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Previous human infections occurred in workers on farms. But community transmission of the virus remains unlikely for now, experts said.
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.