New Research Suggests Bird Flu Can Survive in Some Cheeses

The current outbreak of avian flu (HPAI H5N1) virus is affecting more than just birds; dairy herds across the United States are also becoming infected. If a cow becomes sick with avian flu, its milk supply can be contaminated with the virus. While it’s been confirmed that the virus can survive in raw (unpasteurized) dairy products like milk, yogurt, and soft cheeses, a new study from Cornell University suggests the virus can even survive in aged cheeses.

So, what does this mean for cheese production in the U.S. going forward and, more importantly, how worried should you be about bird flu contamination in your favorite cheeses?

What does the science say?

Scientists at Cornell University noted there was a lack of research around whether or not the avian flu is also destroyed by the cheese-aging process. Their research, which has not yet been peer-reviewed, aims to answer that question.  

In the study—which was funded by the Food and Drug Administration (FDA), as well as New York State—scientists purposefully spiked raw milk with active virus, then used it to make cheese at three different pH levels. The study also tested commercial raw-milk cheese that was confirmed to have been contaminated with the virus. 

Scientists found that, in most cases, the virus was able to survive in cheese that had been aged for 60 days. Currently, U.S. law requires that cheeses made from raw milk be aged for a minimum of 60 days in order to kill off dangerous pathogens like listeria, E.coli, and salmonella. Which means the current aging protocols for other pathogens seem to not apply to this strain of bird flu.

How are cows getting avian flu in the first place?

“Over the past year, almost a thousand dairy herds in the U.S. have been affected, across 17 states,” says Dr. Peter Rabinowitz, professor of environmental and occupied health sciences and director of the University of Washington’s Center for One Health Research. “Dairy farmers are concerned, but fortunately there is not a major impact on production.” For contrast, over 100 million egg-laying birds have been impacted, according to data from the U.S. Department of Agriculture (USDA).

At this point, it’s not entirely clear how the virus is spreading among dairy cows. “Scientists are still researching how avian influenza is transmitted between animals, the environment, and humans,” explains Dr. Rabinowitz. “It does appear that cows have been infected by birds, but also that cow to cow transmission has occurred.” He also mentions that the virus could potentially be spread through contaminated milking equipment or other surfaces.

Ok, so what cheeses are at risk?

“Raw milk and raw milk dairy products (like soft cheese, yogurt, etc.) can contain pathogens that may make you sick, including salmonella, listeria, E. coli, or even avian flu,” says Emily Hovis, assistant teaching professor at the University of Washington’s Department of Environmental & Occupational Health Sciences and a former retail food safety specialist. Luckily, the process of pasteurization kills these harmful germs and makes these dairy products safe to consume.

According to the Centers for Disease Control and Prevention (CDC), 99 percent of milk sold in the U.S. is pasteurized. What you may not realize, however, is that certain aged cheeses are traditionally made from raw milk—especially those imported from Europe—including English cheddar, gruyère, and Parmigiano Reggiano. These cheeses don’t carry the same risk that soft cheeses do. “60-day aged raw milk cheeses are generally considered safe by the FDA,” Hovis says, “the aging process causes pathogenic bacteria to be destroyed.”

As cheese ages, sugars in the milk (lactose) are converted into lactic acid. Hovis explains that this acidity, combined with added salt, and loss of moisture as the cheese ages, creates an environment that bacteria are less able to grow and survive in. 

However, the Cornell University study found that avian flu can withstand these conditions, even if other dangerous pathogens, like E. coli and listeria, cannot.

What does this mean for consumers (and cheese-lovers)?

“This study has not yet been peer reviewed, so I would reference it with caution,” says Hovis. “That said, the findings are concerning and may show that 60-day aged raw milk cheeses may present a public health risk.” Hovis emphasizes the need for more research before we can better understand the risk of avian flu in aged raw milk cheeses. 

Fortunately, Hovis also notes that, as of yet, there have been no known cases of a human contracting avian flu through eating contaminated foods, like eggs or cheese.

While pasteurized dairy products are always the safest option, some findings from the study could offer solutions for making aged raw milk cheeses safer. Interestingly, there was no virus detected in the cheese with the lowest pH. The study also found that “thermization” or heat-treating raw milk to temperatures above 129°F (54°C) for at least 15 minutes is an effective way to inactivate the virus. (Note that this process doesn’t eliminate bacteria in the same way that pasteurization does.)

The FDA is currently testing samples of 60-day raw-milk aged cheeses for avian influenza. While the study is not yet complete, so far, none of the samples have tested positive for the virus. The final results will be published later this spring and will give more insight to the risk that consumers may be facing. 

In the meantime, there are plenty of options when it comes to buying pasteurized cheeses. When in doubt, be sure to check the label for “pasteurized milk”.