MONDAY, Dec. 30, 2024 (HealthDay News) -- Federal health officials have confirmed unsettling new details about the first United States case of severe bird flu, reported recently in a hospitalized patient in Louisiana.

Genetic samples from the patient contained mutations that could, in theory, help the virus better infect human cells.

Importantly, these mutations were observed in the later stages of infection and were not present in the virus samples taken from the backyard poultry flock that infected the patient, a media report from The New York Times shows.

This suggests that the changes occurred as the virus adapted to its human host, rather than spreading widely in nature, the U.S. Centers for Disease Control and Prevention (CDC) said in an update published Dec. 26.

Still, experts are concerned about the broader implications. “Every additional human case gives H5N1 more opportunities to adapt to people,” The Times reported.

“It has the potential to really harm a lot of people,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization in Canada, said.

In a positive finding, the CDC emphasized that there is no evidence the virus has spread from the Louisiana patient to others, and the mutations alone are unlikely to spread from person to person.

CDC scientists also confirmed, however, that the Louisiana virus samples are closely related to candidate vaccine strains already developed for bird flu.

Yet, Rasmussen questioned why those vaccines are not being used to protect high-risk groups, such as farmworkers, who have the greatest exposure.

Sixty-six other cases of H5N1 avian flu have been reported in people across the United States since April of this year, but have resulted in relatively mild symptoms that resolved after treatment.

However, the Louisiana case -- the first-ever reported H5N1 illness in that state -- "marks the first instance of severe illness linked to the virus in the United States," the CDC said in a previous statement.

Most of the other bird-to-human cases have occurred in workers at major poultry farms; "this is the first case of H5N1 bird flu in the U.S. that has been linked to exposure to a backyard flock," the CDC noted.

The agency also stressed that an eventual case of severe H5N1 avian flu in an American was "not unexpected."

"Avian influenza A(H5N1) virus infection has previously been associated with severe human illness in other countries during 2024 and prior years, including illness resulting in death," the CDC stated.

The case does serve as a reminder, however, that anyone with close exposures to birds needs to be careful. "This means that backyard flock owners, hunters and other bird enthusiasts should also take precautions," the agency said.

In a health alert issued Dec. 13, state officials said the case involves a resident of southwestern Louisiana.

"The individual had exposure to sick and dead birds that are suspected to have been infected with H5N1,” the virus that causes bird flu.

Of the 66 human bird flu cases reported in the United States, the majority of them have been in California and Washington, and the infections have mostly surfaced in workers who had been in close contact with infected poultry or dairy cows.

For now, there's no evidence that the bird flu is spreading from person to person, and most cases have largely been mild, with the main symptom being conjunctivitis, or pinkeye. No deaths have been reported.

Earlier this month, U.S. health officials also reported that the strain of bird flu detected in a California child is similar to the strain spreading through livestock, though the patient had no known exposure to infected animals.

While the findings were not definitive, tests "showed that the virus was very similar to viruses detected in both dairy cattle and poultry as well as to A[H5N1] viruses from previous human infections in dairy workers in California," a previous health update from the U.S. Centers for Disease Control and Prevention stated.

“This case does not change CDC’s assessment of the immediate risk to the general public, which remains low at this time,” the agency added.

Meanwhile, California health officials also reported this month that their investigation into how the child may have been exposed to bird flu is continuing. The child received flu antivirals and has since recovered.

Richard Webby, a virologist at St. Jude Children’s Research Hospital who studies influenza, said it is unlikely that community spread explains the California case. Instead, other animals that could have come into contact with the virus, such as cats, dogs or rodents, might be contributing to its spread.

In the California case, no person-to-person spread of the virus has been detected and the child's family members all tested negative.

Bird flu has been spreading in poultry since 2022, and cases in dairy cows began to crop up in March. The virus was discovered in a pig for the first time earlier this year.

Bird flu infections in people -- nearly all among farmworkers -- have now been confirmed in 10 states.

California accounts for the largest share of human bird flu cases in the country, with 37 confirmed infections. Washington has recorded 11 cases, and Colorado 10, CDC data show.

Across the United States, more than 900 dairy herds in 16 states have been infected since the outbreak in dairy cows was first confirmed last spring. Avian influenza has been spreading in wild and domestic birds in the United States for several years.

“We should be very concerned at this point,” Dr. James Lawler, co-director of the University of Nebraska’s Global Center for Health Security, told the New York Times. “Nobody should be hitting the panic button yet, but we should really be devoting a lot of resources into figuring out what’s going on.”

More information

The CDC has more on bird flu.

SOURCES: U.S. Centers for Disease Control and Prevention, health update, Dec. 26, 2024; U.S. Centers for Disease Control and Prevention, health update, Dec. 18, 2024; Louisiana Department of Health, news release, Dec. 13, 2024; U.S. Centers for Disease Control and Prevention, health update, Dec. 10, 2024; California Department of Public Health, news release, Nov. 19, 2024; British Columbia Ministry of Health, news release, Nov. 9, 2024; New York Times