A California duck farm has become the center of attention after U.S. authorities reported that a strain of bird flu, identified as H5N9, was found among sick birds in the flock. The report, published by the World Organization of Animal Health, confirms that this is the first time H5N9 has been detected in American poultry with “highly pathogenic avian influenza” (HPAI), meaning it causes severe disease.
“Clinical signs included increased mortality. State officials have quarantined the affected premises,” the U.S. Department of Agriculture (USDA) stated.
While H5N9 has been detected in birds across the U.S. before, it has typically been in less severe forms. Scientists believe this new detection is the result of a genetic reassortment of H5N1, the dominant strain responsible for bird flu outbreaks in poultry, wild birds, dairy cattle, and even humans in recent years.
H5N1 and H5N9 were both found infecting ducks at the California farm. “The H5N1 virus is widely circulating across North America, and these viruses can reassort in species that are susceptible to influenza viruses,” said Dr. S. Mark Tompkins, director of the Center for Influenza Disease and Emergence Research at the University of Georgia.
Experts agree that this reassortment is not unexpected, as ducks are known to be natural reservoirs for influenza viruses. “H5N1 is reassorted with other avian influenza viruses quite frequently,” said Dr. Andy Pekosz, a professor of microbiology and immunology at Johns Hopkins University.
H5N9: Past cases and human health risks
H5N9 has been identified in U.S. birds before, but these cases were generally classified as “low pathogenic avian influenza” (LPAI), meaning they caused milder symptoms. According to Dr. Tompkins, H5N9 viruses were detected in North American wild birds in 2023 and 2024, as well as in past outbreaks dating back to the 1960s, when turkeys in Wisconsin showed mild respiratory symptoms and reduced egg production.
However, the H5N9 strain found at the California farm is different. It is closely related to a newer variant of H5N1 called H5N1 D1.1, which has dominated bird flu cases in the U.S. over the past year. Despite its detection, experts do not believe this reassortment presents an immediate crisis. “The combination H5N9 is not new, and other versions, like H5N5, also exist,” said Dr. Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai.
The USDA confirmed that H5N9 does not currently pose a greater risk to humans than H5N1. Most human infections with H5N1 have been linked to direct contact with infected animals, and there has been no evidence of human-to-human transmission. “Unless this strain acquires key mutations, it is unlikely to spread efficiently among humans,” the Centers for Disease Control and Prevention (CDC) reported.
At this time, it remains unclear whether authorities are monitoring workers at the Merced County duck farm for potential human infections. A spokesperson for the county has not responded to requests for comment.
The impact of bird flu on California’s poultry industry
The outbreak occurred at a commercial duck meat farm in Merced County, California, with cases first identified on November 23, 2024. In response, 118,954 birds were culled by December 2 to prevent further spread. This is part of a larger effort, as more than 3 million birds have been culled in Merced County over the past year due to bird flu outbreaks—the highest number of any county in the state.
California has been particularly affected by the ongoing bird flu crisis, leading to record-high cullings in the egg industry. Ducks, in particular, are known to be highly susceptible to influenza viruses and can carry the disease without showing symptoms, making them a key factor in the spread of bird flu.
“Unless you have very tight biosecurity, it is very hard to raise ducks without exposure to low-path influenza viruses,” said Dr. David Swayne, a former director of the USDA’s Southeast Poultry Research Laboratory. He emphasized that ducks require the lowest dose of the virus to become infected, and farms that allow outdoor access can become vulnerable to transmission from wild ducks.
Despite concerns, experts believe that H5N9 is unlikely to outcompete H5N1 unless it develops enhanced transmissibility. “So far, I have not heard of any evidence to say that H5N9 has some kind of fitness that is better than H5N1,” Dr. Swayne stated.
For now, the USDA and other health agencies continue to monitor the situation, while poultry farms across the country remain on high alert for potential new outbreaks.
