Federal food safety regulators believe that retail milk is still safe to consume, despite the fact that 1 in 5 samples tested nationwide by the U.S. Food and Drug Administration show traces of highly pathogenic avian influenza’s viral fragments. Most of the positive results showed up in areas where an outbreak of bird flu has been found in dairy cattle herds.

The U.S. Department of Agriculture, the FDA and the Centers for Disease Control and Prevention are partnering with state agencies to investigate the outbreak, which has infected dairy cows in multiple states, according to an FDA advisory. The notice said that the viral infection in the cattle “is causing decreased lactation, low appetite and other symptoms in infected cattle.”

After the milk sampling returned findings of viral fragments in so much of the milk supply, the USDA ordered all dairy cows be tested for bird flu before they can be transported between states. Any positive finding must be reported to federal health officials.

USDA and FDA food safety experts say that current information suggests the commercial supply is safe “because of these two reasons: 1) the pasteurization process and 2) the diversion or destruction of milk from sick cows.”

Pasteurization, a heating process, is done to kill pathogens “to a level that does not pose a risk to consumer health.” The process of pasteurizing milk is more than a century old. The regulators said while pasteurization “is likely to inactivate the virus,” it’s not expected to remove viral particles like those detected, so the finding in milk samples was not especially surprising.

The agencies said they’re vetting their determination that pasteurized retail milk is safe by checking positive findings using another type of test called egg inoculation, which is deemed “a gold standard for determining viable virus.”

In that test, samples of the milk that tested positive are injected into an egg to see if the virus replicates, as Nam Tran, senior director of clinical pathology at UC Davis Health, explained in a news release. UC Davis said the results from that test are more sensitive, but the test takes longer to complete, so results are pending.

The FDA said results from its various studies will be available in a few days to weeks.

History of the outbreak

By late this week, 33 herds in eight states tested positive for bird flu. The states were Idaho, Kansas, Michigan, New Mexico, North Carolina, South Dakota, Ohio and Texas.

There are many strains of avian influenza, but the one that’s the focus of concern is a type A influenza designated H5N1 or A(H5N1). It’s worrisome because while it doesn’t easily spread to humans — in the U.S. there are just two known recent cases — globally more than half of the people who contracted the illness died, per the CDC. The U.S. cases, one this year related to the cattle outbreak and another in 2022, have both produced just mild symptoms.

The virus does not transfer easily from person to person, according to health experts. CDC has reported, however, that while human infections are rare, they “have occurred sporadically worldwide. CDC has been monitoring for illness among people exposed to H5 virus-infected birds since outbreaks were first detected in U.S. wild birds and poultry in late 2021. Human illnesses with H5N1 bird flu have ranged from mild (e.g., eye infection, upper respiratory symptoms) to severe illness (e.g., pneumonia) that have resulted in death in other countries.”

Dr. Dean Blumberg, chief of pediatric infectious diseases at UC Davis Health, said human infections “can be effectively treated with the antiviral oseltamivir (Tamiflu).”

CDC notes that H5 bird flu is widespread among wild birds around the world, including in the U.S.

But health officials are concerned because viruses tend to mutate and a change that made it easier for the strain of influenza to infect humans could be dire. Bird flu has already been detected in a number of types of mammals — examples include mink, red foxes, bears, cows and mountain lions, among others — though primarily in small numbers.

Officials have warned against consuming raw dairy milk, since it’s not known whether H5N1 can be transmitted that way. It is believed that some infected animals, including sea lions, contracted illness through consuming infected wild birds, for example.

Underestimating size of outbreak?

Experts believe the outbreak in cattle may have been underway longer than previously known, in part based on viral genome sequences the USDA released Sunday. As STAT News reported, “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,” said Angie Rasmussen, a virologist studying emerging zoonotic pathogens at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada.

“It may also signal that herds can be infectious with only mild symptoms or no symptoms at all, which would complicate the response and make containment much more difficult,” per the article.

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Some experts question if all the infected herds have been found. A veterinary epidemiologist at Ohio State University, Andrew Bowman, and one of his graduate students figured a complete accounting of the outbreak was unlikely, so they collected 150 commercial milk products around the Midwest and did their own testing.

They told STAT that the samples included milk products from processing plants in 10 different states and they found viral RNA from bird flu in well over a third of the samples.

Per STAT, “The researchers expect additional lab studies currently underway to show that those samples don’t contain live virus with the capability to cause human infections, meaning that the risk of pasteurized milk to consumer health is still very low. But the prevalence of viral genetic material in the products they sampled suggests that the H5N1 outbreak is likely far more widespread in dairy cows than official counts indicate.”

NBC News quoted a flu virologist at St. Jude Children’s Research Hospital, Richard Webby, who noted that the number of milk samples that tested positive for viral fragments “does seem high if the number of infected farms is indeed only 30-odd. Clearly there are more infected animals out there than being reported.”

Of the outbreak, “I think it’s safe to say that it’s longer and much more extensive that has been realized,” Dr. Michael Mina, an epidemiologist and former professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, told reporters during a briefing midweek.

But again, health experts are not unduly worried about human health related to avian influenza in cattle at the moment.

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“CDC has not identified any out-of-the-ordinary influenza-related emergency department visits, even when we compare areas where H5N1 has spread among cattle against areas where it has not,” Dr. Nirav Shah, principal deputy director of the CDC, said during the media briefing.

Safety advice

As for staying safe, CDC recommends that people avoid entirely or use respiratory and eye protection around sick or dead birds and animals, domesticated or wild, as well as “animal feces, litter or materials contaminated by birds or other animals” that might have bird flu infection.

CDC also warns against preparing or eating uncooked or undercooked food or “related uncooked food products, such as unpasteurized (raw) milk or raw cheeses” from animals with suspected or confirmed bird flu.

For farmers or those with livestock or backyard flocks, the safety recommendations extend to surfaces and water that might be contaminated with animal excretion, as well.

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