A Washington resident has been hospitalized with a different strain of bird flu than those seen in past cases, state health officials announced Friday.
The person was infected with a bird flu called H5N5. State and federal health officials say it appears to be the first known human infection with the H5N5 bird flu virus.
The individual has a mixed backyard flock that was exposed to wild birds, making either the domestic birds or the wild ones the most likely source of infection, the Washington State Department of Health said. Public health officials are continuing to investigate.
It is the nation’s first human case of bird flu since February. The older adult with underlying health conditions remains hospitalized. State health officials had announced the preliminary bird flu diagnosis on Thursday. On Friday, they said it had been confirmed.
The H5N5 version is not believed to be a greater threat to human health than the H5N1 virus behind a wave of 70 reported human infections in the U.S. in 2024 and 2025. Most of those have been mild illnesses in workers on dairy and poultry farms.
“These viruses behave similarly,” said Richard Webby, a prominent flu researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. “My gut instinct is to consider it the same as H5N1 from a human health perspective.”
The Centers for Disease Control and Prevention on Friday issued a similar statement that said no information would suggest “the risk to public health has increased as a result of this case.”
The agency is awaiting a specimen from Washington to conduct additional testing.
The distinction between H5N5 and H5N1 lies in a protein involved in releasing the virus from an infected cell and promoting spread to surrounding cells.
“Think along the lines of different brands of car tires. They both do the same job, it’s just each is better tuned for specific conditions, which we don’t fully understand,” Webby wrote in an email to The Associated Press.
H5N5 may have a different preference for which kind of birds it most readily infects, he added.
Bird flu has been detected in a variety of bird populations since January 2022, and in March of last year, it was found in dairy cows for the first time.
Bird flu can infect birds as well as mammals, including pigs, cattle and cats. People can also get infected when they come into close contact with infected animals, which is why farm workers are at high risk for infection.
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