H5N1 Bird Flu in Dairy Cows Raises Human Health Concerns

The prospect of avian flu passing from birds to cows and then to people is causing serious concern among some scientists.

That sequence has already happened twice in the United States since spring 2024, when the country’s first cattle began developing highly pathogenic avian influenza (HPAI).

A team led by researchers at Iowa State University has examined two Texas dairy cows infected with the H5N1 strain, seeking to clarify how such transmission may occur.

H5N1 receptors in dairy cows

In the respiratory tissues and mammary glands of both infected animals, the researchers identified receptors used by numerous flu strains, including those originating in birds, pigs and humans.

When these receptors are present together, a virus infecting a cell could potentially ‘learn its secrets’ and mutate so that it attaches to other receptors in that cell, including receptors commonly found in people.

Although bird flu is fatal among avian species, infection in cattle appears to cause a pronounced but brief, non-fatal decline in milk production. US dairy cows are the first cattle anywhere in the world confirmed to have been infected, and experts still do not know how a virus evolved to infect birds is moving between cows.

The new research suggests that milking may be one route of transmission. By finding H5N1-compatible receptors in bovine mammary glands, it also helps account for the virus’s unusual effects on milk.

Dairy workers report that milk from infected cows becomes thick and discoloured. Scientists suspect that raw, unpasteurised milk could transmit the virus to other mammals, including humans.

US Centers for Disease Control and Prevention (CDC) officials have detected high concentrations of H5N1 in contaminated cow’s milk. However, they state that “sporadic human infections with no ongoing spread will not change the CDC risk assessment for the US general public, which CDC considers to be low.”

What H5N1 infection could mean for people

Fortunately, the two US dairy workers who contracted H5N1 from dairy cows had only mild symptoms, recovered and did not pass the infection to anyone else. Yet scientific analysis of the pathogen found signs of adaptation that were concerning for mammalian bodies.

It remains unclear whether those adaptations developed in the dairy cows or in the workers, but Iowa State scientists are eager to investigate further.

Influenza A viruses, including H5N1, infect cells through receptors composed of sialic acids. Viral strains adapted to birds favour particular sialic-acid receptors, unlike strains adapted to mammals. Influenza A viruses are highly selective about their points of attachment on cells, even at species level.

Researchers at Iowa State University and the University of Georgia are concerned by the pattern and distribution of sialic acids in the respiratory tracts and mammary glands of H5N1-infected dairy cows.

Both areas appear vulnerable to avian influenza and show indications that they may support viral replication.

Why mammary glands may be vulnerable

Like coronavirus, influenza is “inherently error-prone,” the authors write, adding that “infection of new host species gives the virus additional opportunities to replicate and subsequently mutate to be better adapt to novel hosts.”

“The spread of [influenza A virus] to tissues outside of the respiratory tract is rare,” the research team writes.

They suggest that the mildly acidic conditions in cow mammary glands, together with sialic-acid receptors, may make cattle more susceptible to H5N1 infection.

These ideas require further study, but the findings emphasise the “urgent need” to determine how avian influenza is infecting so many mammalian species.

They also serve as an important reminder that consumers should always drink pasteurised milk.

The study was published in Emerging Infectious Diseases.