The United States has recorded its first fatal case of H5N1 bird flu, in a person from Louisiana who contracted the virus from infected chickens and wild birds in a backyard flock. The death was reported Monday by the Louisiana Department of Health.
The unidentified person was described as being over the age of 65 and reportedly had underlying medical conditions. What those conditions were and whether they put the person at increased risk of developing a severe infection from influenza is not clear.
The Centers for Disease Control and Prevention said in a statement that the event, while tragic, does not change its position on the current risk posed by the virus.
“CDC has carefully studied the available information about the person who died in Louisiana and continues to assess that the risk to the general public remains low,” the CDC said in a statement.
Flu experts warned that while this was the first death from H5N1 in the country, there probably will be others.
“I hesitate to make too many predictions about this virus, but I’m very comfortable predicting that we will see more fatal cases if the H5 virus continues to circulate in birds and dairy cattle, which seems likely,” said Richard Webby, a flu virologist who heads the World Health Organization’s Collaborating Center for Studies on the Ecology of Influenza in Animals, located at St. Jude Children’s Research Hospital in Memphis, Tenn.
“The intrinsic capacity of the current batch of H5 viruses to cause disease in many species is pretty much unparalleled in the flu virus world. So even if lethal infections continue to be a small percentage of human infections, we, tragically, have unlikely seen the last.”
Angela Rasmussen agreed, suggesting it had been only a matter of time before the country saw a fatal case. “I was kind of waiting for something like this to happen,” said Rasmussen, a virologist who studies emerging infectious diseases at the University of Saskatchewan’s Vaccine and Infectious Disease Organization in Saskatoon, Canada.
While the H5N1 virus has claimed over 450 lives globally since 2003, this is the first death from the virus in North America. The U.S. has recorded 67 human cases of H5N1, all but one of which occurred in 2024. All of the other cases have involved mild symptoms only.
An analysis of the H5N1 cases in the United States published last week in the New England Journal of Medicine noted that since 2022 there have been 11 human infections in five other countries with the version of the virus currently circulating in dairy cattle in parts of the U.S., clade 2.3.4.4b. Of those, seven people had asymptomatic infection, four had severe or critical illnesses, one of which was fatal. The fatal case occurred in China in 2022.
The mildness of the preponderance of the U.S. cases has puzzled scientists who have followed H5N1 for years. Some have worried that there is a perception emerging that this version of the virus is effectively harmless, causing mainly conjunctivitis — pink eye — among people infected.
Rasmussen has cautioned against that view, suggesting the relative lack of severe infections may be due to who is getting infected — mainly young, healthy farmworkers — and how they are being infected. Their illnesses may not tell us how the virus would behave in people who are more vulnerable to severe disease from flu, she said.
“I didn’t think that it was possible to have basically an H5N1 influenza virus … that mysteriously would only cause mild disease in everybody that it infected,” Rasmussen said. “And to me, that’s the big take home from the Louisiana patient, is that these viruses are capable of causing severe illness. Maybe not at the same frequency that we’ve thought historically about H5N1, but they’re definitely capable of it.”
Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, was more cautious in his assessment of the death of the Louisiana patient, pointing out it has long been known that this virus will kill some portion of the people who become infected with it. What he’s watching for is any indication that the virus is adapting to be able to spread easily from person to person, a prerequisite for the start of a new pandemic.
“We should always be mindful of all cases of influenza and what they might teach us about what’s happening. We also have to be very careful not to overinterpret what one case means,” Osterholm said.
Analysis of virus samples taken from the Louisiana person showed that the virus had developed some mutations that are thought to increase its capability to attach to cells in the human upper respiratory tract. These changes were not seen in viruses from the birds in the person’s backyard flock, a fact that the Centers for Disease Control and Prevention said likely means they developed in the individual, during the course of the infection.
Similar mutations were found in virus samples from a teenager in British Columbia, Canada, who was also rendered severely ill by H5N1. The 13-year-old girl spent several weeks in critical care but is now recovering.
The statement from the Louisiana Department of Health said that no additional cases have been detected among contacts of the person who died. Likewise, health authorities in British Columbia could find no evidence that the infected girl had transmitted the virus to anyone else.
H5N1 has long been at or near the top of the list of viruses that concern pandemic planners. Infections in wild birds have moved it around the world and recent versions of the virus have shown an unsettling ability to infect multiple species of mammals, including dairy cows.
The outbreak in cows, which was first detected in late March, has spread to over 900 herds in 16 states. Forty of the 66 human cases detected in 2024 were among farmworkers exposed to infected cows.