How bird flu virus fragments get into milk sold in stores, and what the spread of H5N1 in cows means for the dairy industry and milk drinkers

By Noelia Silva del Rio, Richard V. Pereira, Robert B. Moeller, Terry W. Lehenbauer, Todd Cornish, University of California, Davis

The discovery of fragments of avian flu virus in about 20% of milk samples from stores in initial testing across the U.S. suggests that the H5N1 virus may be more widespread in U.S. dairy cattle than previously realized.

The Food and Drug Administration, which announced the initial results from its nationally representative sampling on April 25, 2024, was quick to stress that it believes the commercial milk supply is safe. However, highly pathogenic avian influenza virus can make cows sick, and the flu virus’s presence in herds in several states and new federal restrictions on the movement of dairy cows between states are putting economic pressure on farmers.

Five experts in infectious diseases in cattle from the University of California, Davis – Noelia Silva del Rio, Terry Lehenbauer, Richard Pereira, Robert Moeller and Todd Cornish – explain what the test results mean, how bird flu can spread to cattle and the impact on the industry.

What are viral fragments of avian flu, and can they pose risks to people?

It’s crucial to understand that the presence of viral fragments of H5N1 doesn’t indicate the presence of intact virus particles that could cause disease.

The commercial milk supply maintains safety through two critical measures:

• First, milk sourced from sick animals is promptly diverted or disposed of, ensuring it does not enter the food chain.

• Second, all milk at grocery stores is heat treated to reduce pathogen load to safe levels, mainly by pasteurization. Pasteurization has been shown to effectively inactivate H5N1 in eggs, and that process occurs at a lower temperature than is used for milk.

The viral fragments were detected using quantitative polymerase chain reaction testing, which is known for its exceptional sensitivity in detecting even trace amounts of viral genetic material. These fragments are only evidence that the virus was present in the milk.

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