The Thermal Stability of Influenza Viruses in Milk

Highly pathogenic avian influenza viruses (HPAIVs) of the H5N1 subtype (clade 2.3.4.4b) have been detected in raw milk from infected cows. Several studies have examined the time and temperature parameters to ascertain whether influenza viruses in milk can be completely inac-tivated under commercial pasteurization conditions, yielding conflicting results. This study aimed to investigate whether milk could help protect influenza viruses from heat treatment. After heat treatment at 49°C for one hour, the titer reduction of the influenza A/WSN/1933 (A/H1) virus in milk was approximately 1.6 log10TCID50/mL greater than that observed in the Dulbecco's Modified Eagle Medium (DMEM) control media. The influenza D/bovine/CHN/JY3002/2022 (D/Yama2019) virus in milk retained a high residual infectivity (4.68x103 log10TCID50/mL) after treatment at 53°C; however, the virus in DMEM completely lost its infectivity under the same conditions. Moreover, the influenza A/chicken/CHN/Cangzhou03/2023 (A/H5) virus in DMEM could be completely inactivated using any of the three pasteurization methods: 63°C for 30 minutes, 72°C for 15 sec-onds, or 80°C for 15 seconds. For the virus present in milk, only pasteurization at 80°C for 15 seconds completely inactivated it. These results suggest that milk prevents influenza viruses from heat inactivation. Pasteurization at 80°C for 15 seconds is more effective than pasteurization at 72°C for 15 seconds or at 63°C for 30 minutes in completely eliminating the virus in milk.