We have learned a lot about how cows react to highly pathogenic avian influenza (H5N1) since it was first identified on U.S. dairy farms in late March. Now, researchers from Iowa State University have uncovered some answers around how cows are being infected with the disease in the first place.

A major factor in any disease spread is the presence of pathogen receptors. In a study published in the Centers for Disease Control and Prevention’s (CDC) journal Emerging Infectious Diseases, the team discusses how influenza A viruses like H5N1 use a host’s sialic acids as receptors for initial attachment and entry into cells. Sialic acid is a sugar molecule found on the surface of some animal cells. This binding has been observed in avian species, humans, and swine with various influenza A viruses.

The main identifiable symptoms of H5N1 in dairy cattle include reduced milk production and milk that appears thick and almost colostrum-like. H5N1 has also been detected in raw, unpasteurized milk. Still, there has been little research into the presence of viruses in the udder or milk in dairy cows; mastitis pathogens of concern are often bacteria. So, the Iowa State team decided to look into the unusual infection of the mammary gland by H5N1.

What they found was that the udder is an excellent source of sialic acids for the virus to attach to. Without these acids, it would be unlikely and difficult for a flu virus to enter the host. The presence of these receptors highlights how the viruses can adapt and spread between a range of species.

Knowing that the udder is a prime way for the virus to shed, replicate, and enter new hosts underscores the importance of proper milking procedures, thorough washing of equipment, the discarding of infected milk, and pasteurization of raw milk, the researchers said. Pasteurization has repeatedly been shown to inactivate virus particles from milk. It is important to reiterate that the commercial milk supply remains safe.

However, these are important findings as we move forward with H5N1 as a possible concern for dairy cattle. Understanding how the virus infiltrates the bovine body is crucial to developing effective prevention and control methods.

So far, there have been 11 reported cases of a human contracting H5N1 dating back to 2022 when the disease was only observed in birds. Four of those cases were in people who have interacted with infected cattle, while the other seven were in poultry workers. The research team noted that humans have some sialic acid receptors deep in the lungs, and the infections observed are likely due to inhaling large numbers of the virus. However, the symptoms are generally mild, and at least one virologist has postulated that the current H5N1 strain is not well suited for human infection.