A patient in Missouri was hospitalized with bird flu despite having no known contact with animals.
The Centers for Disease Control and Prevention (CDC) confirmed the positive case of avian influenza A (H5) on Friday.
The patient, who had underlying medical conditions, was successfully treated with antiviral medications at the hospital and has since been discharged, according to the Missouri Department of Health and Senior Services (DHSS).
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This marks the 14th person (in three states) to contract bird flu in the U.S. this year — and the first infection to occur without any reported exposure to sick or infected animals, the CDC alert stated.
The prior 13 cases came after exposure to dairy cows or poultry.
Dr. Benjamin Anderson, assistant professor in the Department of Environmental and Global Health at the University of Florida, said the fact that an individual has tested positive for H5 without any reported animal exposure is "very concerning," but noted that very little is known about the case.
"We don’t know if the individual had indirect exposure to people or products from agricultural settings," he told Fox News Digital.
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"We also do not yet know the sequence of the virus to determine if it is related to the strains circulating in dairy cattle or not."
A full epidemiological investigation is necessary to determine all potential paths of exposure, Anderson said.
"This is definitely something we need to pay very close attention to."
Dr. Edward Liu, chief of infectious diseases at Hackensack Meridian Jersey Shore University Medical Center in New Jersey, echoed the concern about human-to-human spread.
"When that happens, you won't see an animal vector or source," he told Fox News Digital.
"One of the underlying concerns is that the avian influenza will adapt itself to be more efficient at replicating in humans and better at spreading in respiratory droplets and secretions."
Liu also emphasized that current testing for avian influenza is limited.
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"Knowing it is influenza A doesn't mean it is avian influenza," he said.
"The confirmation of avian influenza requires another step to send it to the state health department or CDC — so we may be measuring the tip of an iceberg. If our cases of influenza A spike, we need to test more samples to see if some are avian influenza."
Dr. Marc Siegel, senior medical analyst for Fox News and clinical professor of medicine at NYU Langone Medical Center, noted that while there hasn’t been evidence of animal exposure, "most likely there was."
"There has been no evidence of any human-to-human spread at this point," he told Fox News Digital.
"Surveillance, wastewater analysis and control in animals are key."
The biggest concern for bird flu right now, according to Anderson, is the potential for it to evolve and gain human-to-human transmission ability.
"Influenza viruses can evolve over time through the accumulation of small point mutations in their genetic sequence," he said. "However, they can also evolve very rapidly through what is called recombination — when two or more influenza viruses infecting the same host exchange their genetic material and make a new strain of virus."
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The more this strain of influenza virus circulates, the greater the potential for recombination to occur, the doctor said.
"Right now, the highest-risk human groups are individuals with dairy cow and poultry exposure in areas where spillovers of bird flu have been documented," Anderson said.
So far, those who have contracted the dairy cow strain of influenza virus have mostly had mild infections, the doctor noted.
"However, we know these viruses change quickly, so we need to continue to be vigilant in monitoring their circulation."
Another risk is that avian influenza may cause more severe disease than regular influenza, Liu cautioned.
"It is not currently in our seasonal influenza vaccine, although the vaccine may still provide some protection," he said.
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"Luckily, current prescription antivirals, like Tamiflu, can still treat avian influenza."
As with other flu strains, avian influenza will likely pose a greater risk to older patients, young children, immunocompromised patients, and those with chronic lung, cardiac or kidney disease, the doctor added.
Overall, Siegel said, bird flu poses "no current concern unless it mutates further."
"It needs to be monitored closely," he added. "If it starts to pass from human to human, it could be a huge problem, but we aren't there."
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The CDC stated on Friday that the public risk for bird flu infection remains low.
The agency will continue to investigate the specimen from the human patient, and the Missouri DHSS is looking into potential means of exposure.
Fox News Digital reached out to the Missouri DHSS for comment.
