Some Pasteurization Methods May Not Clear H5N1 in Heavily Infected Milk

Heating raw milk spiked with H5N1 to temperatures commonly used in commercial pasteurization resulted in a decrease in infectious virus within seconds to minutes, according to a brief study, though some infectious material remained after one standard method.

At a temperature of 63°C (145.4°F), H5N1 virus experimentally added to raw milk decreased from an initial titer of 10⁶ 50% tissue-culture infectious doses (TCID₅₀) per mL to undetectable levels within 2 minutes, Vincent Munster, PhD, of the National Institute of Allergy and Infectious Diseases (NIAID) in Hamilton, Montana, and colleagues reported.

Heating milk to 63°C for 30 minutes is one standard method of pasteurization, and “has a large safety buffer,” Munster and co-authors wrote in a correspondence to the New England Journal of Medicine. When heated to 63°C, the half-life of infectious virus was 4.5 seconds (95% CI 3.5-5.8), researchers estimated.

However, when they used another standard pasteurization method — heating milk to 72°C (161.6°F) for 15 seconds — they found that small, detectable amounts of infectious virus remained in raw milk samples with initial high viral titers. In samples heated to this higher temperature, virus titers decreased from about 10⁵ to 10¹ TCID₅₀/ml within 5 seconds. After 20 seconds of heating, no viable virus was detected.

Pasteurization definitely inactivates the H5N1 virus, Munster commented to MedPage Today, but the effect is dose- and duration-dependent. “So a higher dose of virus will take longer to inactivate,” he explained.

“The results were generally what we expected,” Munster said, noting that his lab had previously done experimental studies on inactivation of MERS-CoV and SARS-CoV-2, as well. “This is the first time we have been doing this with H5N1, but the virus behaves similarly,” to other viruses they have tested.

In a press release from NIAID, the authors stressed that their findings “reflect experimental conditions in a laboratory setting and are not identical to large-scale industrial pasteurization processes for raw milk.”

“The next step, which the USDA [U.S. Department of Agriculture] and FDA are working on, is performing similar experiments with actual industrial pasteurization equipment and mimic the complete process faithfully,” Munster said. The study of the effects of pasteurization on the virus in other dairy products besides milk is also needed, the authors wrote.

Gastrointestinal infections with H5N1 have occurred in several species, but it is unknown whether ingesting live H5N1 in raw milk could cause infections in humans or how much would be needed, they pointed out.

An outbreak of H5N1 among dairy cows in Texas was first reported in March 2024. To date, at least 90 cattle herds across 12 states have been affected, with three human infections detected in dairy farm workers. Since 2020, H5N1 clade 2.3.4.4b subtype viruses have spread widely among wild birds worldwide, resulting in outbreaks in poultry and other animals. Recently, the clade 2.3.4.4b viruses were identified in dairy cows and in unpasteurized milk in multiple U.S. states, as well as in cats.

“I think it’s important that the public is aware of the current situation with H5N1 in dairy cattle,” Munster stressed. At the moment, the direct risk for humans appears to be low, he said, “but this virus already has had devastating effects on wildlife. It needs a true One Health approach to try to get this under control and minimize the impact on wildlife and production animals.”

Munster and his fellow researchers at NIAID’s Rocky Mountain Laboratories isolated H5N1 (clade 2.3.4.4b) from the lungs of a dead mountain lion in Montana. They mixed viral isolates with raw, unpasteurized cow milk samples and heat-treated the milk at 63℃ and 72℃ for different periods of time. The samples were then cell-cultured.

H5N1 genome copies were quantified using quantitative reverse-transcriptase-polymerase-chain-reaction assay to determine the amounts of live virus remaining at different time periods. Since the study relied on raw milk samples spiked with H5N1 virus, raw milk from H5N1-infected cows may have a different composition or contain cell-associated virus that may impact the effects of heat, the authors noted.

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