The illness shows increased risks from the virus that previously caused eye redness, or conjunctivitis, in infected farm workers.
H5N1 bird flu still represents a low risk to the general public, the US Centers for Disease Control and Prevention said.
CDC has confirmed 61 human cases since April, mostly in workers on dairy farms where the virus infected cattle.
Workers culling infected poultry have also tested positive.
The patient in Louisiana is in critical condition and suffering severe respiratory illness, the Louisiana Department of Health said in a statement.
The person is reported to have underlying medical conditions and is over the age of 65, the department said, putting the patient at higher risk.
The case is the first to be linked to backyard, non-commercial poultry, said Demetre Daskalakis, director of CDC's National Center for Immunization and Respiratory Diseases, on a call with reporters.
The CDC said that a sporadic case of severe H5N1 bird flu illness in a person is not unexpected as has previously been experienced in other countries during 2024 and prior years, including in cases that led to death.
CDC confirms first severe case of H5N1 Bird Flu in the U.S. While an investigation into the source of the infection is ongoing, it has been determined that the patient had exposure to sick and dead birds in backyard flocks.— CDC (@CDCgov) No person-to-person spread of H5 bird flu has been… pic.twitter.com/0jgHKkjQpxDecember 18, 2024
"The mild cases that we've seen in the United States largely reflect that many of the individuals are getting infected by dairy cows and that's very different than getting infected with infected birds," said Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security.
"If you look at the genotype of this patient in Louisiana, it wasn't the cattle strain. It was a wild bird strain."
The CDC said partial viral genome data from the infected patient shows that the virus belongs to the D1.1 genotype, recently detected in wild birds and poultry in the US and in recent human cases in the US state of Washington and Canadian province of British Columbia.
This genotype of the virus is different from the B3.13 genotype detected in dairy cows, human cases in multiple states and some poultry outbreaks in the country, the CDC said.
Bird flu has infected more than 860 dairy herds in 16 US states since March and killed 123 million poultry since the outbreak began in 2022.
There has not been evidence of dairy herds being reinfected once they have cleared the virus, US Department of Agriculture Deputy Under Secretary Eric Deeble said on the press call.
The agency has enrolled 13 states in its newly launched US bulk milk bird flu testing plan, representing nearly half of the country's milk supply, Deeble said.
