The newly reported H5N9 strain was found Monday at a duck farm in Merced County, California, according to the World Organization for Animal Health. The Paris-based organization that studies animal diseases said it's the first verified U.S. case of H5N9 in poultry. The organization said the more common strain H5N1 was also found.

H5N9 is “not commonly seen in poultry in general,” says Eman Anis, assistant professor at the University of Pennsylvania School of Veterinary Medicine. (Anis is part of a lab that conducts national testing of poultry samples for avian influenza.)