MERCED, Calif. – A Merced County duck farm became the site of the country’s first detected case of a new strain of bird flu.

The avian influenza viruses that have widely sickened birds and cattle—as well as infected dozens of people—are referred to as highly-pathogenic avian influenza (HPAI), which means they are highly contagious and often deadly for birds.

Most of the bird flu cases reported so far in the U.S. have belonged to an influenza subtype known as HPAI H5N1. The newly reported subtype in Merced County is called “HPAI H5N9.” Although H5N9 has been reported before in the U.S., this was the first time it was reported as “highly pathogenic.”

A report on the incident published by the World Organization for Animal Health said “this is the first confirmed case of HPAI H5N9 in poultry in the United States.”

“Clinical signs included increased mortality. State officials have quarantined the affected premises,” the report continued.

According to testing done by the National Veterinary Services Laboratories in Ames, Iowa, both highly pathogenic strains – H5N1 and H5N9 – were detected in a commercial flock of ducks in Merced County. Nearly 119,000 ducks were euthanized as a result.

In a statement to KVPR, officials with the California Department of Public Health said they are assessing any potential impacts to public health.

“There are currently no known human cases of H5N9 in California, and risk to the general public remains low for all bird flu strains, including H5N9 and H5N1,” the statement reads.

Complicating the picture, however, is that H5N9 appears to have formed by exchanging genetic material with other virus strains.

California Department of Food and Agriculture spokesperson Steve Lyle wrote in a statement: “The bird flu virus we have seen in wild birds since 2022 in North America can recombine with other strains native to our wild birds, especially in ducks as they are well adapted to bird flu. Because domestic ducks and wild ducks are closely related, viruses seem to move between them more readily than other birds.”

Lyle said the recent H5N9 detection was “not unexpected.”

What are the implications for people?

Health experts warn that as bird flu continues to spread among animal populations, it could evolve to become more transmissible to humans.

So far in California, bird flu has led to the quarantine of more than 700 herds of dairy cattle and the culling of more than 23 million birds in commercial and backyard flocks.

The virus has also been reported in . Public health officials say most cases have been mild, and there’s been no evidence of human-to-human transmission.3Dr. Peter Chin-Hong, a professor and infectious disease specialist at the UC San Francisco School of Medicine, said the good news is that the newly reported H5N9 strain was found in ducks—and not people.

“But the bad news is that it really illustrates that these viruses are just swapping genes left, right, and center,” he said.

The gene-swapping both he and Lyle referred to is called viral reassortment, and it can occur when multiple virus strains infect a host at the same time. Some reassortment is normal, and it’s one of the reasons that the seasonal flu changes and warrants a different vaccine formulation each year.

But when flu subtypes that originated in animals are added to the mix of circulating viruses, Chin-Hong said that reassortment can become less predictable.

“The more transmissions you have, the more this kind of exchange of genetic material can happen from things that we know about and we can treat,” he said, “into something like a Frankenstein…that’s harder to treat and that can make people sicker.”

Indeed, research suggests many well-known global flu pandemics—including the Spanish flu that killed tens of millions of people from 1918 to 1920, and the Hong Kong flu that killed as many as four million people from 1968 to 1970—had their origins in “zoonotic” viruses that originated in animals and crossed into humans. For years this was also the going hypothesis for the origin of COVID-19, though the U.S. intelligence community has signaled more recently that a lab leak origin may have been more likely.

“Since the beginning of the 20th century, zoonotic spillover events have given rise to the generation of multiple, well-documented pandemic [influenza A viruses],” wrote the authors of. Although the detection of H5N9 does not appear to pose an immediate health risk to people, Chin-Hong wonders whether more questions would have been answered had the incoming Trump administration not ordered federal health agencies earlier this month to halt all external communications until February.

“[The Department of Health and Human Services] has issued a pause on mass communications and public appearances that are not directly related to emergencies or critical to preserving health,” Benjamin N. Haynes, director of media relations for the Centers for Disease Control’s (CDC) Office of Communications,. Chin-Hong said there potentially would have been a call with the CDC even two or three weeks ago on the matter.

“The fact that the World Organization for Animal Health had to bring this to light is telling in itself,” he said.