The United States has now seen dozens of human bird flu cases this year, all of them mild—until now.
The Centers for Disease Control and Prevention confirmed Wednesday that a patient in Louisiana has been hospitalized with a severe case of avian influenza caused by the H5N1 virus. This marks the first instance of severe illness linked to the virus in the US.
The virus has decimated poultry flocks and wild birds across the country and has infected more than 800 dairy herds in 16 states. Infected animals have been spreading the virus to people who come into contact with them. Since April, the US has seen a total of 61 reported human cases of bird flu in eight states. Of those, 37 had exposure to sick or infected dairy cows, while 21 had exposure to poultry farms and culling operations. In those cases, people developed conjunctivitis and mild respiratory symptoms and fully recovered.
A severe case is significant because bird flu has previously been associated with severe illness in other countries, including outbreaks that resulted in death in up to 50 percent of cases. From 2003 to 2023, of the 878 people who tested positive for the virus, 458 deaths were reported.
An investigation by the Louisiana Department of Health and CDC has determined that the hospitalized patient, a resident of southwestern Louisiana, had exposure to sick and dead birds in backyard flocks. This is the first case of H5N1 bird flu in the US that has been linked to exposure to a backyard flock, rather than a commercial farm.
“While an investigation into the source of this infection in Louisiana is ongoing, it is believed that the patient that was reported by Louisiana had exposure to sick or dead birds on their property,” said Demetre Daskalakis, director of the CDC’s National Center for Immunization and Respiratory Diseases, during a press briefing on Wednesday. No other details were available on the source of exposure or the patient’s condition.
A person with bird flu in Missouri was hospitalized in late August, but CDC officials say it was due to underlying medical conditions. The patient did not have respiratory symptoms and was not severely ill from their infection. “In the Missouri case, we don’t really have that same sort of data that supports that it was related to their influenza infection,” Daskalakis said.
There are genetic similarities between the virus from the Louisiana patient and the virus from a teenager in Canada who was hospitalized with H5N1. Scientists have categorized the virus in Louisiana as type D1.1, the same type found in the Canadian patient and another case from Washington state. This variant has also been detected in wild birds and poultry in the US.
This is different from the B3.13 type, which has been detected in dairy cows, some poultry outbreaks, and in sporadic human cases in multiple states. CDC scientists are carrying out additional genomic sequencing of the Louisiana patient’s viral sample. Genomic sequencing can identify potentially concerning changes in the virus that would signal an increased ability to infect humans or be transmitted from person to person.
So far, no person-to-person spread of H5N1 bird flu has been detected. The CDC says the immediate risk to the public’s health remains low, but those with work or recreational exposure to infected animals are at higher risk of coming down with the virus. “This means that backyard flock owners, hunters, and other bird enthusiasts should also take precautions,” the agency said in a statement.