H5N9 is a rare subtype of the influenza A virus that can cause highly pathogenic avian influenza (HPAI), also known as bird flu. It's a reassortment strain that originated from the H5N1, H7N9, and H9N2 subtypes.

The United States has reported its first H5N9 bird flu outbreak in California, affecting almost 119,000 ducks. Both H5N9 and the more common H5N1 strains were found. The USDA is conducting investigations and enhanced surveillance,

Cases have been spreading across the country since April 2024 with 67 confirmed as of Thursday, according to the Centers for Disease Control and Prevention (CDC).