Unresolved Avian Influenza Persists in Unpasteurized Dairy for Five Days
Unpasteurized milk might be more hazardous than commonly believed. Researchers have discovered that influenza viruses that contaminate milk can infect individuals for nearly a week.
Researchers from Stanford University conducted this study to determine the severity of being exposed to influenza through tainted milk. They discovered that a particular strain of influenza A virus remained infectious for five days in refrigerated raw milk. These findings suggest that raw milk could serve as a conduit for infectious influenza strains, particularly avian influenza viruses that are currently spreading among dairy cows.
Pasteurization is a straightforward and brief heating process used to eliminate microbes responsible for spoilage or food poisoning in foods and liquids. Numerous studies have demonstrated that unpasteurized, or raw, dairy products can transmit harmful germs. For example, a 2018 study showed that raw milk and cheese products accounted for 96% of all dairy-related foodborne illnesses reported over a five-year period.
The appearance of H5N1, a highly pathogenic avian influenza A (HPAI) strain, in dairy cows this year has increased the risks associated with raw milk. H5N1 has been detected in commercially available raw milk products, and some animals, such as farm cats, are suspected of contracting H5N1 by drinking raw milk. However, researchers at Stanford University claim that their study is the first to examine the longevity of influenza in raw milk under more realistic conditions for consumers.
The researchers introduced an H1N1 strain of influenza A virus into raw milk samples, using a dose similar to those found in contaminated store-brand milk products (which, thanks to pasteurization, were free of infectious viruses). They refrigerated the samples and tracked how long it took for the virus levels to decrease below an infectious threshold. They also tested how pasteurization affected the virus's viability.
The researchers found that pasteurization completely eradicated the presence of any infectious influenza virus. However, the raw milk samples remained infectious for up to five days.
"Overall, our study demonstrates that influenza viruses remain infectious in raw milk, posing a significant human health risk," the researchers wrote in their paper, published this month in the journal Environmental Science & Technology Letters.
The study examined a different strain of influenza A than H5N1. However, other studies suggest that the infectiousness of both strains decreases at a similar rate in milk, and that influenza A viruses in general do not significantly differ in this regard. Therefore, H1N1 can likely serve as a substitute for H5N1.
As of early December, outbreaks of H5N1 among dairy cows have been reported in 16 states this year. California and Nevada reported cases in the past month, and the U.S. has documented 60 human cases of H5N1, most linked to contact with infected cows or poultry.
Initially, human cases have been mild, and the virus has not yet adapted to spread easily between people.
However, more serious cases of H5N1 have been reported as well. The longer these strains are allowed to spread among cows and other mammals, the greater the risk of a highly contagious version of H5N1 emerging and causing a widespread epidemic—a risk that is being amplified by the continued popularity of raw milk. Approximately 4% of Americans consume raw dairy products at least once a year, with 1% consuming them regularly.
"This work highlights the potential risk of avian influenza transmission through the consumption of raw milk and the importance of milk pasteurization," said senior researcher Alexandria Boehm in a statement from the university.
Unfortunately, the same people who love raw milk products are also likely to disregard any warnings about them. Sales of raw milk have skyrocketed since the emergence of H5N1 this year, even in the face of recalls linked to bird flu. Moreover, many advocates of raw milk continue to spread misinformation about its supposed benefits over pasteurized milk, such as boosting people's immune systems.
The study's findings indicate that raw milk could potentially serve as a vector for infectious influenza strains, including avian influenza viruses, in the future due to their longevity in raw milk. Despite the health risks associated with unpasteurized dairy products, there continues to be a significant consumer base for raw milk, with around 4% of Americans consuming raw dairy products at least once a year.
