Mice given raw milk samples from dairy cows infected with the H5N1 flu showed high viral concentrations in their respiratory organs and lower viral concentrations in other vital organs, according to findings published in the New England Journal of Medicine. The results suggest that consuming raw milk poses a risk of H5N1 infection in animals and raise questions about this potential risk to humans.
Since 2003, H5N1 flu viruses have circulated in 23 countries, primarily affecting wild birds and poultry. About 900 human cases are known, mostly among people who had close contact with infected birds. However, in recent years, a highly pathogenic avian influenza virus called HPAI H5N1 has spread, infecting more than 50 animal species. In late March, the U.S. reported a viral outbreak among dairy cows in Texas. So far, 52 cattle herds in nine states are affected, and two cases of infection in farmworkers with conjunctivitis have been identified. Although there is no genetic evidence to date that the virus has the ability to transmit from person to person, health authorities are closely monitoring the dairy cow situation as part of comprehensive pandemic preparedness.
To determine the risk of H5N1 infection from consuming raw milk, researchers at the University of Wisconsin-Madison and the Texas A&M Veterinary Medical Diagnostic Laboratory fed five mice drops of raw milk from infected dairy cows. The animals showed signs of illness, including lethargy, on the first day and were euthanized on the fourth day to determine viral content in organs. Researchers detected high viral concentrations in the animals’ nasal passages, trachea, and lungs, and medium to low viral concentrations in other organs, consistent with H5N1 infections in other mammals.
In addition to the mouse studies, researchers also tested at what temperatures and time intervals the H5N1 virus is inactivated in raw milk from dairy cows. Four milk samples with confirmed high H5N1 levels were tested for 5, 10, 20, and 30 minutes at 63 degrees Celsius (145.4 degrees Fahrenheit) or for 5, 10, 15, 20, and/or 30 seconds at 72 degrees Celsius (161.6 degrees Fahrenheit). At each of the time intervals at 63°C, the virus was successfully killed. At 72°C, viral levels were reduced but not fully inactivated after 15 and 20 seconds. However, the authors emphasize that their laboratory study was not identical to large-scale commercial pasteurization of raw milk and reflected experimental conditions that should be reproduced by direct measurement of infected milk in commercial pasteurizers.
In a separate experiment, researchers stored raw milk infected with H5N1 at 4°C (39.2 degrees Fahrenheit) for five weeks and found only a slight decrease in virus concentration. This suggests that the virus may remain infectious in raw milk even when stored at refrigerated temperatures.
https://www.nejm.org/doi/10.1056/NEJMc2405495
