USDA milk testing shows different strain of H5N1 bird flu in Nevada dairy herds

The U.S. Department of Agriculture announced Wednesday that four dairy herds in Nevada recently determined to be infected with H5N1 bird flu were in fact infected with a different strain of the virus than has been circulating in cows for the past year.

The discovery, experts said, make it clear driving this virus out of cows will be harder than the USDA has estimated.

The version in the Nevada herds is one that has been circulating in wild birds. It is also the version behind the severe infection of a teenager in British Columbia, Canada, last year, and a fatal infection in Louisiana last month.

Since the beginning of the H5N1 outbreak in dairy cows, the USDA has insisted that all the infections — 957 herds in 16 states to date — trace back to a single introduction of the virus into cows in late 2023 or early 2024, possibly in Texas. Break the chain of transmission, the agency argued, and the outbreak would end.

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