Researchers have discovered that the avian influenza (H5N1) virus can persist in certain raw milk cheeses, even after the 60-day aging process mandated by the U.S. Food and Drug Administration (FDA). The findings raise new food safety concerns, particularly for unpasteurized dairy products.

The study, published in Nature Medicine on October 8, shows that while H5N1 can survive for up to 120 days in some raw milk cheeses, highly acidic varieties like feta effectively neutralize the virus. This suggests that acidity may serve as a natural defense against contamination.

Virus Outlasts the FDA's 60-Day Aging Requirement

FDA guidelines require raw milk cheese to be aged for at least 60 days at 35°F (1.6°C) or higher to reduce harmful bacteria. However, researchers found that infectious H5N1 remained detectable even after twice that period - up to 120 days - when aged at 39°F (4°C).

“Previous studies showed that infected cows shed large amounts of the virus in milk,” said lead researcher Diego Diel, professor of virology at Cornell University's College of Veterinary Medicine.“This prompted us to test how long the virus could survive in raw milk cheese.”

Acidity Makes All the Difference

• The team found that virus survival depended heavily on pH levels.
• Cheeses with a pH between 5.8 and 6.6 retained infectious virus.
• Cheeses with a pH below 5.0 - such as feta - showed no trace of the virus.

Since acidity increases during fermentation, cheeses that undergo more extensive bacterial activity are naturally safer.“This acidification process drops the pH and makes the environment hostile to the virus,” explained co-author Nicole Martin, assistant research professor in dairy microbiology.

Ferret Study Reveals How the Virus Infects

To explore infection risks, the researchers tested contaminated cheese and milk on ferrets - a standard animal model for flu transmission. Ferrets that drank contaminated raw milk became infected, but those that ate cheese did not.

This difference may be due to how the virus interacts with mucous membranes.“Liquid milk likely allows more contact with throat tissues than cheese does,” Diel said.

Preventing Contamination in Dairy Production

The researchers recommend preventive measures such as:

• Testing milk before cheesemaking
• Using virus-free milk sources
• Slightly heating (sub-pasteurizing) milk to deactivate pathogens without changing raw cheese flavor

Martin added,“This data gives the dairy industry practical steps to reduce risk during the ongoing H5N1 outbreak.”

Aging Isn't Enough - Acidity Is Key

The study concludes that standard aging rules alone may not guarantee safety from H5N1. Instead, acidity appears to be the deciding factor in whether the virus survives.

“Understanding how acidity forms and how long it lasts during cheesemaking could help artisans design safer processes,” Diel noted.

The research was a collaboration between Cornell University, the FDA, and the New York State Department of Agriculture and Markets, supported by federal and state grants.

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