Stability of influenza viruses in the milk of cows and sheep

In late 2023, H5N1 high-pathogenicity avian influenza virus (HPAIV) started circulating in dairy cattle in the USA. High viral titres were detected in milk from infected cows, raising concerns about onward human infections. Although pasteurisation was shown to effectively inactivate influenza viruses in milk, unpasteurised milk still poses a risk of infection, both from occupational exposure in dairies and from the consumption of raw milk. We therefore assessed how long influenza viruses could remain infectious in milk without heat inactivation. We examined the stability of a panel of influenza viruses in milk, including a contemporary H5N1 HPAIV and a variety of other influenza A and D viruses. We incubated viruses in cows' milk under laboratory conditions: at room temperature to simulate exposure in dairies and at 4 °C to simulate exposure to refrigerated raw milk. Following an isolated report of H5N1 viral RNA detection in milk from a sheep in the UK, we also carried out similar experiments with a laboratory strain of influenza A virus in sheep's milk. Although the survival of influenza viruses in milk was variable, we consistently found that, under laboratory conditions, substantial viral infectivity remained over periods when people might reasonably be exposed to infected milk - for over a day at room temperature and for more than 7 days when refrigerated. Our results highlight the zoonotic risk of H5N1 HPAIV in raw milk from infected animals and reinforce the importance of taking measures to mitigate this risk.