Tiny genetic alterations could help the bird flu virus enter cells in the upper respiratory tract, the C.D.C. said. But there is no sign that mutations are widespread in nature.
After someone in southwest Louisiana was hospitalized with a severe case of bird flu, the first such illness reported in the United States, health workers swabbed the person’s nose and throat, looking for genetic clues about the virus.
On Thursday, federal health officials reported some unsettling results. Some of the genetic samples contained mutations that in theory might help the bird flu virus, H5N1, infect people more easily.
One of those mutations was reported last month in a viral sample taken from a teenager with a severe case of bird flu in British Columbia, Canada. The teenager was placed on a ventilator during a long hospitalization.
Worrying as those severe cases are, the new report about the Louisiana patient contained some reassuring findings, scientists said.
For one thing, the mutations seemed to develop as the virus adapted to its human host. The genetic changes were not present in H5N1 samples from a backyard poultry flock that infected the patient, the Centers for Disease Control and Prevention said.
That suggested that viruses in nature had not yet acquired the concerning mutations. Still, every additional human case gives H5N1 more opportunities to adapt to people, potentially making it more capable of spreading from one person to the next.