Researchers have found that the H5N1 avian influenza virus can survive in raw milk cheese made from contaminated milk, even after the FDA's required 60-day aging period. Highly acidic cheeses, such as feta, showed no detectable virus, highlighting acidity's role in safety. The study, published in Nature Medicine, also indicates that while drinking contaminated milk can infect animals, eating the cheese does not.
A new study reveals potential risks associated with raw milk cheese contaminated by the H5N1 bird flu virus. Led by Diego Diel, a professor of virology at Cornell University's College of Veterinary Medicine, the research examined the virus's stability in cheese products. "In this study, we were specifically looking at the stability or persistence of highly pathogenic avian influenza H5N1 in raw milk cheese products," Diel explained. The work built on prior findings that infected cows shed high levels of the virus in milk and that it survives for extended periods in refrigerated raw milk.
Under FDA guidelines, raw milk cheese must age for at least 60 days at temperatures of 35 degrees Fahrenheit or higher to reduce moisture and harmful bacteria. However, the team detected infectious H5N1 virus in experimental cheeses after 120 days of aging at 39 degrees Fahrenheit, twice the standard period. They created 5-gram test cheeses using raw milk spiked with the virus and analyzed commercial samples, including four cheddar cheeses from a company that all tested positive for H5N1.
Acidity emerged as a critical factor. Cheeses with a pH between 5.8 and 6.6 retained viable virus, while those at pH 5 or below, like feta at around 4.6, showed none. This acidity can result from direct addition of lactic acid or bacterial fermentation converting milk sugars.
Animal experiments using ferrets, which are susceptible to H5N1, provided further insights. Ferrets that drank contaminated raw milk became infected, but those fed the raw milk cheese did not. Diel suggested this difference might stem from milk's fluid allowing greater viral contact with throat mucous membranes, unlike the solid cheese.
Co-author Nicole Martin, an assistant research professor in dairy foods microbiology, stressed the study's value: "The work we've done on H5N1 is critical to providing practical, timely, data-driven knowledge and recommendations to the dairy industry in the face of this outbreak that has affected a large proportion of the milk supply in the U.S., and it allows raw milk cheese makers to reduce risk."
To mitigate risks, Diel recommended testing milk for the virus before cheesemaking or heating it to sub-pasteurization levels to inactivate the pathogen while preserving artisanal qualities. The findings were published on October 8, 2025, in Nature Medicine, with support from the FDA and the New York State Department of Agriculture and Markets. First author Mohammed Nooruzzaman and other collaborators from Cornell and the FDA contributed to the effort.