Dairy cattle in Nevada have been infected with a new type of bird flu that’s different from the version that has spread in U.S. herds since last year, Agriculture Department officials said Feb. 5.

The detection indicates that distinct forms of the virus known as Type A H5N1 have spilled over from wild birds into cattle at least twice. Experts said it raises new questions about wider spread and the difficulty of controlling infections in animals and the people who work closely with them.

“I always thought one bird-to-cow transmission was a very rare event. Seems that may not be the case,” said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital.

A version of the H5N1 bird flu virus known as B3.13 was confirmed in March after being introduced to cattle in late 2023, scientists said. It has infected more than 950 herds in 16 states. The new version, known as D1.1, was confirmed in Nevada cattle, according to USDA. It was detected in milk collected as part of a surveillance program launched in December.

“Now we know why it’s really important to test and continue testing,” said Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada, who helped identify the first spillover.

The D1.1 version of the virus was the type linked to the first U.S. death tied to bird flu and a severe illness in Canada. A person in Louisiana died in January after developing severe respiratory symptoms following contact with wild and backyard birds. In British Columbia, a teen girl was hospitalized for months with a virus traced to poultry.

At least 67 people in the U.S. have been infected with bird flu, mostly those who work closely with dairy or cattle, according to the U.S. Centers for Disease Control and Prevention.

USDA officials said they would post genetic sequences and other information about the new form of the virus to a public repository later this week. Scientists said that would be key to understanding whether the spillover was a recent event or whether the virus has been circulating, perhaps widely, for longer.

“If this turns out to have been something that crossed into cattle a couple months ago, a couple months is a long time not to detect it,” said Michael Worobey, an evolutionary biologist at the University of Arizona who has studied the H5N1 virus in cattle.

He added that it’s important for federal officials to share promptly information about a virus that has the potential to trigger a pandemic that could “make COVID seem like a walk in the park.”

“It’s a vital part of national security, global security, the well-being of people, of animals and of businesses in the U.S.,” Worobey added.