Examining the survival of A(H5N1) influenza virus in thermised whole milk

The recent spillover events of highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b to dairy cattle, and high viral shedding in the milk from infected animals, has created concern that milk and dairy products could be a route for human infection. It has been demonstrated that pasteurization is effective in inactivating A(H5N1) in milk. However, multiple dairy products are made with unpasteurized but thermised milk. Thermisation is a subpasteurisation heat treatment applied to raw milk, and the aim of this study was to examine whether commonly used conditions for thermisation are effective against inactivation of A(H5N1) in whole milk. For this purpose, we artificially inoculated whole raw cow milk with 6.5 log₁₀ EID₅₀ A(H5N1) and heated for 15 s at 60, 63 and 66 °C, the viral infectivity was tested using embryonated chicken eggs. We observed over 4 and 5 log reduction in viral infectivity at 60 and 63 °C, respectively (p < 0.0001) compared to the titre in untreated milk. The viral infectivity was reduced to below the detection limit at 66 °C. We also calculated the D-values, the time required to reduce the viral titer by one log, for each treatment and as expected, we observed a decrease in D-values with increasing thermisation temperature. These data demonstrate that thermisation is effective in reducing the viral load and thus they allow for informed risk assessment of A(H5N1) contaminated dairy products made from thermized milk.