One in Five Milk Samples Has Bird Flu Virus Fragments, Suggesting Cow Infections Are More Widespread Than Thought

The research has not yet found evidence that milk contains infectious virus, and the FDA says the commercial milk supply is safe

A refrigerator of milk in a grocery store
Experts say the pasteurization process likely kills the virus, and tests are likely just detecting remnants of the dead virus. Spencer Platt / Getty Images

One in five samples of grocery store milk tested positive for bird flu virus, according to initial results from a nationally representative survey conducted by the Food and Drug Administration (FDA), the agency announced Thursday.

Preliminary research suggests the virus in grocery store milk is not infectious. “To date, the retail milk studies have shown no results that would change our assessment that the commercial milk supply is safe,” the FDA says in the statement.

The agency’s announcement did not include details on how many cows have been tested. But the early results indicate infections in cows are more widespread than previously thought—so far, outbreaks have been detected in 33 herds across eight states.

“The discovery of bird flu virus fragments in commercial milk is significant, not because it poses a direct threat to public health, but because it indicates a broader exposure among dairy cattle than we previously understood,” John Brownstein, an epidemiologist at Boston Children’s Hospital, says to ABC News’ Mary Kekatos and Youri Benadjaoud. “This calls for an expanded surveillance of both the virus’ presence and its potential impact on food safety.”

“The number does seem high if the number of infected farms is indeed only 30-odd,” Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital, tells NBC News’ Berkeley Lovelace Jr. “Clearly there are more infected animals out there than being reported.”

The FDA first announced that researchers had found evidence of bird flu virus in pasteurized milk on Tuesday. A greater proportion of milk samples tested positive in areas with infected cows, the agency added in its new statement.

Officials are conducting quantitative polymerase chain reaction (qPCR) tests that detect genetic material in the sample, but these do not necessarily indicate the presence of infectious virus. The tests might only be picking up remains of pathogens killed off by the pasteurization process, which involves heating the milk.

Jeanne Marrazzo, director of the National Institute of Allergy and Infectious Diseases, said at a Wednesday press conference that research had not found evidence of live virus in grocery store milk, suggesting pasteurization had killed it, per the New York Times’ Emily Anthes and Noah Weiland.

It’s not surprising to find genetic remains of the virus after pasteurization, “but that by itself does not at all suggest a public health concern,” Michael Osterholm, an epidemiologist at the University of Minnesota, tells Science News’ Tina Hesman Saey.

To determine the presence of live virus, the agency is also conducting tests that involve injecting chicken eggs with samples and monitoring whether the virus replicates.

New genetic evidence also suggests that infections in cows may have started as early as last fall, much longer ago than previously thought, reports Science’s Jon Cohen. Officials confirmed the first dairy cow infections in late March.

“Both of these data—the milk data and the genetic data that shows this has been around since December of last year—suggests that the outbreak is probably much bigger than we know,” Angie Rasmussen, a virologist at the University of Saskatchewan in Canada, tells STAT News’ Megan Molteni.

In response to the threat of bird flu spreading in cows, the Department of Agriculture (USDA) announced Wednesday that dairy cattle will have to be tested for bird flu before crossing state lines, and labs and veterinarians will have to report positive test results in livestock to the USDA.

But these rules don’t apply to healthy-looking cows that are not traveling, meaning that dairy farmers could miss infections in some cattle, Jennifer Nuzzo, an epidemiologist at Brown University, tells STAT News.

“I have not seen evidence that makes me want to discard the fear that testing practices are absolutely shaping what we think we know about this virus,” Nuzzo tells the publication. “We just don’t have the right data right now to tell us what’s going on.”

If the virus spreads widely between cows, that gives it more chances to mutate in a way that could make it more transmissible among humans, writes the New York Times. So far, just one person has tested positive from exposure to dairy cows, and the Centers for Disease Control and Prevention has not changed its assessment that the risk to the general public is low. The patient is recovering after reporting eye redness as their only symptom.

“What we are concerned about is adaptation of the virus to better suit some of the receptors that humans might have, which is why any transmission out of bird populations into any mammal is initial cause for concern,” Meghan Davis, an environmental epidemiologist at Johns Hopkins University, says to Science News.

“We need to do what we can now to understand it and contain it, so it doesn’t turn into a pathogen of pandemic potential,” Keith Poulsen, who studies infectious diseases at the University of Wisconsin, tells STAT News.

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