Dairy cows in Nevada have been infected with a new strain of bird flu virus different from the one circulating in other herds throughout the past year, the U.S. Department of Agriculture (USDA)

U.S. dairy cattle tested positive for a strain of bird flu that previously had not been seen in cows, the U.S. Department of Agriculture said.

Experts said it raises news questions about wider spread and the difficulty of controlling infections in animals and the people who work closely with them.