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.