ATLANTA — The Centers for Disease Control and Prevention (CDC) confirmed that a new human case of the H5N1 strain of avian influenza has been reported in Louisiana — the first in the state and, notably, the first severe case in the country, requiring hospitalization to treat the infected person.

During a press call on Dec. 18, Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases at CDC, described what classified this case as severe as opposed to the other 60 cases previously confirmed in the United States.

“Hospitalization related to influenza is something that we would consider as severe,” he said. “This person was hospitalized because of their other influenza infection, which I think is notable.”

As a distinction, Daskalakis pointed out that a Missouri resident who contracted H5N1 was also hospitalized, but the cause was not determined to be from the infection, so the case was not classified as severe.

Specific symptoms of the patient have yet to have been disclosed by the Louisiana Department of Health. However, what is known is that the patient had exposure to sick and dead birds in backyard flocks. The CDC said an investigation into the source of the infection is still ongoing, but this is the first case of H5N1 in the United States linked to exposure to a backyard flock.

Just a couple days before the CDC’s announcement, the first domestic flock in Louisiana reported an outbreak of highly pathogenic avian influenza (HPAI) in Bossier Parish.

Partial viral genome data from the CDC indicates that the virus that infected the Louisiana patient belongs to the D1.1 genotype. That specific genotype is related to viruses detected in wild birds and poultry in the United States as well as human cases in British Columbia, Canada and Washington state. The D1.1 genotype is different from the B3.13 genotype detected in dairy cows, sporadic human cases in several states as well as some poultry outbreaks in the United States.

A severe case of H5N1 in a person is not unexpected, the CDC said.

“Over the 20 plus years of global experience with this virus, H5 infection has previously been associated with severe illness in other countries, including illnesses that resulted in death in up to 50% of cases,” Daskalakis said.

Despite this progression in severity of an H5 infection found in the United States, the CDC maintains the same stance that it has held since the beginning of the outbreak, that the public risk remains low.

“This case underscores that, in addition to affected commercial poultry and dairy operations, wild birds and backyard flocks also can be a source of exposure,” the agency said. “People with work or recreational exposures to infected animals are at higher risk of infection and should follow CDC's recommended precautions when around animals that are infected or potentially infected with H5N1 avian influenza virus. This means that backyard flock owners, hunters and other bird enthusiasts should also take precautions.”

Since April 2024, the CDC has confirmed a total of 61 reported human cases of H5N1 across eight states.