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Wisconsin study supports belief that milk pasteurization kills avian flu virus

University of Wisconsin-Madison research finds pasteurization kills 99.99 percent of avian influenza

milking cows
Toby Talbot/AP Photo

Wisconsin researchers have provided new evidence that pasteurization of milk is effective at killing the avian flu virus.

Scientists at the University of Wisconsin-Madison and the state’s Veterinary Diagnostic Laboratory used milk samples collected from infected cows on farms in New Mexico and Kansas. 

Simulating the pasteurization process used by the dairy industry, the team found the level of highly-pathogenic avian influenza virus, called H5N1, was reduced by 99.99 percent.

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From the first discovery of H5N1 in dairy cows in March, federal and state officials have said they believe the commercial milk supply is safe because of pasteurization’s effectiveness at killing similar viruses. But there has not been a study of the H5N1 virus specifically in milk or other dairy products.

Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, said the newly-published work offers further support for commercial milk’s safety, even though it’s not an exact replica of what the industry uses.

“It was good preliminary work that we can then put into larger industrial scale testing options, so at least we had a general idea of where we were going and what the efficacy would be,” he said.

The study also heated milk samples to a higher temperature for a shorter period of time, a process that Poulsen said is below federal guidelines for milk. This method failed to completely inactivate the virus. The researchers also found that refrigerating raw milk did not greatly reduce the level of active virus.

Poulsen said these findings will hopefully inform dairy processors making other products like cheese. He said there are different pasteurization temperatures and protocols for each dairy item, and some are even unique to each company. 

“For instance, processing mozzarella cheese for frozen pizza is very different than fresh pizza, or even the little mozzarella balls that you can get for a caprese salad,” he said. “They’re all very different. So that’s why this is great preliminary information.”

Researchers plan further study as virus continues to spread

Poulsen said the team, which was also led by UW-Madison professor Yoshihiro Kawaoka, was also able to identify what parts of milk carried the virus thanks to researchers at the university’s Center for Dairy Research. He said the distinction was critical in addressing previous cattle viruses like foot-and-mouth disease.

The latest study found that the virus was shed in both the cream, or fat component, and the skim, which contains water and sugars.

“That was another really important part, because some dairy components … they kind of take the milk apart, they process it in different ways and then they put it back together,” Poulsen said.

He said the team hopes to continue working with the center to study the effectiveness of different pasteurization methods in the coming months.

Federal officials have done their own research into whether the avian flu virus is ending up in retail dairy products. This spring, the Food and Drug Administration collected nearly 300 samples from store shelves in 17 states and tested them for the virus. Officials said while they did find pieces of H5N1 in milk, they did not find any viable virus.

Wisconsin has not had a case of H5N1 in dairy cattle, but the virus has spread to 58 herds across nine states. Industry fear over the virus has mostly died down because affected herds have seen little to no mortality in cattle.

But Poulsen said the outbreak is likely not going away, prompting the need for more research and testing.

“There’s a very good chance that it will continue to circulate if we don’t stay on top of it,” he said.

Last week, federal officials confirmed the third human case of avian influenza in a Michigan dairy worker. The Centers for Disease Control and Prevention said the employee reported symptoms of infection in their eye and an eye swab tested positive for the virus, while a nasal swab tested negative. Officials said the case was similar to a dairy farm worker in Texas who tested positive for the virus in April.