Pasteurization destroys H5N1 bird flu in milk

Pasteurization effectively eliminates the H5N1 bird flu virus in milk, even in the presence of residual viral proteins.

Recent research published on September 26 in Science Advances indicates that consuming properly pasteurized milk containing remnants of avian influenza does not enhance the risk of infection. The study showcased that heat treatment thoroughly inactivated the highly pathogenic avian influenza H5N1 virus, although some viral genomic material remained detectable. Importantly, these remnants did not lead to illness in mice that ingested the milk regularly. However, the fragments also did not enhance the immune response to subsequent infections.

“Milk that has undergone pasteurization, even if contaminated with H5N1, is safe for consumption; however, raw milk poses a significant risk,” states Stacey Schultz-Cherry, a virologist from St. Jude Children’s Research Hospital in Memphis, Tennessee.

Since the first confirmed case of H5N1 infections in dairy cows within the United States in 2024, the virus has spread to 17 states, including Nebraska for the first time, as reported by the U.S. Department of Agriculture on September 15. This rapid spread raises concerns over potential human infections.

The U.S. Centers for Disease Control and Prevention (CDC) has documented several H5N1 cases in humans, which include one fatality. The CDC has traced these infections back to interactions with wild birds, backyard flocks, and the consumption of unpasteurized or raw milk.

Regulatory agencies in the U.S. maintain that milk pasteurized at 72°C for at least 15 seconds is safe for human consumption. Nevertheless, laboratory tests have identified remnants of viral genomic material, including RNA and hemagglutinin protein—the component that assists H5N1 in attaching to host cells—in such pasteurized milk.

“Finding viral RNA is similar to discovering a footprint or shadow; it implies that something was there, but it is no longer viable or self-operating,” explains Schultz-Cherry.

Despite the absence of disease risk from these fragments, the implications of viral remnants in pasteurized milk remained uncertain. Key questions included whether they could compromise the immune system leading to worse outcomes for subsequent infections or provide immunity against future infections.

To address these concerns, Schultz-Cherry and her research team utilized raw cow’s milk that tested negative for H5N1. They artificially contaminated it with a significant amount of H5N1 to simulate a severe contamination situation, followed by pasteurization at the standard conditions of 72°C for 15 to 30 seconds. Results consistently showed that pasteurization effectively inactivated the virus.

The test subjects, mice, were fed the pasteurized milk for five days and monitored for a total of three weeks. After this period, the mice were exposed to a lethal dose of live H5N1 virus.

The findings revealed that both groups of mice—those receiving the pasteurized milk containing inactivated H5N1 and those that consumed pasteurized milk without the virus—exhibited similar mortality rates. This indicates that the remnants of inactivated virus did not trigger any immune response capable of enhancing protection against future infections or suppressing the immune system.

Although it is comforting to know that pasteurization is effective, Andrew Pekosz, an immunologist at the Johns Hopkins Bloomberg School of Public Health, emphasizes the importance of preventing H5N1 from contaminating the milk supply altogether. He points out that not all home or noncommercial pasteurization processes may adhere to stringent safety standards, and CDC data suggests that raw milk remains a potential conduit for H5N1 transmission to humans.