Samples from human patients infected with bird flu show the virus has mutated in ways that could make it easier for humans to contract, the Centers for Disease Control and Prevention says. 

The virus already has been blamed for a series of animal deaths in America. In a Washington state animal sanctuary, more than 20 wild cats recently died from the virus. The Wild Felid Advocacy Center of Washington said its team was “grieving” the loss of the large cats, which included a Bengal tiger, four bobcats, and four cougars. 

“Cats are particularly vulnerable to this virus, which can cause subtle initial symptoms but progress rapidly, often resulting in death within 24 hours due to pneumonia-like conditions,” the center said on Facebook.  

In a zoo in Arizona, five animals died earlier this month from the virus, including a mountain lion and a cheetah. Meanwhile, Northwest Naturals issued a recall of its raw and frozen cat food after a house cat died in Oregon after it consumed cat food that tested positive for bird flu.

The CDC said on Thursday that samples from a patient in Louisiana, who became the first person in America to have a severe case of bird flu, showed the virus has mutated in a way that helps it bind to the cells in the upper airways of humans. A similar mutation was found in a sample from a teenager in British Columbia. 

Bird flu viruses will typically bind to cells that are generally not found in the upper airways, one of the reasons scientists say it is rare for humans to contract the virus and for human-to-human transmission to occur. 

A professor of microbiology at the University of Pennsylvania’s Perelman School of Medicine, Scott Hensley, told STAT News that the public should not be too alarmed by the news. However, he said it is “enough to raise my eyebrows” and “not great news.”

The CDC said 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.”

A virologist at the Vaccine and Infectious Disease Organization in Canada, Angela Rasmussen, posted on X that it was “good news” that the mutation of the virus occurred in a human because it “reduces the risk of transmission to another person and suggests ‘human-adapted’ viruses aren’t emerging in birds.’”

She also pointed out the CDC said the virus did not “show evidence of mutation at known sites needed for more efficient replication in a human host or resistance to antiviral drugs.”

However, Ms. Rasmussen wrote, “While this sounds like good news, the [bird flu] situation remains grim. There has been an explosion of human cases… More sequences from humans is a trend we need to reverse — we need fewer humans infected, period.”

“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,” she added.

The CDC said the risk to the public from the mutation is low. It also said there has not been a person-to-person transmission of the virus from the patient in Louisiana. It did not provide an update on the patient’s condition.

On December 18, the CDC confirmed that an individual older than 65 in Louisiana had contracted a severe case of bird flu, the first severe case in America. 

The director of the National Center for Immunization and Respiratory Diseases, Dr. Demetre Daskalakis, said, “It is believed that the patient that was reported by Louisiana had exposure to sick or dead birds on their property. These are not commercial poultry, and there was no exposure to dairy cows or their related products.”

The Louisiana patient was identified as having the D1.1 variant of the virus, which is the same type of bird flu found in other human cases in Canada and Washington state. It has also been found in wild birds and poultry in America. 

More than 60 human cases of bird flu have been reported in America in 2024, and 34 of those cases were in California. Symptoms of the virus in humans can include pink eye, fever, fatigue, cough, vomiting, diarrhea, and shortness of breath.

The bird flu can lead to severe illness and death, but as of Friday morning, there have not been cases of human deaths connected to the bird flu in America.