In March 2024, dairy cows in Texas were found to be infected with highly pathogenic avian influenza, or H5N1 bird flu, in the first known case of the virus spreading to livestock. Since then, H5N1 has been detected in about 200 animals (and 3 people) in 12 states, according to the Centers for Disease Control and Prevention. The virus was soon detected in raw milk, prompting researchers to investigate whether dairy products pose a risk to consumers.
“How far does the virus go?” asked Dr. Erica Spackman, a virologist at the U.S. Department of Agriculture (USDA) in Athens, Georgia. To find out, she and her colleagues analyzed nearly 300 dairy products from 132 processors.
The researchers found no infectious virus in the samples, Spackman and his collaborators report this week in the Journal of Virology, “The milk is safe,” he said. “As for bacterial pathogens that are in the milk, or other viruses that might be present in the milk, the sanitation processes that are being implemented are eliminating the pathogens.”
The milk processing process includes multiple layers of protection, Spackman said. Microbiological monitoring of dairy products can identify pathogens, and milk from cows with mastitis or other diseases does not enter the food supply. Finally, heating during the pasteurization process can destroy H5N1 and other more common bacterial pathogens.
Avian influenza primarily infects and spreads among migratory birds and can be transmitted to domestic poultry, but the virus has also been detected in other animals. Recently, these have included cats, dogs and young goats, as well as a polar bear in Alaska and elephants and fur seals in Antarctica. However, the discovery of H5N1 on dairy farms in March came as a surprise: the virus had never before been found in dairy cows.
Shortly after the discovery, diagnostic testing revealed that an infectious form of the virus was present in raw milk, suggesting that the virus passes from cow to milk. That finding prompted the U.S. Food and Drug Administration and the USDA to investigate whether pasteurization effectively eliminated the risks posed to consumers. Between April 18 and April 22, 2024, researchers used real-time PCR to analyze 297 samples of retail pasteurized dairy products, including 23 types of products, collected from 17 states.
“We ran a viability assay to detect live virus and used the highest sensitivity possible to get even the smallest amount of virus, but we couldn’t detect anything,” Spackman said. Using PCR, the researchers identified viral genetic material in 20% of the samples. “It appears that the virus is totally inactivated,” he said.
Spackman said the new findings “give us confidence that what we’ve been doing – pasteurization – is keeping us safe from the unknown.”