This year in in the United States, 14 people tested positive for avian influenza, or bird flu. Nine of these were infected after contact with chickens, and four acquired the virus from exposure to dairy cows. The origin of the remaining, latest case remains a mystery.
The Centers for Disease Control and Prevention confirmed the case on September 6. Initially identified by the Missouri Department of Health and Senior Services, this is the first known case of human bird flu in the country without known exposure to a sick or infected animal. . On Thursday, health officials said they had not determined how the person contracted the virus.
“Right now, the evidence points to this being a one-off case,” Nirav Shah, chief deputy director of the CDC, said at a news briefing.
But the case is worrisome, as it raises the possibility of an alternate source of transmission, either from a human or an unknown source. Health officials say there is no evidence of person-to-person spread at this time. The CDC says its surveillance system has not picked up any unusual flu activity in the country, and the risk to the general public remains low.
“Our flu surveillance system is designed to find needles in haystacks,” Shah said at the briefing. “In this case, we found such a needle, but we don’t know how it got there.”
The Missouri case is the first to be identified through a national influenza surveillance system versus targeted animal testing. This year, the H5N1 flu virus is responsible for decimating chicken flocks across the country and infecting 200 dairies in 14 states—most recently in California. It also spreads to other mammals, including foxes, mice, raccoons, and domestic cats. With more animals carrying the virus, there is a greater potential for human infection.
It’s not known if that happened in the Missouri case, but it’s one avenue health officials say they’re investigating.
“Regardless of the source, it’s concerning, because it suggests there’s a lot of virus out there,” said David Boyd, a virologist at UC Santa Cruz who studies influenza. “This suggests that there is widespread transmission in animal sources.”
On August 22, an adult patient was hospitalized in Missouri for reasons related to underlying medical conditions and also happened to test positive for the flu. The patient’s specimen was then sent to the Missouri State Public Health Laboratory, which determined that it did not match the currently circulating seasonal flu viruses.
That triggered further testing by the CDC, which last week confirmed it was a type of bird flu, or H5. The agency is conducting further testing to determine the subtype of the virus—the “N” part of H5N1. On Thursday, health officials said the patient had a very low concentration of viral genetic material and, as a result, they were unable to sequence a full genome, including the N part of the virus. However, their data show that the specimen is closely related to the H5 virus circulating in dairy cows.
