Bird flu and human flu viruses could mix cow’s udders and trigger a pandemic

Cow’s UDDERS could brew the new stumps of dangerous birds

By Chris Simms Edited by Tanya Lewis

The prospect of an avian influenza epidemic in dairy cattle triggering a pandemic in humans is a step more than scientists thought. New research shows that cow’s udder cells can be infected with human flu and bird flu viruses at the same time, which means that viruses could exchange genes and generate new strains that would be better suited to infected people. The risk that this happens is low, but if this is the case, the consequences could be serious, say the experts.

The H5N1 avian flu is widespread in wild birds, and there have been many epidemics in poultry since 2020. In the United States, a very contagious form of the virus is also spread through herds of dairy cattle, and 70 cases in people have been detected since February 2024. One of these human cases was fatal.

Virologists have been concerned about bird flu for decades because it has such a high mortality rate in humans. Globally, between 2003 and May 2025, 976 cases reported in humans, almost half of which were fatal.

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Fortunately, H5N1 did not historically spread easily between people. But many researchers fear that if it continues to circulate in cattle, it could adapt to better infect mammals in general.

“It is really important to prevent the virus from being able to spread from cow to cow, because, as it does, it adapts to be better to infect cows,” explains Eleanor Gaunt, virologist at the University of Edinburgh. “If the virus adapts to the replica in cows, it probably also improves its ability to reproduce in humans and therefore increase the pandemic risk.”

Although the recent spread of the bird flu in cattle has confirmed that animals could catch this virus, it was not clear if they could catch human versions of flu. Thanks to tests with cells in a laboratory, Gaunt and his colleagues have now shown that not only can the cows’ mammary glands be infected with a human flu virus, but also that many different types of flu are capable of reproducing in cow cells and that a single UDDER cell can be infected with human and avian influenza viruses.

Researchers have concluded that cow’s UDDERs are a site where viruses could exchange genetic equipment – a process called reassortment – and generate new flu viruses that could potentially spread more easily or cause a worst disease. Their work has been published as a pre -printed document on Biorxiv and has not yet been evaluated by peers.

Richard Webby, an expert in animal and human virus at the Research Hospital for children St. Jude in Memphis, who has not been involved in research, the results suggest that cow’s UDDERS could be infected with the two forms of virus. “The concern for this is that we could get a restocking event generating a virus that may have more capacity to infect and transmit between humans,” he said.

It is difficult to predict the properties of any new virus, webby notes. “There is a good chance that it is not worse than the H5 virus that exists. But it could well increase the replication in human respiratory tract, which gives it a little more opportunities for additional adaptation, so it is a concern.”

If a new virus emerges, which looks like nothing of humans before, people will have no immunity, says Gaunt.

Gaunt and his colleagues studied cells in a laboratory, so they have not seen the co-infection occur in a cow in a farm. And infecting a living cow is more difficult, explains Robert de Vries, virologist at the University of Utrecht in the Netherlands, who was not involved in the study either. But he says that cow’s udders could be a potential viral mixture.

Many researchers suspect that human influenza viruses can infect cows. No one has yet detected this, but that does not mean that such infections do not occur, says Vries. “People have not tested it because we consider this species as a non-host,” he adds. Nevertheless, “if you create surveillance for this, you will find it.” There is evidence that certain cows have antibodies against human influenza viruses, which suggests that animals have been infected with these pathogens even if the viruses themselves have not been detected, said Gaunt.

The way in which American cows initially captured bird flu is not known. Scientists suspect contact with wild birds infected with sowing initial infections, but the virus seems to spread among cows via contaminated milking equipment, says Gaunt.

She suggests that a cow could be infected with human flu if a bad agricultural worker did not wear personal protective equipment (if he had access to one) and if the milking equipment was not disinfected between the uses. In-depth cleaning of milking equipment could help stop the spread, she said.

It is not possible that co-infection with avian and human flu is possible, however, says Webby. The bird flu jumping into livestock in the first place is a relatively rare event – as would be a human virus that infects animals, he says. “This would require some rare events that occur at the same time,” he says.

But with the bird flu which circulates largely in American herds, one of the two types of virus is already present in certain cattle, which stimulates the probability of co-infection, says Gaunt. The risk is likely to be low. However, the more cattle there are the influenza of birds, the more the chances of co-infection are high, so it is a risk that should be taken seriously, she said. “When we talk about a pandemic emergency,” adds Gaunt, “then it is generally an incredibly improbable set of circumstances that causes it, whatever the virus we are talking about.”