Six dairy herds in Nevada have tested positive for a newer variant of the H5N1 bird flu virus that’s been associated with severe infections in humans, according to the Nevada Department of Agriculture.

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 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.