Is our milk safe? Here’s what to know about bird flu particles in dairy products.

“To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the agency said in its statement.

Some experts complained that the advisory was vague and did not specify how many samples were collected, or where the infected samples originated.

Here is what you need to know:

There is no reason to stop consuming milk or dairy products, said Dr. Shira Doron, chief infection control officer for Tufts Medicine and hospital epidemiologist at Tufts Medical Center. There’s no research yet specifically measuring the ability of the pasteurization process to eliminate the H5N1 virus. But the process has been shown to work in every other similar virus tested to date. That makes it reasonable to assume it will be effective against H5N1.

“From my own personal standpoint and my family standpoint, I’m not at this point worried. We’re going to continue to consume dairy products, beef, and chicken without any hesitation,” she said. “I don’t think that there is any appreciable risk, at least based on the information I have so far.”

Doron notes that the influenza virus is usually transmitted through the airways, and there is little evidence to date suggesting that it can be contracted through the gastrointestinal system, such as after eating or drinking food contaminated with H5N1.

But Jennifer Nuzzo, a professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health, said transmission of H5N1 through food can’t be ruled out. “They have found the virus in different organs, including the GI tract,” she said. “Is that likely to be a major source of infection? I mean, we’ve not seen major flu outbreaks from gastrointestinal exposure.”

Though it’s too early to tell whether it is possible to contract H5N1 through unpasteurized products, Doron suggests consumers avoid unpasteurized dairy, including raw milk and raw milk cheese, because they are dangerous, even without H5N1.

“It’s never a good idea to consume unpasteurized dairy products, because they could have other live viruses and bacteria that are potentially even more deadly,” she said.

It is not known how the virus, spread from birds to cows. But Michael Worobey, a professor of ecology and evolutionary biology at the University of Arizona, analyzed 239 H5N1 sequences released Sunday night by the USDA for the first time. He said the virus appears to have jumped from a bird to a cow in Texas sometime around the end of 2023. It either had or acquired a mutation that allowed it to be transmitted from cow to cow, he said via email. The transmission mechanism remains a mystery.

“It could well have been a wild bird mixing with cattle and exposing them via feces,” he wrote. “Or a wild bird mixing with domestic poultry, duck, or geese, then the domestic birds exposing cattle. Lots of possibilities that can’t be ruled out yet.”

Nuzzo said some have speculated the disease spread across state lines through the transport of cows between farms. Some have also hypothesized that industrial machinery may somehow be “aerosolizing” the virus during the milking process. But she added: “We just don’t know. It’s all hearsay, really. It’s incredibly frustrating.”

Now that H5N1 has jumped into cattle, it is spreading far and wide, which is why some experts say the presence of genetic fragments of the virus in milk is not surprising. The disease, which is sometimes asymptomatic in cows, can also cause lethargy and a dramatic reduction in milk production. It appears to be far less lethal than it has been in commercial poultry. Most cows recover within two weeks.

As of Wednesday, the USDA had confirmed cases of highly pathogenic bird flu among dairy cattle in eight states: Kansas, Idaho, Michigan, New Mexico, North Carolina, Ohio, South Dakota, and Texas. The virus has not been reported in dairy cattle in Massachusetts.

Bird flu is an influenza virus that primarily infects birds. It was first identified in China in 1996 in domestic waterfowl. In the years since, outbreaks of the highly infectious disease have resulted in the death or destruction of 60 million birds. Human cases are extremely rare.

The current strain of H5N1 is believed to have surfaced in 2020 in Europe, before spreading around the globe, killing millions of birds in the wild and on commercial poultry farms. By the summer of 2022, it had spread to mammals, resulting in the deaths of tens of thousands of seals and sea lions — including more than 180 gray and harbor seals off the coast of Maine. The virus has also been found in cats, forest animals, raccoons, and skunks. In February, the virus was detected in a non-poultry, backyard flock of birds in Essex County. There have been additional reports of dead and dying wild Canada geese with symptoms of the current bird flu strain in Essex County and surrounding coastal areas.

In March, the US Department of Agriculture found it in cows for the first time on farms in Texas, Kansas, New Mexico, Idaho, and Michigan.

To date, only a single dairy worker has been identified as infected with the highly pathogenic strain of H5N1, and his only symptom was conjunctivitis, which Doron called “reassuring.”

But outbreaks of previous strains in humans have been more deadly.

In order for the virus to spread among humans, its shape would likely need to change to allow it to hook onto proteins on the surface of human cells, known as receptors, long enough to inject its genetic cargo into cells located in the human lungs, said Brown’s Nuzzo.

She suggested the best way to prevent the spread would be to equip farm workers with personal protective equipment, including eye protection and masks and to require the equipment for those involved in milking. She also recommended vaccinating workers to prevent them from getting sick in the first place.

“That’s probably one of the single best things we can do to prevent this virus from evolving to become a pandemic strain,” she said.

She expressed frustration with the slow pace of information being put out by the federal government.

“I will just say it feels that the evidence generation process feels to me more oriented around minimizing damage to the reputation of the dairy industry and less about trying to stay ahead of the virus,” she said.

The virus can be picked up by regular flu tests — though further testing in the hospital would be needed to confirm H5N1. Regular flu medications should work against it, though the current annual flu vaccines will not, Doron said. Unlike COVID, the genetic sequence has already been isolated, at least two candidate vaccines are ready to go, and production could be ramped up relatively quickly.

Adam Piore can be reached at adam.piore@globe.com.