An elderly patient in Louisiana is in "critical condition" with severe avian influenza, US authorities announced Wednesday, the first serious human case in the country as fears grow of a possible bird flu pandemic.

The U.S. Centers for Disease Control and Prevention said on Wednesday that a patient has been hospitalized with a severe case of H5N1 infection in Louisiana, marking the first known instance of a severe human illness linked to the bird flu virus in the United States.

Health officials have confirmed the first severe case of H5N1 bird flu in the United States this year. The Centers for Disease Control and Prevention (CDC) announced the infection on Fri., Dec. 13, following contact with sick and dead birds from a backyard flock.