Effects of Temperature and Composition of Dairy Milk on Influenza A Virus Stability

In response to the highly pathogenic avian influenza (HPAI) A(H5N1) outbreak in cattle, Center for Research on Influenza Pathogenesis and Transmission (CRIPT) and University of Pennsylvania CEIRR (Penn-CEIRR) investigator Daniel Perez, Ph.D., and his lab at the University of Georgia, assessed the stability of four influenza A virus strains in retail and raw dairy milk after a range of temperature treatments mimicking pasteurization conditions. The authors submitted this work to Emerging Infectious Diseases, and their results are summarized below.

The authors explored the impact of temperature and colostrum content on the viability of four influenza A virus strains, which included a laboratory-adapted A(H1N1), wild-type HPAI A(H5N1) from turkeys, and two reverse genetics derived A(H5N1) strains, one HPAI from the Texas outbreak and a low pathogenic avian influenza (LPAI) virus. In one set of experiments, pasteurized milk samples (or control medium) of varying volumes (2, 20, or 200 µL) were spiked with one of the four virus strains at a concentration of 108 TCID50/mL. Samples were then uniformly heated at 63°C for 30 minutes, 72°C for 20 seconds, or 91°C for 20 seconds to mimic different pasteurization protocols. To investigate whether milk composition also impacted virus stability, the researchers incubated three of the influenza strains (H1N1, turkey HPAI H5N1, and TX cattle-derived HPAI H5N1) at temperatures ranging from at -20°C to 63°C in either retail pasteurized milk, high colostrum unpasteurized raw milk, low colostrum unpasteurized raw milk, or control medium.

Viral titers were not significantly different between milk samples and the control medium, and strain specific differences were not observed across any experimental conditions. Heat treatment at 63°C for 30 minutes significantly reduced viral stability at all sample volumes tested while samples treated at 72°C or 91°C for 20 seconds exhibited virus titer reductions that were inversely proportional to the sample volume (i.e., higher reductions in lower volumes). All reductions were significant except for 200 µl samples heated at 72°C for 20 seconds. In terms of milk composition, high colostrum content increased viral inactivation compared to low colostrum or retail milk. However, the HPAI H5N1 strains tested were stable for at least four days when kept at temperatures ranging from -20°C up to room temperature in all milk types, while 63°C heat exposure inactivated the virus within 24 hours.

The study reveals standard pasteurization protocols significantly reduce influenza virus stability in dairy milk and that a low-temperature long-time approach (i.e., 63°C for 30 minutes) is the most effective. These results, combined with their findings that influenza A viruses are stable in dairy milk for multiple days across a wide range of temperatures, highlight the importance of commercial milk pasteurization as a vital process that ensures retail milk remains safe to consume even in the presence of emerging viral threats.

Caceres C. et al. (2024) Thermal Inactivation of Influenza A Virus in Dairy Milk: Implications for Pasteurization and Milk Safety. Submitted to Emerging Infectious Diseases. This article is a draft manuscript and has not been certified by peer review. It reports new medical research that has yet to be evaluated and so should not be used to guide clinical practice.