A bird flu sample isolated from a Chilean man who fell ill last month contains two genetic mutations that are signs of mammalian adaptation, officials at the Centers for Disease Control and Prevention said Friday. Experimental animal studies have previously shown that the mutations, both in what is known as the PB2 gene, help the virus replicate better in mammalian cells.
The risk to the public remains low, health officials said, and no other human cases have been linked to the Chilean man, who remains hospitalized.
In addition, the sample lacked other critical genetic changes that scientists believe would be necessary to efficiently spread the virus, known as H5N1, 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 that we believe H5 must undergo in order 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 person sequences in Chile have one of those kinds of changes. But we also know that of those three sets of changes, this is the easiest for the virus to implement.”
PB2 mutations have been found in other mammals infected with this version of the virus, as well as in some humans infected with other versions of H5N1. The mutations most likely emerged in the Chilean patient over the course of his infection, experts said.
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“We understand they are a step towards human adaptation and increased risk to humans,” said Anice C. Lowen, a flu virologist at Emory University. “So it’s definitely concerning to see them.”
But these mutations alone are probably not enough to produce a virus that easily spreads among humans, she added.
“Those genetic changes have been seen before in past H5N1 infections and have not resulted in spread between humans,” Vivien Dugan, acting director of the influenza division of the CDC’s National Center for Immunization and Respiratory Diseases, said in a statement.
“Nevertheless, it is important to continue to carefully look at each case of human infection, as well as other spillover events in mammals, and to monitor viral evolution in birds,” said Dr. Dugan. “We must remain vigilant for changes that could make these viruses more dangerous to humans.”
The sample was sequenced overnight by the National Influenza Center in Chile and uploaded to GISAID, an international database of viral genomes, CDC officials said.
The Chilean Ministry of Health reported the matter to 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 worsened, the report said. WHO
The investigation into the case continues and how the man became infected remains unclear. But the virus had recently been discovered in birds and sea lions in the region where the man lives.
“According to the preliminary findings of the local epidemiological study, the most plausible hypothesis about transmission is that it occurred through environmental exposure to areas where either diseased or dead birds or marine mammals were found near the whereabouts of the case,” the agency reported. WHO. last week.
It is the 11th reported human case of H5N1 since January 2022, none of which has been associated with human-to-human transmission, according to the CDC. Since H5N1 was first discovered in birds in 1996, there have been hundreds of human infections worldwide, mostly in people who have been in close contact with birds.
Still, 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 sparking another pandemic. An H5N1 outbreak on a Spanish mink farm last fall suggests the virus may be adapting to spread more efficiently among 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,” said Dr. Lowen.
This version of the virus has spread rapidly through wild birds in America, leading to regular outbreaks in poultry. The virus is so widespread in birds that it has repeatedly spread to mammals, and “ongoing sporadic human infections are expected,” the CDC wrote in a recent technical report.