Cows might host both human and bird flus

Cows have entry portals for both human and bird flus. This new finding may mean that cattle could host both types of flus at once. The two viruses could then swap parts to create a new type of influenza, maybe even one that could cause a pandemic.

Cattle have three types of influenza A receptors — entry portals viruses use to infect host cells — veterinarian Charlotte Kristensen and colleagues report May 3 at bioRxiv.org. One portal is like a receptor that decorates human cells. Another is like that found in chickens. And the third resembles receptors from ducks.

One of those avian receptors may have allowed a virus from a wild bird to infect a dairy cow in Texas, leading to the current H5N1 bird flu outbreak in cattle.

Researchers have long thought cattle were resistant to influenza A, says Kristensen, of the University of Copenhagen. Some cows have been infected in lab studies, and there have been suggestions that cows might sometimes catch human seasonal influenza. But it surprised many experts when H5N1 bird flu was found in dairy cattle and in grocery store milk this spring (SN: 4/25/24).

In the new study, Kristensen and colleagues tested whether cows have receptors that bird flu viruses can use to infect their cells. Not only did the team find such entry portals but also found that the receptors, especially the duck version, were abundant in the mammary glands, at low levels in the respiratory tract and at very low levels in the brain. That fits with the description of the illness in cows, Kristensen says, which affects milk production but doesn’t seem to make most cows very sick.

It’s still unclear how an infection gets to the udder. It’s possible the virus enters through the teat via contaminated milking machines. But none of the receptors are present in the ducts leading up to the milk-producing glands, Kristensen says. The virus might get to the mammary glands through the blood, but so far there are no reports of the virus in cows’ blood, she says.

“These are just preliminary results,” Kristensen says, “but still important, because right now we really [don’t] know very much about cattle and influenza viruses.”