Raw cheese made with milk from dairy cattle infected with bird flu can harbor infectious virus for months and may be a risk to public health, according to a new study from researchers at Cornell University that was funded by the US Food and Drug Administration.
Raw milk cheeses are those made with milk that hasn’t been heat-treated, or pasteurized, to kill germs.
Although federal law prohibits the sale of raw milk across state lines, sales of raw milk cheese are legal nationwide as long as it’s aged at least 60 days before landing on store shelves. This requirement, which has been in place since 1949, is thought to cut the risk of contamination, since it allows development of natural acids and enzymes, which were believed to kill off pathogens.
The new study shows that this aging process may not inactivate the H5N1 virus, however, and it underscores the risk of consuming raw or undercooked foods during the bird flu outbreak, which continues to infect dairy cattle, poultry and a growing number of other animal species.
The same group of researchers previously found that H5N1 virus remained infectious in refrigerated raw milk for up to eight weeks.
Dr. Diego Diel, who led the study, says he thinks the virus may be so stable in milk and cheese because it’s protected by the complex matrix of molecules around it.
“The protein and fat content in the cheese and milk provide a good environment for the virus to survive at refrigeration temperature,” said Diel, an associate professor of virology at Cornell.
Robert F. Kennedy Jr., secretary of the US Department of Health and Human Services, has said food doesn’t pose 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,” Kennedy told Fox News in an interview that aired Tuesday.
But that’s only partly true. Cats and other animals have been infected by raw cow’s milk and raw pet food, and there have been at least three confirmed human infections in which investigators were unable to determine the source of the person’s exposure to the H5N1 virus.
There have been no confirmed reports of people getting sick from eating or drinking bird-flu contaminated foods, including raw milk cheese, but dairy workers have been infected by getting splashed with raw milk in their faces or eyes.
It’s still not entirely clear whether humans can be infected by eating or drinking contaminated food, Diel said, noting that his study wasn’t really designed to answer that question.
“I do think it is possible. There is a risk of infection,” he said. “It obviously depends on the dose, how much of that contaminated product is ingested.”
It may also depend on the strain of virus in the milk or cheese. Diel and his team tested the stability of the B3.13 virus, which originally infected cattle in the US in March 2024. B3.13 has caused only mild disease in people. However, a new strain, D1.1, which has been associated with some severe human cases, has also crossed into cattle, further increasing the risk.
Assessing the risk of raw cheeses
For the study, the researchers made mini cheeses with milk they spiked with H5N1 virus. They made these cheeses at three pH levels: 6.6, 5.8 and the most acidic formula, 5.0.
The researchers then tested the cheeses over time to see whether any virus capable of infecting cells remained. They injected samples into fertilized chicken eggs, considered the gold standard test.
Levels of the virus remained high for the first seven days after the cheese was made and then dropped a bit in the cheeses produced at pH levels 6.6 and 5.8. But importantly, the virus remained infectious for the full two-month aging period, showing that it was remarkably stable in the cheese over a long period.
Those results were confirmed by some real-world examples. The researchers tested samples of raw milk cheese from a farm that had inadvertently produced them with milk from cows infected with bird flu. These findings also showed that levels of virus stayed high and remained infectious over the full two months of aging.
“Our study demonstrates that HPAI H5N1 virus exhibits remarkable stability throughout the cheese making process,” the authors wrote in their preprint study, posted ahead of peer review Friday.
However, the study suggested that making raw milk cheese more acidic may kill the virus and make the cheese safer to eat. No live virus was detected in the cheese produced at the lowest pH, 5.0. Prior studies have also shown that common pasteurization methods inactivate the virus.
Pasteurized products are safer
Experts said the new study was consistent with tests of the stability of the virus in milk.
“We have also observed that milk can change the pH needed to inactivate the virus,” said Dr. Seema Lakdawala, an associate professor of microbiology and immunology at Emory University, who has been studying the transmission of H5N1.
“We and others have repeatedly observed that milk will prevent the degradation of the virus on surfaces, and it increases the pH stability, such that a much lower pH than normal is needed to inactivate the virus,” said Lakdawala, who was not involved in the new study.
The FDA posted the findings of the study Friday along with preliminary results from its own ongoing sampling study of raw cheese.
The FDA sampling study, which was announced in December, has tested 110 cheese samples taken from store shelves across the country. So far, 96 of those samples were negative by polymerase chain reaction testing, or PCR, which detects live virus as well as inactive viral particles, indicating that those cheeses probably weren’t made with contaminated milk. The results from the remaining 14 samples are pending, the agency said.
“The FDA is continuing to work with Federal and state partners to address the ongoing outbreak of HPAI A (H5N1) in dairy cattle. The FDA has sampled a total of 464 pasteurized dairy products, including milk cheese butter and ice cream, all were negative for viable H5N1. In addition, multiple research studies have confirmed that pasteurization inactivates the virus,” the statement said.
Still, experts say the study is a good reminder of the importance of surveillance – and of not consuming raw milk products.
“This is another reason we should push to ensure there is no bird flu in milk products and only consume pasteurized dairy products,” Lakdawala said.