Cattle were once thought to be resilient to avian influenza virus (AIV) infections, until the recent widespread influenza A(H5N1) virus infections in dairy cattle in several states in the Unites Stares (US) since March 2024. Surprisingly, high viral loads were found in the milk of infected cows, but not in the respiratory tract. Consumption of unpasteurised influenza A(H5N1)-contaminated milk represents a new niche of possible public health concern for an avian virus. Cats on the dairy farms?and laboratory mice got infected and died after drinking influenza A(H5N1)-contaminated unpasteurised colostrum and milk. To date, four dairy farm workers have been reported with influenza A(H5N1)-associated conjunctivitis and upper respiratory influenza-like illness.

Little is known about the efficiency of H5N1 clade 2.3.4.4b, particularly non-US isolates, to replicate in bovine cells and remain infectious in milk with different fat contents. Here we investigated the effect of different fat contents on the thermostability and duration of heat inactivation of H5N1 influenza viruses and assessed the replication of recent German H5N1 viruses of clade 2.3.4.4b in bovine kidney and lung cells.