Milk glands in many mammals have receptors for H5N1 ^{CEFR B2}

A continuing outbreak of highly pathogenic avian influenza (H5N1) has affected more than 184 million domestic poultry since 2022 and, after spreading to dairy cattle in spring 2024, has reached more than 1,000 milking cow herds. Researchers have asked whether other mammals could also host the virus. A new study in the Journal of Dairy Science examined mammary gland tissue from pigs, sheep, goats, beef cattle, alpacas and humans. The research team included scientists from the ISU College of Veterinary Medicine and the USDA’s National Animal Disease Center in Ames.

All mammary tissues examined had sialic acid receptors, a sugar on cell surfaces that influenza viruses use to attach to and enter cells. Because these receptors are preferred by both avian influenza and seasonal human influenza, the authors warn there is concern that different influenza types could mix and adapt across species. A previous study by many of the same researchers found that dairy cattle udders have high sialic acid levels, which helped explain rapid spread among dairy herds.

Only a few sporadic H5N1 infections have been reported so far in the animals examined, and coauthor Todd Bell noted that limited testing leaves gaps: "If we don’t look, we don’t know." In infected dairy herds, sick cows can produce milk that contains the virus. The USDA is conducting nationwide surveillance testing of raw cow milk samples, and pasteurization kills influenza viruses, so store-bought milk is considered safe. The researchers say concerns about raw milk should extend to other mammalian livestock and that heightened surveillance and efforts to limit spread are important to reduce the risk of further dangerous adaptations. The current outbreak has produced 71 confirmed human infections and two deaths, and H5N1 has had a past human fatality rate of around 50%.