Cows Have Been Infected With a Second Form of Bird Flu

Dairy cows in Nevada have been infected with a new form of bird flu that is distinct from the version that has been spreading through herds over the last year, the U.S. Department of Agriculture announced on Wednesday.

The finding indicates that the virus, known as H5N1, has spilled from birds into cows at least twice — leading to these two sets of infections — and that it could continue to do so. It also suggests that the virus may pose a persistent risk to cows and to the people who work closely with them.

Before last year, scientists did not know that cows were susceptible to this type of influenza.

“This is not what anyone wanted to see,” said Louise Moncla, an evolutionary biologist who studies avian influenza at the University of Pennsylvania. “We need to now consider the possibility that cows are more broadly susceptible to these viruses than we initially thought.”

The news was announced in a news release from the Animal and Plant Health Inspection Service, a division of the Department of Agriculture. Federal agencies have not held a news briefing on bird flu since President Trump took office.

The virus that has been spreading through the nation’s dairies is a version of H5N1 known as B3.13, which has infected more than 950 herds in 16 states. Scientists believe that it initially jumped to cows from birds about a year ago, somewhere in the Texas panhandle. That transition took scientists by surprise, and this new one even more so.

“I was kind of under the belief that the bird-to-cow movement was a pretty rare event,” said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital.

The fact that it has happened again is “a little bit of a ‘wow’ to me,” he added.

The cows in Nevada were infected with a version of the virus known as D1.1, which has been spreading in wild birds and poultry. It was initially detected in milk collected from a silo as part of a national milk testing strategy announced by the U.S.D.A. late last year.

The D1.1 form of H5N1 has also shown itself to be dangerous to people. Of the 67 Americans known to have become ill with H5N1 so far, the only one who died was infected with this version. That person, a Louisiana resident older than 65, had cared for sick and dying birds and died in early January.

In November, a 13-year-old Canadian girl also became infected with the D1.1 virus, but it is unclear where she might have acquired it. Her only risk factor was obesity, but she, too, became seriously ill and was placed on life support because of organ failure. She eventually recovered.

Avian influenza is so called because it is best adapted to infecting birds. But in both these individuals, the virus gained mutations during the course of infection that might allow it to better infect people.

“It is possible that the virus is more permissive for human adaptive mutations,” said Scott Hensley, an immunologist at the University of Pennsylvania.

Reassuringly, the virus did not seem to spread from either person to anyone else. Still, its evolution indicated that it was capable of gaining the ability to efficiently spread among people.

So far, at least, the spread of D1.1 to cows “doesn’t change the average person’s life,” Dr. Moncla said. But it poses risks for dairy workers and the dairy industry, experts said. It also suggests the possibility that cows already infected once with B.3.13 could become ill a second time with D1.1, Dr. Webby said.

“It’s no longer just one virus,” he said. “This, to me, suggests that it’s going to be a lingering problem.”

Since January 2022, when H5N1 was detected in wild aquatic birds in the United States, the virus has affected more than 153 million commercial, backyard and wild birds, resulting in record prices on eggs.

It has also struck dozens of mammalian species, including cats both wild and domesticated, raccoons, bears and sea lions.