This year Fourteen people have tested positive for avian influenza in the United States — nine of whom contracted the virus through contact with poultry and four through contact with dairy cows. The cause of the remaining most recent cases remains a mystery.
The Centers for Disease Control and Prevention confirmed the case on Sept. 6. The case, first discovered by the Missouri Department of Health and Human Services, is the first in the nation to involve avian flu in a human with no history of contact with sick or infected animals. Health officials said Thursday they have not determined how the person contracted the virus.
“At this point, the evidence indicates that this is a one-off incident,” CDC principal deputy director Nirav Shah said at a press conference.
However, this case is concerning as it suggests the possibility of another source of infection from humans or an unknown source. Health officials said there is no evidence of person-to-person transmission at this time. The CDC said its surveillance system has not detected any unusual influenza activity in the country and the risk to the general public remains low.
“Our influenza surveillance system is designed to find a needle in a haystack,” 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 have been detected through the country’s national influenza surveillance system rather than through testing in animals. This year, the H5N1 influenza virus has wiped out poultry flocks across the country and infected 200 dairy herds in 14 states, with California being the most recent. The virus has also spread to other mammals, including foxes, rats, raccoons and house cats. As more animals carry the virus, the chances of infection with humans increase.
It’s unclear if that happened in the Missouri case, but it’s one of the ways health officials say they’re investigating.
“That’s concerning because it suggests that there’s a lot of virus here, wherever it originated,” said David Boyd, a virologist at the University of California, Santa Cruz, who studies influenza. “It points to widespread zoonotic transmission.”
On August 22, an adult patient in Missouri was hospitalized for reasons related to an underlying medical condition and incidentally tested positive for influenza. Samples from the patient were subsequently sent to the Missouri State Public Health Laboratory and found to be a match to no currently circulating seasonal influenza viruses.
That prompted the CDC to conduct additional testing, which confirmed last week that it was a type of avian flu, or H5. The agency was conducting additional testing to identify the virus’ subtype, or the “N” part of H5N1. Health officials said Thursday that the patient’s concentration of viral genetic material was so low that they were unable to produce a complete genome, including the N part of the virus. But their data shows that the sample is closely related to the H5 virus circulating among dairy cows.
