Bird Flu In Your Fridge? H5N1 Found In Raw Milk Cheese

New research reveals that cheeses made from raw milk contaminated with the H5N1 avian influenza virus can harbour infectious particles, raising alarms about potential transmission to consumers through unpasteurised dairy products. 

Published on October 8 in Nature Medicine, the Cornell University-led study found the virus persists throughout the cheese-making process and even after 120 days of ageing, far beyond the US FDA's required 60-day minimum, depending on the milk's acidity levels. However, highly acidic varieties, such as feta, showed no detectable virus, suggesting that lower pH environments may neutralise the threat effectively.

"In this study, we were specifically looking at the stability or persistence of highly pathogenic avian influenza H5N1 in raw milk cheese products," said senior author Diego Diel, professor of virology in the Department of Population Medicine and Diagnostic Sciences and director of the Virology Laboratory at the Animal Health Diagnostic Centre (AHDC), all in the College of Veterinary Medicine (CVM).

"This research was initiated due to previous work demonstrating high levels of virus shedding in milk from infected cows and the fact that we have previously shown that the virus survives in refrigerated raw milk for extended periods of time," he said.

A new study has found that raw milk cheese made from H5N1-contaminated milk can retain active bird flu virus even after 120 days of ageing, double the FDA's 60-day requirement. The virus survived in cheeses with a pH between 5.8 and 6.6, such as camembert, but not in more acidic varieties like feta, which has a pH of 4.6 or lower. The standard 60-day ageing process, intended to reduce pathogens, may not be enough for avian influenza. Researchers suggest preventive steps like testing milk before use or applying mild heat treatment below pasteurisation levels, which could inactivate the virus while preserving raw milk cheese characteristics. The findings raise concerns about food safety and current ageing standards.

"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 cheesemakers to reduce risk," said study co-author Nicole Martin, assistant research professor in dairy foods microbiology and director of the Milk Quality Improvement Program in the Department of Food Science, in the College of Agriculture and Life Sciences.