Genetic sequences of H5N1 bird flu viruses collected from a person in Louisiana who became severely ill show signs of development of several mutations thought to affect the virus’ ability to attach to cells in the upper airways of humans, the Centers for Disease Control and Prevention reported on Thursday.
One of the mutations was also seen in a virus sample taken from a teenager in British Columbia who was in critical condition in a Vancouver hospital for weeks after contracting H5N1.
The mutation seen in both viruses is believed to help H5N1 adapt to be able to bind to cell receptors found in the upper respiratory tracts of people. Bird flu viruses normally attach to a type of cell receptor that is rare in human upper airways, which is believed to be one of the reasons why H5N1 doesn’t easily infect people and does not spread from person-to-person when it does.
Scott Hensley, a professor of microbiology at the University of Pennsylvania’s Perelman School of Medicine, cautioned against reading too much into data from two severe cases, though he admitted the CDC’s report was “enough to raise my eyebrows.”
“It’s not great. It’s not great news,” Hensley told STAT.
The CDC reported its scientists had compared viruses collected from the unidentified Louisiana patient to one from infected poultry on the person’s property. The mutations seen in the patient’s samples were not present in the virus from the birds, which suggests the mutations were developing during the course of the person’s infection.
“The changes observed were likely generated by replication of this virus in the patient with advanced disease rather than primarily transmitted at the time of infection,” the report stated.
That is believed to have been the case with the British Columbia patient as well, though health officials there had no source virus to study because they could never determine how the teen became infected.
Hensley said it would have been more concerning if the mutations had been seen in the virus from the birds, because it would have suggested viruses in nature were acquiring these changes.
Angela Rasmussen, a virologist who specializes in emerging infectious diseases, agreed that the news would have been worse if the mutations had been seen in the virus from the Louisiana patient’s poultry. But she called the current H5N1 situation “grim,” noting there has been an explosion of human cases.
“More [genetic] sequences from humans is a trend we need to reverse — we need fewer humans infected, period,” said Rasmussen, who works at the University of Saskatchewan’s Vaccine and Infectious Disease Organization in Saskatoon, Canada, said on the social media site X (formerly known as Twitter).
“We don’t know what combination of mutations would lead to a pandemic H5N1 virus and there’s only so much we can predict from these sequence data. But the more humans are infected, the more chances a pandemic virus will emerge.”
There have been at least 65 confirmed human cases of H5N1 bird flu this year in the United States, since the ongoing outbreak of H5N1 in dairy cows was first confirmed in late March. About 60% of the cases have been people who were infected after having contact with infected cows; most of the others have been infected by contact with infected poultry, either while culling birds at affected commercial operations or, in the case of the person from Louisiana, through contact with an infected backyard flock. In two cases the source of infection was not found.
The version of the virus that is circulating in cows is slightly different from the one found in wild birds and in most of the poultry farm outbreaks. The cattle virus is clade 2.3.4.4b genotype B3.13 while the version in birds is 2.3.4.4b genotype D1.1 or D1.2.
Both the Louisiana and the British Columbia cases were caused by genotype D1.1 viruses. They are the only two severe infections reported in North America in 2024.
The current condition of the Louisiana case is not known. “We are not providing updates about the patient’s condition at this time,” Emma Herrock, communications director of Louisiana’s Department of Health, said Thursday via email.
Hensley said his laboratory is working to test if the mutations seen in the virus from the British Columbia teen do actually increase binding to human cells, as is believed. He expects to have results of that work in early January.
Even if they do, it isn’t clear that would be enough to allow the virus to easily infect people, and to transmit from person to person, he said. It is thought that other changes in other parts of the virus would be needed. “We know attachment is a prerequisite but it might not be enough,” he said.
There was no onward transmission seen from the British Columbia teenager. And Louisiana has found no secondary cases among contacts of the person who was hospitalized.
The CDC said it is working with the Louisiana Department of Public Health to generate genetic sequences of samples taken later in the person’s infection, to see if additional mutations developed.
— Megan Molteni contributed reporting.
Correction: An earlier version of this article misstated the genotype of the H5N1 virus circulating in cows.