A genetic analysis suggests the bird flu virus mutated inside a Louisiana patient who contracted the nation’s first severe case of the illness, the U.S. Centers for Disease Control and Prevention said this week.

Growing concerns surrounding the transmission of H5N1 bird flu. The CDC reports that samples collected from a severely ill patient in Louisiana showed signs of mutation, as they differ from the samples collected from sick and dead birds believed to have infected the man in the first place.

The CDC has confirmed a total of 66 human cases of bird flu across the United States as of Friday, although the number is believed to be higher.