Stability and Infectiousness of H5N1 Bird Flu Virus in Raw Milk Cheese Lasts for Months, Reveals Research
Rewritten Article:
Chewing on raw cheese made from contaminated cow's milk could potentially harbor the dangerous bird flu virus for months, posing a potential threat to public health, according to a recent study conducted by researchers at Cornell University and funded by the US Food and Drug Administration (FDA).
Raw milk cheeses are typically made without heat treatment, also known as pasteurization, which kills bacteria and other germs. Although shipping raw milk across state lines is illegal, the sale of raw milk cheese is federally allowed as long as it is aged for a minimum of 60 days before hitting the market. This aging period is believed to lessen the risk of contamination by facilitating the development of natural acids and enzymes, which were once thought to eliminate pathogens.
However, this study indicates that the aging process might not eliminate the H5N1 bird flu virus. This discovery emphasizes the risk of consuming raw or undercooked foods during bird flu outbreaks, as dairy cattle, poultry, and an expanding array of other animal species continue to be infected.
The same research team had previously found that the H5N1 virus remained infectious in refrigerated raw milk for up to eight weeks. Dr. Diego Diel, the study's lead researcher, suggests that the virus might persist in milk and cheese due to the protective shield formed by the complex matrix of molecules surrounding it.
"The protein and fat content in cheese and milk offer a perfect environment for the virus to survive at refrigeration temperature," said Diel, an associate professor of virology at Cornell.
The risk of humans acquiring bird flu from contaminated food is often downplayed, with Robert F. Kennedy Jr., secretary of the US Department of Health and Human Services, asserting that food doesn't pose a bird flu risk. However, studies have shown that cats, along with other animals, have been infected through raw cow's milk and raw pet food. There have also been at least three confirmed human infections where the source of exposure to the H5N1 virus could not be determined.
Though no reports of people falling ill from consuming bird-flu contaminated foods, including raw milk cheese, have been confirmed, dairy workers have contracted the virus after being splashed with raw milk in their eyes or faces. Although it's not yet clear whether humans can become infected by eating or drinking contaminated food, Diel suspects that the risk exists.
"I believe it is possible. There is a risk of infection," he said. "It depends on the dose - how much contaminated product is ingested."
The strain of the virus used in the study was the B3.13, which originated from cattle infected in the US in March 2024. While the B3.13 strain has only caused mild infections in humans, a new strain, D1.1, has crossed into cattle, further increasing the risk.
To assess the risk of raw cheeses, the researchers produced mini-cheeses using milk contaminated with the H5N1 virus at three pH levels: 6.6, 5.8, and the most acidic formula, 5.0. The researchers then tested the cheeses over time to determine if any virus capable of infecting cells remained. Samples were injected into fertilized chicken eggs, widely considered the gold standard test.
After seven days, levels of the virus remained high in all cheeses but dropped slightly in the cheeses produced at pH levels 6.6 and 5.8. However, the virus remained infectious for the full two-month aging period, demonstrating impressive stability in the cheese over a long period.
Real-world samples of raw milk cheese from a farm that unintentionally produced them with infected cow's milk were also tested, yielding similar results. The findings suggest that making raw milk cheese more acidic may eliminate 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 research has also shown that common pasteurization methods inactivate the virus.
"Our study shows that the HPAI H5N1 virus exhibits remarkable resilience throughout the cheese-making process," the authors wrote in their preprint study, posted ahead of peer review.
Nevertheless, the study suggests that making raw milk cheese more acidic may destroy the virus and render the cheese safer to consume. Despite the ongoing bird flu outbreak in dairy cattle, the FDA has tested 464 pasteurized dairy products, including milk, cheese, butter, and ice cream, all of which tested negative for the live H5N1 virus. The FDA encourages the consumption of pasteurized dairy products in light of the study's findings.
- The aging process of raw milk cheese might not eradicate the H5N1 bird flu virus, as suggested by a recent study, potentially increasing the risk of public health during bird flu outbreaks.
- The protein and fat content in cheese and milk provide an ideal environment for the H5N1 bird flu virus to survive at refrigeration temperature, according to the study's lead researcher, Dr. Diego Diel.
- The researchers found that making raw milk cheese more acidic may destroy the H5N1 virus and make the cheese safer to consume, based on their experiments with cheeses of varying pH levels.
