H5N1 Avian Influenza: The Risk Associated with Raw Milk Has Been Confirmed

The virus remains infectious for weeks in the milk of infected cows, but standard pasteurization appears to be sufficient. With the H5N1 avian influenza virus now circulating in dairy herds in North America, an article published in the New England Journal of Medicine sheds light on a sensitive issue: what happens to the virus in milk, and does pasteurization actually neutralize it? The authors compared the behavior of the H5N1 virus in milk from infected cows and in milk from healthy cows that had been experimentally contaminated in the laboratory. The result is clear: in raw milk from infected cows, the virus is remarkably stable. When stored at 4 °C, it remains detectable and infectious for up to twenty-two weeks, with a very slow decline in activity. In contrast, when the virus is simply added to milk from healthy cows, it loses its infectivity much more rapidly, disappearing within two to three weeks. This difference suggests that the conditions of natural infection in the udder (milk composition, cell count, and any associated viral particles) offer the virus a form of protection. The authors also tested different milk heating processes. Conventional pasteurization—whether the slow method (around 63 °C for several minutes) or the high-temperature, short-time method (approximately 72 °C for about 30 seconds), reduces the viral load to below the detection limit. However, the experiment also shows that the H5N1 virus present in raw milk from infected cows is more heat-resistant than the virus added to milk from healthy cows, particularly at the lowest temperatures used in heat treatment. The conclusion is twofold. On the one hand, industrial dairy products, which undergo controlled pasteurization, are still considered safe with regard to the H5N1 virus. On the other hand, raw milk and unpasteurized products appear to be potential vectors for the transmission of avian influenza, both to humans and to pets or livestock for which this milk may be intended.