The United States have now seen dozens of case of human bird flu this yearthey were all gentle—until now.

The Centers for Disease Control and Prevention confirmed Wednesday that a patient in Louisiana was 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 ravaged poultry and wild bird flocks across the country and infected more than 800 dairy cows in 16 states. Infected animals spread the virus to people who come in contact with them. Since April, the US has seen totals 61 reported human cases of bird flu in eight states. Of these, 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 recovered completely.

A severe case is significant because bird flu has previously been associated with severe illness in other countries, including outbreaks that resulted in deaths of up to 50 percent of cases. From 2003 to 2023, of the 878 people who tested positive for the virus, 458 died.

An investigation by the Louisiana Department of Health and CDC determined that the hospitalized patient, a resident of southwest Louisiana, had exposure to sick and dead birds in backyard flocks. This is the first case of H5N1 bird flu in the US linked to exposure in a backyard flock, rather than a commercial farm.

“While the investigation into the source of this infection in Louisiana continues, it is believed that the patient 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. Diseases, during the press briefing last 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 said it was due to underlying medical conditions. The patient had no respiratory symptoms and no severe illness from their infection. “In the case of Missouri, we don’t really have the same kind of data that supports that it’s related to influenza infection,” Daskalakis said.

There are genetic similarities between the virus from the Louisiana patient and the virus from a Canadian teenager who was hospitalized with H5N1. Scientists classified the Louisiana virus as type D1.1, the same type found in the Canadian patient and another case from Washington state. This variant has also been found in wild birds and chickens in the US.

This is different from the B3.13 strain, which has been detected in dairy cows, some poultry outbreaks, and in human cases in several states. CDC scientists conducted additional genomic sequencing of the Louisiana patient’s viral sample. Genomic sequencing can identify potential changes in the virus that indicate an increased ability to infect people or be transmitted from person to person.

So far, no human-to-human spread of H5N1 bird flu has been detected. The CDC says the immediate public health risk remains low, but those with occupational or recreational exposure to infected animals are at higher risk of contracting the virus. “This means backyard flock owners, hunters, and other bird enthusiasts should also take precautions,” the agency said in a statement.