Officials in Missouri on Friday identified the first American infected with bird flu who had no known exposure to the infected animals or birds that have been plaguing the nation’s farms for more than six months.
The case brings this year’s number of human bird-flu infections to 14. Previous human cases were all acquired from interactions with infected dairy cattle or poultry. The new patient raises the unsettling possibility that the virus, called H5N1, may be spreading undetected among people.
How the person became infected is unknown, but Missouri health officials said they are trying to pinpoint the source.
“The route of transmission is going to determine how much more escalated the risk of the disease is to the general public,” said Dr. Nahid Bhadelia, director of Center on Emerging Infectious Diseases at Boston University.
The case was picked up by routine seasonal flu surveillance conducted in Missouri, where the person was hospitalized, and not during an investigation on a farm. The state has not reported any infections on its cattle farms, but has detected the virus in some commercial and backyard flocks, as well as in wild birds.
Since March, the bird flu virus has been found in nearly 200 dairy herds in 14 states, although testing has not been conducted widely enough to gauge the full spread. California, the nation’s largest milk producer, found bird flu in three cattle herds last week.
By now, H5N1 is known to have infected a very wide range of wild animals and birds. It’s possible the person in Missouri was unknowingly exposed to one of these species, or to infected milk, or even to a backyard bird feeder, said Michael Osterholm, an infectious disease expert at the University of Minnesota.
“There are a lot of other avenues to look at here,” he said.
The individual, who has an underlying medical condition, was hospitalized on Aug. 22. The patient was treated with antiviral medications, recovered and was sent home, Missouri officials said. They declined to offer further details.
The Centers for Disease Control and Prevention posted a statement online but directed queries to Missouri’s health department. Missouri officials did not immediately respond to questions.
Dr. Osterholm said he was reassured that no new cases have been identified in the days since.
A fraction of flu tests at the nation’s hospitals are routinely sent along to other labs for further testing, part of the nation’s influenza surveillance program. Missouri’s health department identified a subtype of influenza that includes H5N1, and sent the sample to the C.D.C. for further analysis.
This is the first time that routine flu surveillance has picked up H5N1, although it has detected other novel flu strains. Dr. Bhadelia said she has been pushing for a greater percentage of flu samples taken at hospitals to be included in the routine surveillance.
“We are always later than we think we are in outbreaks,” she said, adding that routinely testing more samples would allow officials to “cast a wider net.”
While this individual was hospitalized, she noted, others who are infected may have few to no symptoms and may not seek care at all.
Some studies have identified farm workers who carry antibodies to the virus — suggesting that they were exposed to the pathogen — but could not recall being sick.
The C.D.C., which completed its initial testing on Thursday, is conducting genetic analyses of the sample. That will reveal whether and how the virus might differ from the version circulating in dairy cattle.
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