Bird Flu Virus May Survive Aging Process in Raw Milk Cheese

MONDAY, March 17, 2025 (HealthDay News) — There's a new reason to steer clear of raw cheese: New research shows it can harbor the infectious bird flu virus for months.

"There is a risk of infection," lead study author Dr. Diego Diel, an associate professor at Cornell University, told CNN. "It obviously depends on the dose, how much of that contaminated product is ingested."

Though it's illegal to sell raw milk across state lines, since 1949 it's been legal to sell raw milk cheese provided it has been aged for at least 60 days. That period allows for development of natural acids and enzymes that have been thought to destroy pathogens.

The new research — funded by the U.S. Food and Drug Administration (FDA) — demonstrates that this aging alone may not inactivate the H5N1 bird flu virus. The same team had previously shown that the H5N1 virus stayed infectious for up to eight weeks in refrigerated raw milk.

"The protein and fat content in the cheese and milk provide a good environment for the virus to survive at refrigeration temperature," Diel said. 

For the study, his team made mini cheeses that they laced with the H5N1 virus. The cheeses were produced at three pH levels, from the least acidic, 6.6, to the most acidic, 5.0. 

Researchers then injected samples of the cheeses into fertilized chicken eggs to see how long any virus able of causing infection remained. 

For the first seven days, levels of virus remained high, then fell in the two least acidic cheeses. Significantly, levels remained infectious for the full two-month aging period.

"Our study demonstrates that HPA1 H5N1 virus exhibits remarkable stability throughout the cheese making process," the authors wrote, suggesting that making raw milk cheese more acidic may make it safer to eat.

No live virus was detected in the most acidic cheese (pH 5.0). Other research has shown that pasteurization inactivates the virus.

Dr. Seema Lakdawala, an associate professor of microbiology and immunology at Emory University who also studies H5N1 transmission, reviewed the findings.

"We have also observed that milk can change the pH needed to inactivate the virus," Lakdawala told CNN, adding that "a much lower pH than is normal is needed to inactivate the virus."

Despite the new findings, the head of the U.S. health system, has said food doesn't present a bird flu risk.

"The disease is not passed through food so you cannot get it -- as far as we know, you cannot get it from an egg or milk or meat from an infected animal," Robert F. Kennedy Jr., secretary of the U.S. Department of Health and Human Services, told Fox News last week.

While there are no confirmed cases of people being sickened from eating or drinking virus-tained foods, including raw milk cheese, dairy workers have been infected when raw milk splashed their faces or eyes.

Diel told CNN  it's not entirely clear whether people can be infected by consuming contaminated food and that his study was not designed to answer that question.

The findings are posted on the preprint server bioRxiv. Until it is published in a peer-reviewed journal, findings should be considered preliminary.

The FDA also posted the findings on Friday along with early results from an ongoing sampling study of raw cheese announced in December.

Of 110 cheese samples taken from stores across the country, 96 tested negative for live virus as well as inactive viral particles. The agency said that indicates they were probably not made with contaminated milk. Results of 14 samples are pending.

"The FDA is confident that pasteurization is effective at inactivating H5N1, and that the commercial, pasteurized milk supply is safe," the agency said in a statement. 

Checking the product label may help -- but it's not foolproof, however.

"While there is no express Federal requirement for the labeling of cheeses to disclose on the ingredient list whether the milk used in manufacture was raw or pasteurized, many cheeses do disclose on the ingredient list (or elsewhere on the product label) if the milk used is pasteurized or raw," the statement said. "The FDA is not aware of any H5N1 illnesses to date from the consumption of aged raw milk cheese products."

More information

There's more about H5N1 bird flu at the Cleveland Clinic.

SOURCES: CNN, March 14, 2025; U.S. Food and Drug Administration, news release, March 14, 2025