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 inactivated completely 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, which was significantly lower than that (3 log₁₀TCID₅₀/mL) 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 (4.68 × 10³ 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 inactivated completely using any of the three heat treatment methods: 63 °C for 30 min, 72 °C for 15 s, or 80 °C for 15 s. For the virus present in milk, only heat treatment at 80 °C for 15 s completely inactivated it. These results suggest that milk prevents influenza viruses from pasteurization inactivation.