Madison Reporter

Consuming raw cow’s milk containing the H5N1 avian influenza virus poses an infection risk, but a laboratory process simulating high-temperature pasteurization reduces the virus in infected milk by more than 99.99%, according to a team led by University of Wisconsin–Madison scientists. Their findings were reported on May 24 in a letter published in the New England Journal of Medicine.

The group, which included researchers from Texas A&M University, found that mice consuming untreated milk infected with H5N1 subsequently became ill with influenza. Small amounts of the virus persisted in untreated milk for weeks when kept at standard refrigeration temperatures. This research comes as the H5N1 avian influenza virus continues to spread from birds into mammals, with infections reported in dairy herds across the United States this spring.

Yoshihiro Kawaoka, a professor in the UW–Madison Department of Pathobiological Sciences, and Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, led the study. The researchers tested two methods of heat inactivation on four H5N1-positive milk samples taken from infected cows in New Mexico and Kansas. Both methods aimed to simulate common pasteurization techniques, though they differed from standard industry practices.

In one method, they heated infected raw milk to 145.4 F for a minimum of five minutes and up to 30 minutes. This reduced virus levels by more than 99.999%. The second method involved heating raw milk to 161.6 F for up to 30 seconds, which inactivated more than 99.99% of the virus within the samples but did not completely eliminate it.

“We must emphasize that the conditions used in our laboratory study are not identical to large-scale industrial treatment,” says Kawaoka. “It’s important to remember that our heat-treatment findings may not perfectly translate to real-world conditions.”

To gauge possible risks posed by consuming raw milk containing H5N1 virus, the team orally inoculated mice with untreated milk samples. The mice began showing symptoms after one day.

While no mice died within the experiment’s four-day window, subsequent analyses showed high levels of virus in their respiratory tracts and significant amounts in their mammary glands—aligning with high levels found in infected cows’ milk.

To date, only two people have had confirmed H5N1 infections in the U.S., both farm workers who worked closely with dairy cattle—one case was reported on May 22nd in Michigan and another earlier case occurred in Texas. The primary symptom was eye inflammation. While epidemiologists have not confirmed transmission modes, the Centers for Disease Control and Prevention suggest likely routes include infected milk splashing into workers’ eyes or contact between contaminated hands and eyes.

The latest findings indicate potential risk if people consume raw milk from an infected cow.

“Do not drink raw milk,” advises Kawaoka.

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