A recent study from Stanford University indicates that the influenza virus can remain active in refrigerated raw milk for up to five days. This research appears against the backdrop of ongoing outbreaks of bird flu, a different subtype of the influenza virus, which has raised alarms about the possibility of a new pandemic. While some people view raw milk as a wholesome alternative to pasteurized dairy, this study suggests significant hidden risks.
“Our findings emphasize the potential danger of avian influenza being transmitted through raw milk consumption and highlight the crucial role of pasteurization,” stated Alexandria Boehm, the senior author of the study and a professor at the Stanford Doerr School of Sustainability and the Stanford School of Engineering.
Each year, over 14 million Americans drink raw milk. Unlike its pasteurized counterpart, raw milk does not undergo heating processes to eliminate harmful pathogens. Supporters of raw milk believe it contains more beneficial nutrients, enzymes, and probiotics, which may enhance immune and gut health.
However, the Food and Drug Administration has linked raw milk to over 200 illness outbreaks and, along with the Centers for Disease Control and Prevention, warns that pathogens such as E. coli and Salmonella found in raw milk can pose “serious” health risks, particularly to vulnerable groups such as children, the elderly, pregnant women, and individuals with compromised immune systems.
The science behind the risk
The study investigated how a specific strain of human influenza virus persisted in raw cow’s milk when kept at standard refrigeration temperatures. The virus strain, known as H1N1 PR8, was found to survive and remain infectious for as long as five days.
“The fact that the influenza virus can remain infectious in raw milk for days raises red flags about its potential transmission pathways,” commented Mengyang Zhang, a co-lead author of the study and a postdoctoral scholar in civil and environmental engineering. “The virus might contaminate surfaces and other materials in dairy facilities, endangering both animals and humans.”
Moreover, the researchers discovered that the RNA of the influenza virus—molecules that carry genetic material but do not pose any health threat—could still be detected in the raw milk for at least 57 days. In contrast, pasteurization effectively destroyed the infectious influenza in the milk and decreased the viral RNA by nearly 90%, although some RNA remained. While RNA itself isn’t harmful, tests based on RNA are often used for environmental monitoring of pathogens like influenza.
“The long persistence of viral RNA in both raw and pasteurized milk holds significant implications for food safety evaluations and environmental monitoring, especially since many surveillance methods rely on RNA detection,” explained Alessandro Zulli, a co-lead author and postdoctoral scholar in civil and environmental engineering.
This research was inspired by a previous project, supported by the Stanford Woods Institute for the Environment’s Environmental Venture Projects program, which examined human norovirus and the virus subfamily causing COVID-19.
Why it matters now
In the U.S., flu viruses infect over 40 million people and lead to more than 50,000 fatalities each year. These viruses can jump from animals to humans, as was the case with swine flu, which resulted in around 1.4 billion human infections worldwide during the 2009-2010 outbreak.
Though bird flu has not yet shown to be particularly harmful to humans, there is a possibility it could mutate into a more dangerous form. The recent discovery of bird flu in cattle raises concerns regarding its potential transmission through milk and other dairy products.
The authors of the study stress the need for improved monitoring systems, especially in light of the ongoing spread of bird flu among livestock.
The research complements prior work by some of the same researchers who pioneered the method of utilizing wastewater for detecting avian influenza. Their earlier findings indicated that commercial and industrial dairy waste serves as a major source. By examining wastewater, public health officials could monitor viral activity in nearby cattle populations.
“We never imagined that wastewater could be effectively used to detect and respond to zoonotic pathogens in communities,” Boehm remarked. “It’s impressive to see our wastewater detection methods being adopted across the United States and internationally.”
Acknowledgments
Boehm also holds the position of professor of Oceans in the Stanford Doerr School of Sustainability and is a senior fellow at the Stanford Woods Institute for the Environment.
The study also includes co-authors Catherine Blish, the George E. and Lucy Becker Professor in Medicine at the Stanford School of Medicine, and Sehee Jong, a research assistant in civil and environmental engineering.
This research was funded by the Stanford Woods Institute for the Environment and the Sergey Brin Family Foundation.
