H5N1 Bird Flu Virus Found Infectious in Raw Milk Cheese for Months, Posing Public Health Risks

A recent study from Cornell University, funded by the U.S. Food and Drug Administration (FDA), has revealed alarming findings regarding the H5N1 bird flu virus in raw milk cheese. According to the study, raw cheese made with milk from dairy cattle infected with bird flu can harbor the infectious virus for months, raising significant concerns about public health.

Understanding Raw Milk Cheese and Its Regulations

Raw milk cheese is made from milk that has not been heat-treated or pasteurized to kill harmful germs. While federal law prohibits the sale of raw milk across state lines, raw milk cheese is legally sold nationwide, provided it is aged for at least 60 days. This regulation, established in 1949, was based on the assumption that natural acids and enzymes developed during aging would kill pathogens. However, the new research indicates that this process may not be sufficient to inactivate the H5N1 virus.

The Study’s Findings: H5N1 Virus Stability in Cheese

The Cornell study highlights the risk of consuming raw or undercooked foods during the ongoing bird flu outbreak. Researchers had previously found that H5N1 remained infectious in refrigerated raw milk for up to eight weeks. Now, they have demonstrated that the virus can persist in raw milk cheese for at least two months.

Dr. Diego Diel, the study’s lead researcher, suggests that the virus remains stable in milk and cheese due to the protective molecular composition of these dairy products. The protein and fat content create an environment where the virus can survive, even under refrigeration.

Public Health Concerns and Official Statements

Despite these findings, some officials have downplayed the risks of transmission through food. U.S. Health and Human Services Secretary Xavier Becerra stated that “the disease is not passed through food,” indicating that there is no known evidence of transmission via milk, eggs, or meat. However, studies suggest otherwise. Cases of infected cats and other animals that consumed raw cow’s milk or pet food have been documented, and there have been at least three confirmed human infections where the source of exposure was unclear.

Although no confirmed cases of human illness from consuming bird flu-contaminated foods have been reported, dairy workers have contracted the virus after exposure to raw milk splashes. This raises questions about whether ingestion of contaminated dairy products could pose a risk to humans. Dr. Diel acknowledges that while his study was not designed to confirm human infection through consumption, the possibility remains, particularly depending on the virus strain and the ingested dose.

The Impact of Viral Strains on Transmission Risk

The study examined the stability of the B3.13 strain, which first infected U.S. cattle in March 2024. While this strain has caused only mild illness in humans, a newer strain, D1.1, associated with severe human cases, has also spread to cattle, increasing potential risks. Understanding how different strains behave in dairy products is crucial for assessing public health risks.

Experimental Approach: Testing Virus Stability in Cheese

To analyze the persistence of H5N1 in cheese, researchers created mini cheeses using milk spiked with the virus. The cheese was produced at three different pH levels—6.6, 5.8, and 5.0—to simulate varying acidity levels. They then tested the cheese over time using a gold-standard method: injecting samples into fertilized chicken eggs to detect infectious viruses.

Findings showed that virus levels remained high for the first seven days after production, then slightly decreased in cheeses at pH levels of 6.6 and 5.8. However, the virus remained infectious throughout the full two-month aging period. Interestingly, cheese made at the lowest pH level (5.0) showed no detectable live virus, suggesting that higher acidity may help neutralize the virus.

The study also confirmed its laboratory findings through real-world testing. Researchers analyzed raw milk cheese from a farm that unknowingly used milk from H5N1-infected cows. The results mirrored laboratory experiments: the virus remained stable and infectious throughout the two-month aging period.

The Role of Pasteurization in Mitigating Risks

Experts emphasize that pasteurization remains the most effective method for eliminating H5N1 and other pathogens from dairy products. Dr. Seema Lakdawala, a microbiologist at Emory University, noted that milk’s composition can alter the pH needed to inactivate the virus. Her research, along with others’, has demonstrated that milk prevents the degradation of viruses and enhances their stability. This highlights the importance of pasteurization in ensuring the safety of dairy products.

FDA’s Response and Surveillance Efforts

In response to the ongoing outbreak, the FDA has intensified monitoring efforts. The agency recently posted preliminary results from its sampling study of raw cheese, launched in December. Among 110 cheese samples tested from store shelves nationwide, 96 tested negative for H5N1 using polymerase chain reaction (PCR) testing, while results from the remaining 14 are pending. Additionally, tests on 464 pasteurized dairy products—including milk, cheese, butter, and ice cream—confirmed the absence of viable H5N1 virus, reinforcing the effectiveness of pasteurization.

Why This Study Matters: Public Health Implications

The findings underscore the potential dangers of consuming raw milk products, particularly during a bird flu outbreak. While there is no confirmed evidence of human transmission through food, the virus’s persistence in raw milk cheese suggests that the risk cannot be dismissed. These findings reinforce the importance of regulatory measures, consumer awareness, and continued surveillance to protect public health.

The Need for Caution and Further Research

This study serves as a stark reminder of the risks associated with raw milk cheese, especially in the context of emerging infectious diseases like H5N1 bird flu. While more research is needed to determine the exact risk of human infection from consuming contaminated dairy products, the safest approach remains avoiding raw milk products. Regulatory agencies, researchers, and the public must remain vigilant in monitoring and mitigating potential threats to food safety and public health.