Bird Flu Sample from Chilean Man Showed Some Signs of Adaptation to Mammals

A sample of avian influenza isolated from a Chilean man who fell ill last month contains two genetic mutations that are signs of adaptation to mammals, officials from the Centers for Disease Control and Prevention said on Friday. In experimental animal studies, the mutations, both of which are in what is known as the PB2 gene, have previously been shown to help the virus replicate better in mammalian cells.

The risk to the public remains low, health officials said, and no additional human cases have been linked to the Chilean man, who remains hospitalized.

Moreover, the sample was missing other critical genetic changes that scientists believe would be necessary for the virus, known as H5N1, to spread efficiently among humans, including mutations that would stabilize the virus and help it bind more tightly to human cells.

“There are three major categories of changes we think H5 has to undergo to switch from being a bird virus to being a human virus,” said Richard J. Webby, a bird flu expert at St. Jude Children’s Research Hospital. “The sequences from the person in Chile have one of those classes of changes. But we also know that of those three sets of changes, this is the easiest one for the virus to make.”

PB2 mutations have been found in other mammals infected with this version of the virus, as well as in some people infected with other versions of H5N1. The mutations most likely emerged in the Chilean patient over the course of his infection, experts said.

“We understand them to be a step on the path to adaptation to humans and increased risk to humans,” said Anice C. Lowen, an influenza virologist at Emory University. “So certainly it’s concerning to see them.”

But these mutations alone are probably not sufficient to produce a virus that spreads easily among humans, she added.

“Those genetic changes have been seen previously with past H5N1 infections, and have not resulted in spread between people,” Vivien Dugan, acting director of the influenza division at the C.D.C.’s National Center for Immunization and Respiratory Diseases, said in a statement.

“Nevertheless, it’s important to continue to look carefully at every instance of human infection, as well as other mammalian spillover events, and to track viral evolution in birds,” Dr. Dugan said. “We need to remain vigilant for changes that would make these viruses more dangerous to people.”

The sample was sequenced by the National Influenza Center in Chile and uploaded to GISAID, an international database of viral genomes, overnight, C.D.C. officials said.

Chile’s Ministry of Health reported the case to the World Health Organization on March 29. The patient, a 53-year-old man, developed respiratory symptoms, including a cough and a sore throat, and was hospitalized when his condition deteriorated, according to the W.H.O.

Investigation into the case is continuing, and how the man became infected remains unclear. But the virus had recently been detected in birds and sea lions in the region where the man lives.

“According to the preliminary findings of the local epidemiological investigation, the most plausible hypothesis about transmission is that it occurred through environmental exposure to areas where either sick or dead birds or sea mammals were found close to the residence of the case,” the W.H.O. reported last week.

It is the 11th reported human case of H5N1 since January 2022, according to the C.D.C., none of which have been associated with human-to-human transmission. Since H5N1 was first detected in birds in 1996, there have been hundreds of human infections globally, mostly in people who were in close contact with birds.

Still, experts have long been worried about the possibility that avian influenza, which is well adapted to birds, might evolve to spread more easily among humans, potentially setting off another pandemic. An H5N1 outbreak on a Spanish mink farm last fall suggests that the virus is capable of adapting to spread more efficiently among at least some mammals. And every human infection gives the virus more opportunities to adapt.

The mutations documented in the Chilean patient are a “step in the wrong direction,” Dr. Lowen said.

This version of the virus has spread rapidly through wild birds in the Americas, sparking regular outbreaks in farmed poultry. The virus has become so widespread in birds that it has repeatedly spilled over into mammals, and “continued sporadic human infections are anticipated,” the C.D.C. wrote in a recent technical report.