H5N1 bird flu samples collected from a severely ill patient in Louisiana show signs of mutations that may make the virus more transmissible to humans, the Centers for Disease Control and Prevention reported Thursday.
The unidentified patient, who was hospitalized with serious respiratory symptoms, is believed to have contracted bird flu from exposure to sick and dead birds in a backyard flock. The patient has not been identified but is reportedly over 65 with underlying health problems and is the second person in the United States who has been hospitalized with the virus. The Louisiana patient was infected with a strain of the virus different from the one affecting dairy cows and causing sporadic cases in farmworkers in the U.S.
Genetic analysis of two samples collected from the Louisiana patient suggest that the virus may have the ability to attach to cells in the human upper airway.
That’s worrisome, experts say, because bird flu viruses like H5N1 typically attach to cell receptors found in birds and other animals, but not commonly in humans. This is why bird flu typically doesn’t infect humans or spread person to person. One of the mutations was also seen in a sample from a British Columbia teenager who was hospitalized with bird flu, the CDC said.
While the findings show that the virus has the capacity to mutate in ways that could make it more transmissible to humans, experts stopped short of suggesting it's on the verge of a pandemic.
“There’s no evidence that there’s been spread from this person to others, and that’s a good thing,” said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center. “It clearly means that we have to keep our attention on this, and if anything, ratchet it up even more.”
Dr. Paul Offit, a vaccine and infectious disease expert at Children’s Hospital of Philadelphia, said there’s no evidence the mutations were “functional,” meaning the virus could actually attach to the upper airway and replicate and spread to others.
“I think if there were clear and definitive evidence that the virus has mutated to the point that it can bind to the binding receptors in the upper respiratory tract, meaning the lining of the nose, the lining of the throat, the lining of the windpipe and therefore reproduce itself in the upper respiratory tract, that would be worrisome,” Offit said. “But that’s not what the report said.”
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In the report, the CDC said the detection “underscores the importance of ongoing genomic surveillance in people and animals, containment of avian influenza A(H5) outbreaks in dairy cattle and poultry, and prevention measures among people with exposure to infected animals or environments.”
The agency said that the risk to the general public hasn’t changed and remains low.
As of Friday, 65 confirmed human cases of H5N1 have been reported in the U.S., with the majority linked to exposure to infected poultry or dairy cows, according to the CDC. That number is likely an underestimate, however, as a CDC report published in November found evidence of asymptomatic bird flu infections in farmworkers. There is no evidence of person-to-person spread associated with any of the cases.
The Louisiana patient was infected with a strain called D1.1, which is closely related to other viruses recently detected in wild birds and poultry in the U.S. and in recent human cases in British Columbia, Canada and Washington state.
The CDC said its analysis found no changes associated with markers that would mean antivirals, such as Tamiflu, wouldn’t work against the virus. That’s one of the agency’s criteria for deploying a bird flu vaccine.
Angela Rasmussen, a virologist who specializes in emerging infectious diseases, noted on the social media platform X that the mutations identified in the patient weren’t present in samples from birds, indicating the changes developed within the patient after infection and aren’t circulating in wildlife.
She said that’s “good news” because “it reduces risk of transmission to another person and suggests ‘human-adapted’ viruses aren’t emerging in birds.”
Still, “the H5N1 situation remains grim” as human cases continue to rise, Rasmussen posted.
“We don’t know what combination of mutations would lead to a pandemic H5N1 virus and there’s only so much we can predict from these sequence data,” she said. “But the more humans are infected, the more chances a pandemic virus will emerge.”
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