Inactivation of avian influenza virus in yogurt made from raw milk

In March 2024, highly pathogenic avian influenza (HPAIV) H5N1 was first detected in U.S. dairy cattle and has since spread to herds across at least 17 states. Infected cows typically present with mastitis, decreased milk production, and poor milk quality with high viral loads in milk. While commercial pasteurization of milk effectively inactivates avian influenza virus (AIV), growing consumer interest in raw milk and derived products raises public health concerns due to the risk of zoonotic transmission. Standard yogurt production includes an initial heating step at 82 °C for 30 min to denature milk proteins which also inactivates AIV. However, some home yogurt recipes omit this initial heating step. This project determined whether AIV present in raw milk could remain viable through fermentation and persist in the final yogurt product. Raw milk (ca. pH 6.7) was spiked with AIV (ca. 6.6 log₁₀ 50 % egg infectious doses (EID₅₀) per mL and inoculated with a commercial starter culture to produce yogurt. The viability of the virus was determined before and after fermentation (ca. 7.3 h) at 42 °C with resultant pH drop ≤4.4. A significant (p < 0.05) reduction of viable AIV (≥4.1 log₁₀ EID₅₀) was observed in both the yogurt and the control samples of raw milk incubated at 42 °C but without starter culture (ca. pH 6.63). Viral inactivation was likely due to a combination of incubation at a sublethal temperature, pH below 4.4, and microbial degradation. Thus, properly fermented yogurt has a negligible risk of transmitting AIV to humans.