A highly fatal form of avian influenza, or bird flu, has been confirmed in U.S. cattle in Texas and Kansas, the Department of Agriculture announced on Monday.

It is the first time that cows infected with the virus have been identified.

The cows appear to have been infected by wild birds, and dead birds were reported on some farms, the agency said. The results were announced after multiple federal and state agencies began investigating reports of sick cows in Texas, Kansas and New Mexico.

In several cases, the virus was detected in unpasteurized samples of milk collected from sick cows. Because pasteurization kills viruses, officials emphasized that there was little risk to the nation’s milk supply.

“At this stage, there is no concern about the safety of the commercial milk supply or that this circumstance poses a risk to consumer health,” the agency said in a statement.

Outside experts agreed. “It has only been found in milk that is grossly abnormal,” said Dr. Jim Lowe, a veterinarian and influenza researcher at the College of Veterinary Medicine at the University of Illinois at Urbana-Champaign.

In those cases, the milk was described as thick and syrupy, he said, and was discarded. The agency said that dairies are required to divert or destroy milk from sick animals.

The cattle infections come on the heels of the nation’s first detection of highly pathogenic bird flu in goats, which was announced by Minnesota officials last week.

So far, the flu samples from sick cows have not contained genetic mutations that are known to make the virus more likely to infect humans, the agriculture agency said, adding that the risk to the general public remained low.

“There’s still no cause to panic,” said Stacey L. Schultz-Cherry, a virologist and influenza expert at St. Jude Children’s Research Hospital. “It just looks like it’s another spillover event due to contact with diseased wild birds.”

Still, she noted, cows were not thought to be among the species that were particularly susceptible to avian influenza, and the cases were another worrisome turn in a global bird flu outbreak that has devastated wild bird populations over the last few years.

The outbreak has been caused by a new form of bird flu virus, known as H5N1, that emerged in Europe in 2020. Wild birds can spread the virus, through their feces and oral secretions, to farmed poultry and other animals. Outbreaks often flare up in the spring and summer, when migrating birds are on the move.

Although avian influenza viruses are adapted to spread primarily among birds, the new version of H5N1 has become so widespread in wild birds that it has also repeatedly spread to mammals, especially scavenging species, such as foxes, that might feed on infected birds.

Infections of mammals, which give the virus new opportunities to evolve, are always a source of some concern, Andrew Bowman, a molecular epidemiologist and influenza expert at Ohio State University, said. Scientists have long been concerned that a bird flu virus that evolved to spread more efficiently among mammals, including humans, could set off the next pandemic.

At this point, Dr. Bowman said, it remains unclear whether the infected cows have all picked up the virus directly from birds or whether the virus is also spreading from cow to cow.

“That’s a question that’s going to have to get resolved quickly,” he said. “If we have transmission cattle to cattle, that’s a different story. That certainly makes me a little more nervous.”

Additional testing and analysis are underway. “This is a rapidly evolving situation, and U.S.D.A. and federal and state partners will continue to share additional updates as soon as information becomes available,” the agency said.