A new study has found that avian influenza virus, also known as bird flu, can survive in certain types of raw milk cheese made from contaminated milk, raising potential health concerns for consumers.
The research, published in Nature Medicine, highlights how the virus can persist in cheese products long after they have been aged.
The study was led by Professor Diego Diel, a virologist at Cornell University’s College of Veterinary Medicine. His team wanted to understand how long highly pathogenic avian influenza H5N1 could remain infectious in raw milk cheese.
Previous research had already shown that infected cows shed high levels of the virus in their milk and that the virus can survive for long periods in refrigerated raw milk.
Federal regulations in the United States require raw milk cheeses to be aged for at least 60 days at or above 35°F before sale.
This rule is intended to reduce harmful bacteria. However, Diel’s team found that H5N1 virus remained infectious even after 120 days of aging at 39°F, showing that the standard practice may not be enough to eliminate this virus.
Cheese acidity turned out to be a key factor. Cheeses with a pH between 5.8 and 6.6, such as cheddar or camembert, still contained viable virus when made from contaminated milk.
In contrast, highly acidic cheeses with a pH of 5 or less, such as feta, showed no detectable virus.
Acidity in cheese is created by adding cultures of lactic acid bacteria or directly adding acid, which lowers the pH and can stop pathogens from surviving.
Possible solutions to reduce risk include testing raw milk for H5N1 before cheese production, or heating milk at lower-than-pasteurization temperatures to kill the virus while keeping some of the qualities valued in raw milk cheeses.
“This work is critical for giving the dairy industry timely and practical knowledge to lower risks in the face of the current outbreak,” said Nicole Martin, co-author of the study and director of Cornell’s Milk Quality Improvement Program.
The researchers also conducted animal tests with ferrets, which are highly susceptible to bird flu. Some animals became infected after drinking contaminated raw milk, but none became infected after eating contaminated raw milk cheese. The team suggests that the liquid milk may provide more direct contact with throat and nasal tissues, while cheese has less opportunity to interact with these cells.
In addition, the researchers developed an experimental model to produce small batches of cheese using raw milk deliberately spiked with H5N1. They also tested cheddar samples sent by federal regulators, which were suspected of being made with contaminated raw milk. All four commercial cheddar samples tested positive for H5N1.
The findings highlight the need for stricter monitoring of raw milk products and suggest that acidity may play a protective role in making some cheeses safer.
However, the presence of infectious virus in commonly consumed cheeses underscores the importance of continued vigilance during outbreaks of avian influenza.
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Source: Cornell University.
