Pasteurization may not clear bird flu from heavily infected milk

Researchers reported on Friday that small amounts of infectious bird flu virus were still detectable in raw milk samples spiked with high amounts of the virus after undergoing standard pasteurization methods.

However, the study, conducted by researchers from the U.S. government’s National Institute of Allergy and Infectious Diseases (NIAID) Rocky Mountain Laboratories and published in the New England Journal of Medicine, emphasized that these findings should not be extrapolated to draw conclusions about the safety of the U.S. milk supply.

The experiment involved raw milk samples spiked with virus, simulating laboratory conditions. The researchers noted that the composition of raw milk from cows infected with H5N1 influenza and the presence of virus inside cells could affect the effectiveness of heat treatment.

In March, U.S. dairy cows were found to be infected with bird flu. The U.S. Food and Drug Administration (FDA) conducted a survey on pasteurized retail milk samples and estimated that a portion of the U.S. milk supply contained strands of the virus. Despite this, the FDA maintained that pasteurized milk remains safe for consumption.

The virus used in the experiments was isolated from a deceased mountain lion's lungs, combined with raw, unpasteurized cow milk samples, and subjected to heat treatment at temperatures of 63°C (145.4°F) and 72°C (161.6°F) for varying durations.

The study found that after treatment at 72°C for 20 seconds—five seconds longer than the standard industry pasteurization time at that temperature—very small amounts of infectious virus were detected in one of three samples.

The authors cautioned that while this indicated the potential for residual infectious virus, standard industry pasteurization of 30 minutes at 63°C successfully eliminated infectious virus within 2.5 minutes.

They stressed that their experimental conditions differ from large-scale industrial pasteurization processes and called for further research to replicate their findings using commercial pasteurization equipment.

Furthermore, the researchers noted that it remains unclear whether consuming active H5N1 virus in milk could lead to illness in humans.