken developed antibodies that indicated there had been an infection. Those scientists used the same strain of H5N1 as was found to have infected the cows in the U.S., which goes by the awkward moniker 2.3.4.4.b.

But more recently, scientists from the USDA reported that they were able to experimentally infect pigs, and some of the infected pigs transmitted the virus to uninoculated pigs housed with them, when using sample viruses taken from mammals — raccoons and red foxes.

Is this picture likely to change?

Almost undoubtedly.

“We’re doing science here. And science changes. And when the science changes, then our assessment may change,” said Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention.