In a potential sign that the deadly bird flu virus is transforming, a resident in Missouri has tested positive for avian influenza without making contact with an infected animal.
This is the 14th human case of bird flu reported in the United States this year, and the first case without a known exposure to sick or infected animals, according to the Centers for Disease Control and Prevention . All other human cases have been linked to farms with infected cattle or poultry.
The CDC did not mention how the person in Missouri might have been infected. The patient, who was not identified, was hospitalized Aug. 22 after testing positive for influenza A. They were treated with influenza antiviral medications and have since recovered.
While a specimen from the patient originally tested positive for influenza A, it was negative for seasonal flu A virus subtypes. That finding triggered additional testing. The Missouri State Public Health Laboratory sent the specimen to CDC, which confirmed the highly pathogenic H5 subtype, also known as bird flu.
The CDC is continuing to analyze the specimen to determine if it is the same strain that has infected other humans this year . Authorities said the patient reported no exposure to animals, and no ongoing transmission among close contacts was identified.
H5 is primarily found in wild birds and poultry, and recently in dairy cows and other animals, and can occasionally infect humans through close contact with infected animals or contaminated environments.
H5 outbreaks in cattle have not been reported in Missouri, but outbreaks of H5 have been reported in commercial and backyard poultry flocks in 2024, the CDC said. H5N1 bird flu has also been detected in wild birds in the state in the past.
The CDC said there has been no sign of unusual influenza activity in people, including in Missouri. Based on available data, the CDC's current assessment is that the risk to the general public from bird flu remains low.
"The results of this investigation will be particularly important in light of the current lack of an obvious animal exposure," the CDC said in a statement. "It is important to note that, while rare, there have been novel influenza A cases where an animal source cannot be identified. The main concern in these situations is that no onward transmission is occurring."
There's currently no evidence that the virus can spread among people, but researchers are staying on high alert.
People with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other infected animals, are at greater risk of infection from H5N1 and should take precautions.
Human illnesses with bird flu viruses have ranged from mild (eye infection, upper respiratory symptoms) to severe illness (such as pneumonia) that have resulted in death in other countries, according to the CDC.
The symptoms of infection in humans include fever, cough, sore throat and muscle aches. Other early symptoms may include abdominal pain, chest pain and diarrhea. The infection may progress quickly to severe respiratory illness (difficulty breathing or shortness of breath, pneumonia) and neurologic changes (altered mental status, seizures).
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