First U.S. detection of virulent H5N9 strain, at a California duck farm, draws scrutiny as evidence of genetic reassortment that could trigger human outbreaks.

Bird flu cases are still rising in the U.S. as the virus continues to devastate poultry farms. More than 145 million chickens, ducks, turkeys and other fowl have been slaughtered across the United States since the outbreak began in January 2022. As for humans, 67 people have been infected, according to the Centers for Disease Control.

A new strain of bird flu has been confirmed at a duck farm in California, the first time the variant has been discovered in poultry in the United States, an international agency said.