High levels of H5N1 influenza found in mice fed raw milk from infected dairy cows

Mice given raw milk samples from dairy cows infected with H5N1 influenza experienced high levels of virus in their respiratory organs and lower levels of virus in other vital organs, according to findings published in the journal New England Journal of Medicine. The results suggest that consumption of raw milk by animals poses a risk of H5N1 infection and raises questions about its potential risk in humans.

Since 2003, H5N1 influenza viruses have circulated in 23 countries, mainly affecting wild birds and poultry, with around 900 cases in humans, mainly among people who have had close contact with infected birds. However, in recent years, a highly pathogenic avian influenza virus called HPAI H5N1 has spread and infects more than 50 animal species, and in late March, the United States reported a viral outbreak among dairy cows in Texas. To date, 52 cattle herds in nine states have been affected and two human infections have been detected in farm workers with conjunctivitis. Although the virus has so far shown no genetic evidence of acquiring the ability to spread from person to person, public health officials are closely monitoring the situation in dairy cows as part of overall pandemic preparedness efforts.

To assess the risk of H5N1 infection from consuming raw milk, researchers from the University of Wisconsin-Madison and the Texas A&M Veterinary Medical Diagnostic Laboratory fed five mice raw milk droplets from infected dairy cattle. Animals demonstrated signs of disease, including lethargy, on day 1 and were euthanized on day 4 to determine virus levels in the organs. The researchers found high levels of virus in the animals’ nostrils, trachea and lungs and moderate to low levels of virus in other organs, consistent with H5N1 infections found in other mammals.

In addition to the mouse studies, the researchers also conducted tests to determine what temperatures and time intervals inactivate the H5N1 virus in raw milk from dairy cows. Four milk samples with confirmed high levels of H5N1 were tested at 63 degrees Celsius (145.4 degrees Fahrenheit) for 5, 10, 20, and 30 minutes, or at 72 degrees Celsius (161.6 degrees Fahrenheit) for 5, 10, 15 , 20 and/or 30 seconds. Each of the time intervals at 63℃ successfully killed the virus. At 72℃, virus levels decreased but were not completely inactivated after 15 and 20 seconds. The authors emphasize, however, that their laboratory study was not identical to large-scale industrial pasteurization of raw milk and reflects experimental conditions that should be replicated with direct measurement of infected milk in commercial pasteurization equipment.

In a separate experiment, researchers stored H5N1-infected raw milk at 4℃ (39.2 degrees Fahrenheit) for five weeks and found only a small decrease in virus levels, suggesting that the virus in raw milk may continue being infectious when kept at refrigerated temperatures.

To date, the US Food and Drug Administration concludes that the totality of the evidence continues to indicate that the commercial milk supply is safe. While laboratory studies provide important and useful information, there are limitations that challenge inferences about real-world commercial processing and pasteurization. The FDA conducted an initial survey of 297 retail dairy products collected at retail outlets in 17 states and represented products produced at 132 processing locations in 38 states. All samples were negative for viable viruses. These results underscore the opportunity for additional studies that closely replicate real-world conditions. The FDA, in partnership with the USDA, is conducting pasteurization validation studies, including the use of a homogenizer and a continuous flow pasteurizer. Additional results will be available as soon as they are available.

The National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, funded the work of the University of Wisconsin-Madison researchers.