California officials have declared a state of emergency over the spread of bird flu, which is tearing through dairy cows in that state and causing sporadic illnesses in people in the U.S.

New research shows that the highly contagious bird flu, found in the nation’s first severe human case, shows signs of a rare mutation, amid growing concern over pets contracting the disease. A genetic analysis suggests the bird flu virus mutated inside a Louisiana patient who contracted the first severe case of the illness in the United States.

The Atlanta-based CDC says the case is the first bird flu infection in the United States that has been linked to backyard birds.