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 Wednesday.

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.