H5N1 bird flu found to persist in raw milk and cheese, raising new food safety concerns

USA – Highly pathogenic avian influenza (H5N1) can remain infectious in raw milk and certain raw-milk cheeses for weeks or even months, according to new research from Cornell University, intensifying concerns about food safety, livestock health and the virus’s growing ability to infect mammals.

The findings, published late last year in the journal Nature Medicine, show that H5N1 remains viable in refrigerated raw milk for up to eight weeks and can persist in raw-milk cheeses for far longer than previously assumed. 

In controlled experiments, ferrets that consumed H5N1-contaminated raw milk became ill, while those fed raw-milk cheese did not.

However, researchers cautioned that the virus was still infectious in cheese under specific production conditions.

The study found that H5N1 survived for up to 120 days in raw-milk cheeses at pH 6.6 and 5.8. A pH of 5 inactivated the virus, but such acidity is not typical in most cheesemaking processes.

“The stability of the virus in raw-milk cheeses under these conditions is remarkable, as the virus survived much longer than the minimum aging period of 60 days required by FDA for raw milk cheeses,” said Dr. Diego Diel, director of Cornell University’s Animal Health Diagnostic Center, in an email.

Cornell researchers were among the first to detect the infectious H5N1 virus in milk from infected cows and participated in the initial identification of the virus in U.S. dairy cattle. 

According to Diel, the team began studying raw and pasteurised dairy products immediately after the virus was detected in milk.

Researchers are now working to determine the minimum level of acidification required to reliably inactivate H5N1, raising the possibility that small changes in cheesemaking could reduce infection risks. 

The findings point to a potential need for increased testing or treatment of raw dairy products, particularly those intended for human consumption.

Growing concern over cattle infections

The research comes as scientists intensify efforts to understand how H5N1, commonly known as bird flu, is adapting to infect dairy cattle in the United States. 

The virus was first identified in U.S. dairy herds in March 2024, marking a significant departure from historical patterns, as influenza viruses have not traditionally caused outbreaks in cattle.

Researchers believe the emergence reflects H5N1’s ability to jump from birds to mammals, with recent variants appearing better adapted to infect bovine cells and tissues than earlier strains. 

Current data suggest the strain circulating in U.S. dairy cattle is among the most capable of infecting cow cells identified to date.

“Because the virus continues to circulate among multiple hosts, producers should closely monitor for any clinical signs that may indicate infection,” Diel said.

“Early detection enables timely implementation of control strategies to limit spread, minimise economic losses and reduce public health risks.”

While no sustained human transmission has been detected, scientists warn that influenza viruses evolve rapidly, and increased adaptation to mammals could theoretically raise the risk of further spillover. 

Experts say ongoing surveillance, genomic analysis and functional testing are critical to protecting livestock, ensuring food safety and strengthening global pandemic preparedness.

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