Cheese made from contaminated raw milk was found to contain infectious avian influenza virus, raising potential health concerns for consumers, according to new research. The study also found that no virus was detected in samples of raw milk cheese with high acidity. Feta cheese, which is naturally more acidic, served as an example of this safer variety.
The findings were published on October 8 in Nature Medicine.
Tracking How the H5N1 Virus Behaves in Cheese
"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 Center (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.
Virus Survived Twice as Long as Expected
Under Food and Drug Administration guidelines, raw milk cheese must be aged for at least 60 days at or above 35 degrees Fahrenheit. This process helps reduce moisture and destroy harmful bacteria that may be present in unpasteurized milk. However, the researchers detected infectious H5N1 virus after 120 days of aging at 39 degrees Fahrenheit, suggesting that the standard aging period may not fully eliminate viral contamination.
Acidity Plays a Key Role in Cheese Safety
A pH level below 7 is considered acidic. Most cheeses have a pH between 5.4 (as in cheddar) and 7 (as in camembert). Some, like feta, can be as low as 4.6 or even lower.
When the pH of raw milk cheese was between 5.8 and 6.6, the virus remained viable. No virus was detected in cheeses with a pH of 5 or below. The results highlight acidity as a key factor in inactivating the pathogen during cheesemaking.
Reducing Contamination Risks
According to Diel, steps to prevent contamination could include testing milk before cheesemaking and using only virus-free milk. Another option is to heat milk to sub-pasteurization temperatures, which could inactivate the virus while maintaining the raw milk cheese characteristics valued by artisanal producers.
Nicole Martin, study co-author and assistant research professor in dairy foods microbiology, emphasized the importance of this work: "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."
Animal Tests Shed Light on Transmission
The study also included an animal experiment using ferrets, which are highly susceptible to H5N1. The animals were fed contaminated raw milk and raw milk cheese from the study. Some ferrets that drank the raw milk became infected, but those that ate the raw milk cheese did not.
Diel suggested that the difference may relate to how the virus interacts with the body. The fluid consistency of milk may allow the virus to make greater contact with mucous membranes in the throat, while cheese likely provides less exposure time for infection to begin.
Testing Cheese Samples for the Virus
To further examine virus stability, the team created small 5-gram experimental cheeses in the lab using raw milk spiked with H5N1. They also analyzed commercial cheese samples submitted to Cornell by FDA officials who suspected contamination.
"All four samples of company-made cheddar that we received tested positive for H5N1," Diel said.
Understanding How Acidity Is Created
Cheese becomes acidic either through direct acidification or by adding lactic acid-producing bacteria that convert milk sugars into lactic acid. "This acid drops the pH of the milk and depending on how far this fermentation is allowed to proceed determines how low the pH goes," Martin said.
The researchers used direct acidification in their experiments, adding lactic acid to H5N1-spiked milk to produce cheeses with varying levels of acidity for testing.
Research Collaboration and Support
Mohammed Nooruzzaman, an assistant research professor in the Department of Population Medicine and Diagnostic Sciences (CVM), is the paper's first author. Co-authors include postdoctoral associate Pablo Sebastian Britto de Oliveira; research associate Salman Butt; Samuel D. Alcaine, associate professor of food science (CALS); and Stephen Walker at the FDA's Office of Dairy and Seafood Safety.
The study was supported by the FDA and the New York State Department of Agriculture and Markets.