How are H5N9 and H5N1 different? What to know after California bird flu outbreak

A California duck farm made headlines this week after the World Organization of Animal Health published a report by U.S. authorities that a strain of bird flu that scientists call H5N9 had been found among sick birds in the flock.

This is far from the first time that H5N9 has been found in birds around the country. But the U.S. Department of Agriculture said it was the first time that H5N9 had been found in American poultry sick with "highly pathogenic avian influenza," meaning it caused severe disease.

"Clinical signs included increased mortality. State officials have quarantined the affected premises," the USDA said of the flock.