Stanford study reveals flu virus remains infectious in refrigerated raw milk

Raw milk, hailed by some as a natural and nutritious alternative to pasteurized dairy, may come with hidden dangers, according to a new Stanford University study. The research, published December 12 in Environmental Science & Technology Letters reveals that influenza or flu virus can remain infectious in refrigerated raw milk for up to five days. The findings come at a time when outbreaks of bird flu in dairy cattle have raised concerns about the potential for a new pandemic.

"This work highlights the potential risk of avian influenza transmission through consumption of raw milk and the importance of milk pasteurization," said study senior author Alexandria Boehm, the Richard and Rhoda Goldman Professor of Environmental Studies in the Stanford Doerr School of Sustainability and the Stanford School of Engineering. 

More than 14 million Americans consume raw milk annually. Unlike pasteurized milk, raw milk is not heated to kill potentially harmful pathogens. Proponents of raw milk claim that it leaves more beneficial nutrients, enzymes, and probiotics than in pasteurized milk, and can boost immune and gastrointestinal health.

The Food and Drug Administration has tied raw milk to over 200 outbreaks of illnesses, and - together with the Centers for Disease Control and Prevention - warns that germs, such E. coli and Salmonella, in raw milk present “serious” health risks, especially for children, the elderly, pregnant women, and people with weakened immune systems.

The science behind the risk

The researchers explored the persistence of a strain of human influenza virus in raw cow’s milk at typical refrigeration temperatures. The flu virus, called H1N1 PR8, survived and remained infectious in the milk for up to five days. 

“The persistence of infectious influenza virus in raw milk for days raises concerns about potential transmission pathways,” said study co-lead author Mengyang Zhang, a postdoctoral scholar in civil and environmental engineering. “The virus could contaminate surfaces and other environmental materials within dairy facilities, posing risks to animals and humans.”

Additionally, the researchers found that flu virus RNA – molecules that carry genetic information but are not considered a health risk – remained detectable in the raw milk for at least 57 days. By comparison, pasteurization completely destroyed infectious influenza in the milk and reduced the amount of viral RNA by almost 90%, but didn’t eliminate the RNA entirely. Although exposure to influenza virus RNA does not pose a health risk, RNA-based testing methods are often used to conduct environmental surveillance of pathogens like influenza. 

“The prolonged persistence of viral RNA in both raw and pasteurized milk has implications for food safety assessments and environmental surveillance, particularly because many of the techniques used in environmental surveillance detect RNA,” said study co-lead author Alessandro Zulli, a postdoctoral scholar in civil and environmental engineering.

The research grew out of an earlier project – funded by the Stanford Woods Institute for the Environment’s Environmental Venture Projects program – focused on human norovirus and the subfamily of viruses responsible for the COVID-19 pandemic.

Why it matters now

In the U.S. alone, flu viruses infect more than 40 million people and kill more than 50,000 every year. These types of viruses can spread from animals to humans, as in the case of swine flu, which led to as many as 1.4 billion human infections globally in 2009-2010. 

Although bird flu has not proven as dangerous to people yet, it could mutate to become so. Recent detection of bird flu in cattle has raised questions about its potential transmission through milk and other dairy products. 

The study’s findings underscore the importance of improving monitoring systems, particularly as bird flu continues to spread among livestock, according to the study’s authors.

The study complements earlier research involving several of the same researchers that pioneered the use of wastewater for detection of avian influenza. That analysis revealed commercial and industrial dairy waste as primary sources. By analyzing wastewater, public health officials could detect virus activity in nearby cattle populations.

“We never thought that wastewater could be used to detect and respond to zoonotic pathogens circulating in the community,” Boehm said. “It has been amazing to watch our work on detection in wastewater scaled across the United States and the world.”

Acknowledgments

Boehm is also professor of Oceans in the Stanford Doerr School of Sustainability and a senior fellow at the Stanford Woods Institute for the Environment.

The study’s coauthors also include 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.

The study was funded by the Stanford Woods Institute for the Environment and by the Sergey Brin Family Foundation.