A bold warning about a hidden risk in our food system: avian influenza is not just a bird problem—it can live in the mammary glands of several livestock and even humans, with potential for cross-species transmission. This ongoing outbreak has ravaged over 184 million domestic poultry since 2022 and, after jumping to dairy cattle in spring 2024, has affected more than 1,000 milking cow herds.
A new Iowa State University study shows that the mammary glands of additional production animals—pigs, sheep, goats, beef cattle, and alpacas—also carry high levels of sialic acids. These sugar molecules on cell surfaces act like docking stations that influenza viruses use to attach and invade host cells. In other words, these animals’ udders and mammary tissues are biologically capable of harboring avian influenza, raising questions about how easily the virus could spread within farms and potentially to humans.
The researchers’ main goal was to assess whether these animals could transmit the virus to each other and to people, and the findings suggest there is a potential pathway for transmission. Rahu l Nelli, the study’s lead author and a research assistant professor at the Veterinary Diagnostic and Production Animal Medicine department, explains that sialic acid serves as the entry point for the virus.
A related study last year found that dairy cattle udders exhibit high sialic acid levels, which helped explain the rapid spread of H5N1 among dairy herds. The current study, published November 27 in the Journal of Dairy Science, involved a collaboration between Iowa State University’s College of Veterinary Medicine and the USDA National Animal Disease Center in Ames. It also identified these same receptors in human mammary tissue, underscoring a potential cross-species risk.
While only a few sporadic H5N1 infections were identified in the animals studied, officials note that testing isn’t widespread for all species, which limits how precisely we can gauge risk. Dr. Todd Bell, a veterinary pathologist and co-author, emphasizes: if we don’t look, we won’t know what’s happening.
In dairy herds, H5N1 infections can cause sick cows to produce milk contaminated with the virus, prompting nationwide surveillance of raw milk by the USDA. Pasteurization kills influenza viruses, so commercially sold milk remains safe, but Nelli warns that raw milk from other animals is consumed by some people, compounding the public health concern.
The presence of the virus in milk from infected cattle likely contributes to the virus’s spread and raises the risk of human exposure. Unlike respiratory transmission, which requires close contact, milk is distributed widely, potentially carrying the virus into communities.
All mammary tissues examined displayed sialic acid receptors preferred by both avian and seasonal human influenza strains, which raises the worrying possibility that different influenza viruses could mingle and adapt across species. H5N1 has shown a historically high fatality rate in humans, though current confirmed human cases around this outbreak have resulted in a relatively small number of deaths so far. Experts stress the importance of staying ahead of the virus to prevent further replication and evolution into something more dangerous.
Source:
Journal reference:
Nelli, R. K., et al. (2025). Exploring influenza A virus receptor distribution in the lactating mammary gland of domesticated livestock and in human breast tissue. Journal of Dairy Science. doi: 10.3168/jds.2025-26950.
