NIAID Experiments Show H5N1 Levels Plummet After Heat Treatment, but Not to Zero in Some Cases

By Lisa Schnirring

Experiments by researchers from the National Institute of Allergy and Infectious Diseases (NIAID) on milk samples spiked with H5N1 avian influenza found that levels rapidly decline when heated to pasteurization temperatures, but small amount of infectious virus remain.

The team reported its findings today in a letter to the New England Journal of Medicine, and they emphasized that the findings reflect experimental lab conditions and don't reflect large-scale industrial pasteurization of raw milk.

Using virus isolated and grown from a mountain lion that died from H5N1 in Montana, they mixed the virus with raw cow milk samples, then heated them to 63°C (145.4°F) and 72°C (161.6°F), the temperatures most commonly used in commercial pasteurization, for different periods. Then they cultured the milk sample to see if any infectious virus remained.

Findings should not be used to judge safety of US milk supply

Heating the milk to 63°C led to a tenfold decrease in H5N1 levels after 2.5 minutes, and they noted that standard pasteurization of 30 minutes would eliminate infectious virus. Heating the milk to 72°C decreased virus levels fourfold within 5 seconds, but they detected very small levels of infectious virus after 20 seconds in one out of three samples.

"This finding indicates the potential for a relatively small but detectable quantity of H5N1 virus to remain infectious in milk after 15 seconds at 72℃ if the initial virus levels were sufficiently high," the group wrote.

Researchers emphasized that the conditions should be replicated with commercial pasteurization equipment and that the study's findings should not be used to judge the safety of the US milk supply.

The US Food and Drug Administration still concludes that the US milk supply is safe. Its initial findings on 297 retail dairy product samples were negative for viable virus. In light of recent H5N1 outbreaks in dairy herds, federal health officials have reiterated their warnings about illness risks from drinking raw milk.