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.

A version of the H5N1 virus that has killed one person and severely sickened another has been detected in milk samples collected from Nevada dairy herds.

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.

A new strain of the H5N1 bird flu virus has been detected in milk samples collected from dairy cows in Nevada.

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.

The USDA said four Nevada dairy herds were infected with an H5N1 bird flu strain that has circulated in wild birds, making prospects for containment in cows appear dim.

The U.S. Department of Agriculture has detected a bird flu strain in dairy cattle that previously had not been seen in cows, the agency's Animal and Plant Health Inspection Service said 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.

Dairy cows in Nevada have been infected with a new strain of bird flu, marking the second strain detected in herds since March.

A version of the avian influenza virus different from the one rampaging through herds since last spring has been detected in dairy cows in Nevada, USDA says.