Bird flu sample taken from Chilean man showed signs of mammalian adaptation

A bird flu sample isolated from a Chilean man who fell ill last month contains two genetic mutations that are signs of mammalian adaptation, Centers for Disease Control and Prevention officials said Friday. In experimental animal studies, the mutations, both of which are in the so-called PB2 gene, have already 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.

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

“We believe that H5 must undergo three broad categories of changes to transition from an avian virus to a human virus,” said Richard J. Webby, an avian flu expert at St. Jude Children’s Research Hospital. “The footage from the person in Chile has one of those categories of changes. But we also know that of these three sets of changes, this is the easiest 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 arose in the Chilean patient during his infection, experts said.

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

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

“These genetic changes have already been observed with past H5N1 infections and have not resulted in spread between people,” said Vivien Dugan, acting director of the influenza division at the National Center for Immunization and Respiratory Diseases of the CDC, in a statement.

“Nevertheless, it is important to continue to carefully examine each case of human infection, as well as other mammalian spread events, and to monitor viral evolution in birds,” said Dr Dugan. “We need to stay alert to changes that would make these viruses more dangerous for people.”

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

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

The 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 area where the man lives.

“According to the preliminary results of the local epidemiological investigation, the most plausible hypothesis regarding transmission is that it occurred by environmental exposure to areas where sick or dead birds or marine mammals were found nearby. of the residence of the case,” the WHO reported. last week.

It is the 11th human case reported H5N1 since January 2022, according to the CDC, 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 worldwide, mostly in people in close contact with birds.

Yet experts have long worried about the possibility that avian flu, which is well adapted to birds, could evolve to spread more easily among humans, potentially triggering another pandemic. An epidemic of H5N1 on a spanish mink farm last fall suggests that the virus is able to adapt to spread more efficiently in at least some mammals. And each 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 spread rapidly among wild birds in the Americas, triggering regular outbreaks in farmed poultry. The virus has become so widespread in birds that it has repeatedly spread to mammals, and “continued sporadic human infections are anticipated,” the CDC wrote. in a recent technical report.