In a recent study published in the New England Journal of Medicine, researchers found that mice administered raw milk samples from dairy cows infected with H5N1 influenza experienced high virus levels in their respiratory organs and lower virus levels in other vital organs. This poses a potential risk for H5N1 infection in animals and raises concerns about its potential risk in humans. H5N1 influenza viruses have been circulating since 2003, primarily affecting wild birds and poultry, with about 900 human cases reported, mostly among those with close contact with infected birds. However, in recent years, a highly pathogenic avian influenza virus called HPAI H5N1 has spread to infect more than 50 animal species.
In late March, the United States reported a viral outbreak among dairy cows in Texas, with 52 cattle herds across nine states affected. Two human infections with conjunctivitis were also detected in farm workers. While the virus has not shown genetic evidence of being able to spread from person to person, public health officials are closely monitoring the situation as part of pandemic preparedness efforts. To assess the risk of H5N1 infection from consuming raw milk, researchers fed droplets of raw milk from infected dairy cattle to mice, which showed signs of illness and high virus levels in their nasal passages, trachea, and lungs.
The researchers also conducted tests to determine how different temperatures and time intervals would inactivate the H5N1 virus in raw milk. They found that the virus was successfully killed at 63 degrees Celsius for varying time intervals, while at 72 degrees Celsius, virus levels were diminished but not completely inactivated after 15 and 20 seconds. However, the study was not identical to large-scale industrial pasteurization of raw milk, and further research is needed to replicate these findings under commercial pasteurization conditions. Additionally, storing raw milk infected with H5N1 at refrigerated temperatures for five weeks showed only a small decline in virus levels, suggesting that the virus may remain infectious in raw milk under refrigeration.
Despite these findings, the U.S. Food and Drug Administration maintains that the commercial milk supply is safe, with no viable virus found in a survey of 297 retail dairy products collected in 17 states. Further studies are being conducted to closely replicate real-world conditions, including pasteurization validation studies using homogenizers and continuous flow pasteurizers. The National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, funded the research conducted by the University of Wisconsin-Madison researchers. This study highlights the importance of understanding the potential risks associated with consuming raw milk from infected animals and the need for further research to ensure the safety of the commercial milk supply.