Impact of pH and temperature in dairy processing on the infectivity of H5N1 avian influenza viruses.

Highly pathogenic avian influenza viruses of subtype H5N1 (clade 2.3.4.4b) can cause a mastitis-like disease in dairy cows. The presence of high amounts of infectious H5N1 virus in milk has raised significant concerns about the safety of raw milk products. In this study, the effect of temperature and pH on the stability of H5N1 viruses was investigated. We found that both bovine and avian H5N1 viruses remained infectious when incubated in milk at 4 °C for four weeks. When the viruses were incubated in milk at 21 °C, infectivity of avian H5N1 decreased only slightly and of bovine H5N1 moderately. The avian H5N1 virus was stable at 50 °C for 30 min but was inactivated at higher temperatures (55 °C for 10 min, 60 °C for 1 min, or 72 °C for 30 s). Bovine and avian H5N1 viruses were stable at pH levels between 6.0 and 10.0, but were partially inactivated at pH 5.0 and completely inactivated at pH 4.0. Both H5N1 viruses were completely inactivated when incubated with yoghurt at pH 4.2. Incubation of the avian H5N1 virus with soft and semi-hard cheese at pH 5.0-5.3 reduced infectious titers by 5.1 and 3.9 log10, respectively. In contrast, the infectivity of bovine H5N1 was only minimally reduced following incubation with semi-hard cheese. In conclusion, H5N1 viruses are efficiently inactivated by pasteurization and most thermisation procedures. However, in untreated raw milk bovine H5N1 virus may survive cheese-making processes if the production temperature stays below 50 °C.