As avian influenza spreads among dairy cattle in the U.S., health officials are monitoring the risk of human infection through contaminated milk or meat.

While the federal government maintains a low risk to the public and the safety of commercially sold milk, the U.S. Food and Drug Administration (FDA) has detected fragments of the H5N1 bird flu virus in some milk samples. These fragments are inactive, meaning they cannot produce live virus, and have not led to infections in exposed animals.

Both agencies emphasize that pasteurization effectively inactivates the virus. However, the duration and temperature of pasteurization play a crucial role in its effectiveness.

A study published in the New England Journal of Medicine by researchers from the National Institute of Allergy and Infectious Diseases and the University of California, Los Angeles investigated the inactivation of H5N1 by pasteurization. They tested raw milk at two typical pasteurization temperatures: 63°C (145°F) and 72°C (161°F).

The study found that the lower temperature inactivated the virus within two minutes, suggesting that commercial pasteurization, which typically heats milk to 63°C for 30 minutes, is sufficient. At the higher temperature, inactivation occurred within 20 seconds in most cases.

“This is the first study examining the stability, inactivation, and efficiency of heat treatment of H5N1 in the lab setting,” said Vincent Munster, chief of virus ecology at the National Institute of Allergy and Infectious Diseases’ Rocky Mountain Laboratories. “We were able to build information on how well these viruses are inactivated by pasteurization, which was previously unknown due to the lack of H5N1 in milk.”

While the findings are encouraging regarding the effectiveness of pasteurization, the FDA and U.S. Department of Agriculture are conducting further studies to confirm the inactivation of H5N1 in real-world milk processing. Munster notes that the effectiveness of pasteurization is dependent on both time and the initial viral concentration. Milk with high concentrations of the virus may require longer heating to ensure complete inactivation. Since pasteurization facilities often process milk from various farms, batches may have differing viral loads. Uniform pasteurization settings might not consistently inactivate all the virus present, especially in milk with high H5N1 concentrations. “The next step is to confirm that industrial-scale pasteurization functions as intended,” he says.

Continued research is crucial to understanding the virus’s path from infected dairy cows to the milk supply. “Even with highly efficient inactivation, H5N1 should not be present in our milk,” says Munster. “We should prioritize countermeasures to prevent H5N1-positive milk from entering dairy processing plants. Eliminating H5N1 from milk will eliminate the need for its inactivation.”