The patient, a Louisiana man over the age of 65 who had underlying medical conditions, contracted the virus from a backyard flock and wild birds, the Louisiana Department of Health noted in reporting his death January 6. Further investigation showed no person-to-person transmission of the virus.

The patient, the country’s first severe case, was hospitalized with respiratory symptoms in December, and state health officials reported he had highly pathogenic avian influenza, or H5N1, on December 13.

Since early 2024, there have been 66 confirmed bird flu cases in people in the United States. The currently circulating strain has been widespread among cattle, poultry and wild birds. Most human cases have been in farm workers and resulted in mild symptoms, such as pink eye, fever and a cough.

Genetic analyses of viral specimens from the Louisiana patient showed that he had caught what’s called the D1.1 virus subtype, which has been circulating in poultry and wild birds in the United States. A 13-year-old girl in British Columbia, Canada, hospitalized with critical illness from bird flu in November, also had this version of the virus. She no longer needs intensive support but remains hospitalized. Another virus subtype, called B3.13, is spreading primarily in dairy cows.

Within the Louisiana patient, the virus acquired mutations that could potentially increase person-to-person transmission of the virus, according to the U.S. Centers for Disease Control and Prevention. But the genetic changes aren’t too concerning because they arose later in the course of infection, when he was already diagnosed and taking precautions to prevent spread, says virologist Angela Rasmussen of the University of Saskatchewan in Saskatoon, Canada.