Infectious H5N1 flu virus in raw milk decreases rapidly through heat treatment

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Friday, June 14, 2024

In laboratory studies, the amount of infectious H5N1 influenza viruses in raw milk decreased rapidly after heat treatment.


In laboratory studies by scientists at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, the amount of infectious H5N1 influenza virus in raw milk decreased rapidly when heat treated. However, small, detectable amounts of infectious virus remained in raw milk samples with high virus content when treated for 15 seconds at 72 degrees Celsius (161.6 degrees Fahrenheit) – one of the dairy industry's standard pasteurization methods. The study authors emphasize, however, that their results reflect experimental conditions in a laboratory and are not identical to large-scale industrial pasteurization processes for raw milk. The results were published today in the New England Journal of Medicine

At the end of March 2024, US authorities reported an outbreak of the highly pathogenic avian influenza virus HPAI H5N1 among dairy cows in Texas. So far, 95 cattle herds in 12 states have been affected, with three human infections detected in farm workers with conjunctivitis. Although there is no genetic evidence to date that the virus has the ability to be transmitted from person to person, health authorities are closely monitoring the situation of dairy cows as part of comprehensive pandemic preparedness.

Because there is little data on the susceptibility of avian influenza viruses to the pasteurization methods used in the dairy industry, scientists at NIAID's Rocky Mountain Laboratories attempted to quantify the stability of the H5N1 virus in raw milk. These tests were conducted at various times at 63 °C (145.4 degrees Fahrenheit) and 72 °C, the temperatures most commonly used in commercial pasteurization processes in the dairy industry. The scientists isolated HPAI H5N1 from the lungs of a dead cougar in Montana. They then mixed these virus isolates with raw, unpasteurized cow's milk samples and heat-treated the milk for varying periods of time at 63 °C and 72 °C. The samples were then grown in cell culture and tested to determine if and how much live virus was still present.

They found that 63℃ caused a significant decrease (1010-fold) of infectious H5N1 virus levels within 2.5 minutes and note that a standard pasteurization of 30 minutes would eliminate infectious viruses. At 72″C they observed a decrease (104-fold) of infectious virus within five seconds, but very low levels of infectious virus were detected in one of three samples after up to 20 seconds of heat treatment. “This finding indicates that a relatively small but detectable amount of H5N1 virus in milk can still be infectious after 15 seconds at 72°C if initial virus levels were sufficiently high,” the authors note.

The scientists emphasize that their measurements reflect experimental conditions, should be reproduced with direct measurements of infected milk in commercial pasteurization plants, and should not be used to draw conclusions about the safety of the U.S. milk supply. Another limitation of their study was the use of raw milk samples that were enriched with the H5N1 virus, while raw milk from cows infected with H5N1 flu may have a different composition or contain cell-associated viruses that can cause heat effects. The authors conclude that while gastrointestinal infections with the HPAI-H5N1 virus have occurred in several mammalian species, it is still unknown whether ingestion of live H5N1 viruses in raw milk can cause disease in humans.

To date, the U.S. Food and Drug Administration concludes that all evidence continues to indicate that the commercial milk supply is safe. While laboratory studies provide important, useful information, there are limitations that make it difficult to draw conclusions about actual commercial processing and pasteurization. FDA conducted an initial investigation of 297 dairy products collected from retail locations in 17 states, representing products produced at 132 processing sites in 38 states. No viable virus was detected in any of the samples. These results highlight the opportunity to conduct further studies that closely replicate real-world conditions. FDA is conducting validation studies on pasteurization in collaboration with the USDA—including the use of a homogenizer and a continuous flow pasteurizer. Additional results will be provided as they become available.


F Kaiser et al. Inactivation rate of the highly pathogenic avian influenza virus H5N1 (clade in raw milk at 63 and 72 degrees Celsius. The New England Journal of Medicine DOI: 10.1056/NEJMc2405488 (2024).


Vincent Munster, Ph.D., and Emmie de Wit, Ph.D., principal investigators in NIAID's Laboratory of Virology at Rocky Mountain Laboratories in Hamilton, Montana, are available to discuss the results.


To arrange interviews, please contact the NIAID News and Science Writing Branch, (301) 402-1663, [email protected].

NIAID conducts and supports research—at NIH, throughout the United States, and around the world—to understand the causes of infectious and immune-mediated diseases and to develop better means of preventing, diagnosing, and treating these diseases. News releases, fact sheets, and other NIAID-related materials are available on the NIAID website.

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