The strain of bird flu is distinct from what has previously been found in dairy cattle. The finding means the virus has again spilled over from birds to cows, and it raises some worrying questions.

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.