Source of person's recent bird flu case remains a mystery — and experts say that's concerning

Over a dozen cases of H5N1 have been reported among people in the United States this year. Current evidence suggests this bird flu virus spreads to humans only from infected animals, such as dairy cows and poultry, and not from person to person.

However, the latest case of human infection — reported Sept. 6 by the Centers for Disease Control and Prevention (CDC) — occurred in a person in Missouri who'd had no known exposure to relevant Animals.

The CDC reports that there is no evidence of the person passing the virus to close contacts, and says "the risk to the general public from H5N1 remains low." The agency maintained that the risk is low even after two health care workers got sick after interacting with the patient — one tested negative for flu and one has yet to be tested for H5N1 or antibodies.

"I agree with the CDC that it remains low-risk," said Dr. Nahid Bhadelia, founding director of the Center on Emerging Infectious Diseases at Boston University. "The good news is you haven't seen an uptick in influenza cases," as that would show up in surveillance for seasonal flu, she added. For the moment, that should help quell public concern that this could turn into an outbreak.

Nonetheless, Bhadelia and another expert told Live Science that the Missouri case raises other concerns — namely, that the U.S. is not tracking the spread of H5N1 closely enough to head off a potential epidemic.

Related: H5N1: What to know about the bird flu cases in cows, goats and people

The Missouri case

The Missouri patient was hospitalized in August, marking the country's 14th H5N1 case overall and its first without any connection to sick Animals at the person's workplace. The patient, who had underlying medical conditions, experienced chest pain, diarrhea, vomiting and weakness. They were not severely ill during their hospital stay and have recovered.

The patient initially tested positive for iNFLuenza A, the broad group of viruses to which H5N1 belongs. This group also includes seasonal flu viruses, like H1N1, but the patient tested negative for those subtypes. Further testing then revealed H5N1.

In a Sept. 13 update, the CDC reported that it had partially analyzed the genome of the virus; there was not enough genetic material for a complete sequence. The results showed that the virus that infected the Missouri patient bears high similarity to those circulating in cattle and hasn't mutated enough to be well adapted for infecting human lungs. Such mutations could pave the way for human-to-human transmission.