Infectious Bird Flu Can Linger in Refrigerated Raw Milk for 5 Days

Raw milk might be more dangerous than assumed. Researchers just found evidence that influenza viruses that end up in raw milk can infect people for almost up to a week.

Scientists at Stanford University conducted the study, which aimed to quantify the threat of being exposed to influenza through contaminated milk. They found that one particular strain of influenza A virus was still infectious after five days in refrigerated raw milk. The findings suggest that raw milk is a viable transmission route for similar influenza strains—particularly bird flu viruses that are now actively spreading among dairy cows.

Pasteurization is the simple and brief heating of foods and liquids to kill off microbes that can cause spoilage or food poisoning. Unsurprisingly, extensive research has shown that unpasteurized, or raw, dairy products can spread harmful germs. A 2018 study, for instance, found that raw milk and cheese products accounted for 96% of all reported foodborne illnesses traced back to dairy over a five-year span.

The emergence of H5N1, a strain of highly pathogenic avian influenza A (HPAI), in dairy cows this year has made raw milk even more of a gamble. People have already found H5N1 in commercially sold raw milk products. And some animals, particularly cats living on farms, are suspected of having contracted H5N1 by drinking raw milk. But the Stanford University researchers say theirs is the first research to examine the persistence of influenza in raw milk under more real-world conditions for people.

The researchers seeded raw milk samples with an H1N1 strain of influenza A. They used a starting dose of the virus similar to the doses seen in contaminated store-brand milk products (thanks to pasteurization, though, no infectious virus was found in these products). Then they kept the samples refrigerated at a typical temperature and tracked how long it took for levels of viable virus to drop off before it wouldn’t be able to infect someone any longer. They also tested how pasteurization would affect the viability of the virus.

As with other research, they found that pasteurization fully removed the presence of any infectious influenza virus. But it took up to five days for the raw milk samples to no longer be infectious.

“Overall, our study demonstrates that influenza viruses remain infectious in raw milk, where it could pose a significant human health risk,” the researchers wrote in their paper, published this month in the journal Environmental Science & Technology Letters.

The study did look at a different strain of influenza A than H5N1. But other studies have shown that the infectiousness of both strains seems to decay at about the same rate in milk, and that influenza A viruses in general don’t differ much from each other in that respect. So H1N1 is likely a good surrogate for H5N1.

As of early December, outbreaks of H5N1 among dairy cows have occurred in 16 states this year, though only California and Nevada reported cases in the past month. In the U.S., 60 human cases of H5N1 have been documented, most linked to contact with infected cows or poultry.

So far, human cases have generally been mild, and the virus isn’t believed to have adapted to spread easily between people yet.

But there have been more serious cases of H5N1 reported as well. And the longer these strains are allowed to spread among cows and other mammals like us, the greater the risk that a nightmare version of H5N1 will emerge and set off a widespread epidemic—a risk that’s only being amplified by the continued popularity of raw milk. About 4% of Americans are thought to 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 consumption of raw milk and the importance of milk pasteurization,” said senior researcher Alexandria Boehm in a statement from the university.

Unfortunately, the very same people who love raw milk products are also likely to ignore any warning about them. Sales of raw milk have only skyrocketed since the emergence of H5N1 this year, even in the face of recalls tied to bird flu. And many adherents of raw milk continue to spread misinformation about its supposed benefits over pasteurized milk, such as it boosting people’s immunity.