Raw milk has long been touted as a more natural and nutritious alternative to pasteurized dairy, with proponents claiming that it contains higher levels of beneficial nutrients, enzymes, and probiotics. These advocates argue that raw milk can boost immune and gastrointestinal health. However, a new study from Stanford University, published on December 12 in Environmental Science & Technology Letters, highlights a significant risk associated with consuming raw milk: the potential for the transmission of infectious influenza viruses. The study reveals that influenza viruses, including strains of avian influenza, can remain infectious in refrigerated raw milk for up to five days. This finding raises concerns about the possible public health risks associated with raw milk consumption, especially as bird flu outbreaks among livestock, including dairy cattle, increase.
The Study’s Key Findings
The study, led by Alexandria Boehm, a professor of environmental studies at the Stanford Doerr School of Sustainability and the Stanford School of Engineering, explored how long influenza viruses, particularly H1N1 PR8 (a strain of the human influenza virus), could persist in raw cow’s milk stored at typical refrigeration temperatures. The researchers discovered that the virus could remain viable and infectious for up to five days, potentially providing a pathway for the virus to spread.
“The persistence of infectious influenza virus in raw milk for days raises concerns about potential transmission pathways,” said Mengyang Zhang, the study’s co-lead author and a postdoctoral scholar in civil and environmental engineering. Zhang emphasized that such prolonged virus survival could lead to contamination of surfaces and materials within dairy facilities, putting both animals and humans at risk.
Furthermore, the study also detected influenza virus RNA in raw milk for up to 57 days. While the RNA itself does not pose a direct health risk—since it does not represent an infectious particle—it could still play a role in environmental surveillance. RNA-based testing is commonly used by health authorities to track the presence of viruses, and its persistence in raw milk could make it harder to detect the virus’s activity in dairy environments.
The Dangers of Raw Milk
Unlike pasteurized milk, which is heated to a high temperature to kill harmful pathogens, raw milk is unheated and can contain a wide variety of dangerous bacteria and viruses. Over the years, the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) have warned consumers about the potential risks of drinking raw milk, citing over 200 documented outbreaks of illness linked to raw milk consumption. Pathogens like E. coli, Salmonella, and Listeria can contaminate raw milk, posing significant health risks, especially for vulnerable populations such as children, pregnant women, the elderly, and those with weakened immune systems.
In addition to the known bacterial risks, this latest study adds another layer of concern: the possibility of influenza transmission. If bird flu or other avian influenza strains were to enter the milk supply, the consequences could be severe, particularly given the global scale of annual flu outbreaks. Each year, influenza viruses infect tens of millions of people and cause tens of thousands of deaths in the United States alone. The potential for zoonotic transmission, where a virus jumps from animals to humans, makes the risk of consuming raw milk even more concerning.
The Importance of Pasteurization
This study underscores the critical role that pasteurization plays in safeguarding public health. Pasteurization—the process of heating milk to a specific temperature for a set period to kill harmful pathogens—has been one of the most effective public health measures in preventing the spread of foodborne illnesses. The researchers found that pasteurization completely destroyed infectious influenza viruses in milk, significantly reducing the potential for transmission. Even though pasteurization did not completely eliminate the viral RNA, it reduced it by almost 90%, underscoring the effectiveness of the process in reducing risk.
“The data from our study clearly show that pasteurization remains a vital public health tool,” said Boehm. “Not only does it eliminate harmful pathogens, but it also significantly reduces the risk of viruses like influenza making their way into the human population.”
Implications for the Dairy Industry and Public Health
As the study points out, the risk of influenza transmission through raw milk is especially pressing as avian influenza continues to spread among livestock worldwide. In recent years, there have been increasing concerns about the potential for bird flu to mutate and become more easily transmissible to humans. While avian influenza has not yet led to a major human pandemic, its ability to jump from birds to mammals, including pigs, and the recent detection of the virus in dairy cattle, suggest that vigilance is necessary to prevent further zoonotic outbreaks.
“These findings highlight the importance of improving monitoring systems for avian influenza, especially as it continues to spread among livestock,” said Boehm. “By improving surveillance, we can better anticipate and mitigate potential risks before they reach the human population.”
The research also builds on earlier work conducted by the same team, which explored the use of wastewater for detecting avian influenza in the environment. The team had previously demonstrated that wastewater from commercial and industrial dairy facilities could serve as an early warning system for the presence of the virus in nearby cattle populations. By analyzing wastewater samples, public health officials can detect the virus and take proactive measures to prevent its spread.
“We never thought that wastewater could be used to detect and respond to zoonotic pathogens circulating in the community,” Boehm added. “It has been amazing to watch our work on detection in wastewater scaled across the United States and the world, helping authorities better track and respond to outbreaks.”
Why This Matters Now
The growing risk of zoonotic diseases like avian influenza calls for urgent action to strengthen monitoring and control systems within the agricultural and food industries. The emergence of new diseases and the evolution of existing pathogens highlight the need for more robust surveillance techniques, particularly as climate change, global travel, and agricultural practices continue to alter the dynamics of infectious disease transmission.
In addition to its implications for public health, the study also emphasizes the need for continued research into the safety of food production and consumption. While raw milk remains popular among certain health-conscious consumers, it is important to consider the potential risks that come with its consumption. Raw milk’s association with serious illness outbreaks, combined with this new evidence about influenza transmission, should encourage consumers to think carefully about the safety of their milk choices.
Source: Stanford University