Pasteurizing Milk Inactivates Highly Infectious Bird Flu: USDA Report

H5N1 was discovered in dairy cows in March—marking the first time dairy cattle have become infected. It has since been found in 200 animals and three people.

Cows graze in a field at a dairy farm in Petaluma, Calif., on April 26, 2024. Justin Sullivan/Getty Images

Pasteurizing milk kills highly pathogenic bird flu virus H5N1, though some viral RNA may remain, a study conducted by researchers at the U.S. Department of Agriculture (USDA) finds.

The study, published Wednesday in the Journal of Virology, evaluated nearly 300 pasteurized milk samples. About 20 percent contained noninfectious virus RNA, and the authors determined that the milk did not pose a risk to public health.

Before this study, the USDA had detected the new H5N1 bird flu in raw milk. Therefore, researchers wanted to determine if pasteurization was sufficient to eliminate the virus.

Pasteurization is a sanitation and food preservation method often used in milk processing. Raw milk is quickly heated for 15 seconds to 161 degrees Fahrenheit to kill pathogenic viruses and other microbes.

“Milk is safe,” Erica Spackman, a virologist at the USDA, said in the news release. “Just like bacterial pathogens that occur in milk, or other viruses that could occur in milk, the sanitation processes that are in place are getting rid of the pathogens.”

“We did a viability assay to detect live virus ... but couldn’t detect anything,” Ms. Spackman said. “It looks like the virus is just totally inactivated.”

Bird Flu in Milk

Bird flu primarily infects and spreads among migratory birds and can be transmitted to domestic poultry, though the virus has been found in other animals.

H5N1 was discovered in dairy cows in March—marking the first time dairy cattle have become infected. Since then, the virus has been found in about 200 animals and three people across 12 states, according to the U.S. Centers for Disease Control and Prevention (CDC).

Commercial milk is often collected from several dairy farms before being routed for bulk pasteurization and processing. When the virus was discovered in raw milk, there was some concern that, because of the complexity of milk processing, the whole milk supply could have been contaminated.

It is then shipped to multiple states, meaning a “product could have been produced by cows in one state, then processed in a different state, and then sold commercially in a third state,” Ms. Spackman and her team wrote.

Researchers used real-time PCR tests between April 18 and April 22, 2024, to assess 297 samples of pasteurized retail milk products from 17 states prone to bird flu exposure.

Although viral RNA was present in some of the samples, suggesting genetic components of the virus had entered the milk supply, the pathogen would have been killed through two processes.

“First, cows rapidly develop antibodies after infection which are present in milk and will inactivate the virus,” the authors wrote. “Second, virus is inactivated by pasteurization and possibly by the high shear forces of homogenization.”

Reassurance in Pasteurization Process

Ms. Spackman said the new findings “give us reassurance that what we have been doing—pasteurization—is keeping us safe from what we don’t know about.”

The CDC recommends against consuming raw milk. Drinking infected raw milk should not be done to build immunity against the virus, the CDC advises, adding that consuming contaminated raw milk could make a person ill.

“It’s important to understand that raw milk can be a source of foodborne illness,” the CDC writes. “While good practices on farms can reduce contamination, they cannot guarantee safety from harmful germs.”