Bird Flu Virus Detected in Pasteurized Milk, as U.S. Moves to Test More Dairy Cows

The FDA maintains that the commercial milk supply is safe, and it plans to report results of further tests in the coming days and weeks

Black and white cows
Bird flu was first detected in U.S. dairy cows in March. So far, infections have been detected in 33 herds across eight states. Derek Davis / Portland Press Herald via Getty Images

The Food and Drug Administration (FDA) has found evidence of bird flu virus in pasteurized milk, but it says the milk supply is still safe to drink, according to a statement from the agency on Tuesday.

While the tests identified genetic material from the bird flu virus in milk, the FDA suggests these are leftover particles from inactive virus that had been killed during pasteurization, rather than infectious virus. The agency is continuing tests to get more information.

“To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the FDA’s statement says.

“There is no evidence to date that this is infectious virus, and the FDA is following up on that,” Lee-Ann Jaykus, a food microbiologist and virologist at North Carolina State University, tells Jonel Aleccia of the Associated Press (AP).

Milk sold in grocery stores in the United States has undergone pasteurization, in which it is heated to kill viruses and bacteria. This process is expected to kill the avian flu virus—which was detected in dairy cows last month—but it likely won’t remove all traces of viral particles, per the FDA.

This residual genetic material poses little risk to milk drinkers, David O’Connor, a virologist at the University of Wisconsin-Madison, tells the New York Times’ Emily Anthes, Apoorva Mandavilli and Noah Weiland.

“The risk of getting infected from milk that has viral fragments in it should be nil,” he tells the publication. “The genetic material can’t replicate on its own.”

Still, officials are conducting additional tests to make sure infectious virus is not present in the milk. The FDA is currently injecting chicken eggs with the viral samples to see whether they replicate, which it calls the “gold standard” for detecting active virus.

“Pasteurization kills much sturdier viruses than influenza, so we expect it to work,” Andrew Pekosz, a molecular biologist who studies respiratory viruses at Johns Hopkins University, tells Stat News’ Helen Branswell, Nicholas Florko, Megan Molteni and Rachel Cohrs Zhang. “But it’d be great to have the data.”

The FDA says it will make results from these studies available in the coming days or weeks.

Of greater concern is that the virus is “showing up in a lot more samples, meaning the infection is more widespread in dairy herds than we thought,” a public health official who spoke on the condition of anonymity to share information that wasn’t public, says to the Washington Post’s Lena H. Sun, Rachel Roubein and Dan Diamond.

Following the FDA’s announcement, the Biden administration is now requiring dairy cows to be tested for bird flu before they are transported between states.

A month ago, officials confirmed that dairy cows had been infected with bird flu in Texas and Kansas, with symptoms including decreased lactation and low appetite. As of this week, officials have detected infections in 33 dairy herds across eight states: Idaho, New Mexico, South Dakota, Kansas, Texas, Michigan, Ohio and North Carolina, per the Centers for Disease Control and Prevention (CDC).

In 2022, avian flu was detected in wild birds in the U.S. for the first time since 2016. Over the last couple of years, the virus has been identified in more than 9,000 wild birds in the U.S., and it has affected more than 90 million domestic birds across 48 states. Worldwide, more than 131 million domestic poultry died due to infection or culling to stop the spread of avian flu in 2022 alone. Reports of mammals with avian flu raised concerns that the virus could evolve and jump to humans.

But so far, analyses of the viruses detected in dairy cattle have found no changes that would make it more infectious to humans, according to the U.S. Department of Agriculture (USDA). Only one person has been infected after exposure to sick cattle, and pink eye was their only symptom. The risk to the general public remains low, per the CDC.

Nearly all of the commercial milk supply made at U.S. dairy farms follows the Pasteurized Milk Ordinance, “which includes controls that help ensure the safety of dairy products,” the FDA says.

Dairy producers have been diverting and destroying milk from sick cows to keep the milk supply safe, the agency adds. And in addition to being pasteurized, milk from any individual cow becomes diluted as it mixes with milk from other cows.

“With a virus like this, I would have to believe even if you had the highest levels of virus activity you could ever imagine in the actual milk from the udder of an infected cow, it would be diluted millions of times over going into pasteurization,” Michael Osterholm, an epidemiologist at the University of Minnesota, tells the New York Times.

Still, scientists have recently argued the USDA isn’t sharing enough information about the spread of bird flu into cattle and that it is sharing information too slowly, writes CNN’s Brenda Goodman.

Flu is a “fairly wimpy virus,” so it’s “fairly readily inactivated,” Richard J. Webby, a virologist at St. Jude Children’s Research Hospital, tells the Washington Post. “But that’s something that has to be tested.”

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