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.

The government is investigating a rare strain of avian flu, H5N9, detected on a duck farm in central California, according to the World Organisation for Animal Health. It marks the first reported outbreak of this strain in U.

The H5N9 strain of avian influenza is much more rare than the H5N1 which has been responsible for most of the reported human cases and the first human death.