A new strain of bird flu, known as H5N9, has been confirmed for the first time in the United States, creating heightened concern over its potential risks to both animal and human health. The strain was detected at a duck farm located in Merced County, California, where approximately 119,000 birds have already died from the more common H5N1 variant of avian influenza.
According to the U.S. Department of Agriculture (USDA), this discovery was documented and reported to the World Organisation for Animal Health (WOAH) earlier this week. This marks the very first confirmed case of HPAI H5N9 within U.S. poultry lines.
"This is the first confirmed case of HPAI H5N9 in poultry in the United States," the USDA stated, noting comprehensive epidemiological investigations and enhanced surveillance following the outbreak.
H5N9 is part of the larger H5 lineage of avian influenza viruses, which can be particularly deadly to birds and have shown tenacity when it emerges among poultry populations. The more prevalent H5N1 strain has led to extensive culling operations which have impacted hundreds of millions of birds across the country.
Over the past year, the Centers for Disease Control and Prevention (CDC) confirmed 66 human cases of bird flu, with the first death recorded this January 2025, highlighting the ever-present risk these viruses pose. So far, experts have assured the public there is no evidence of significant human infections stemming from the H5N9 strain, but the potential for future mutations causes concern.
Dr. David J. Cennimo, an associate professor at Rutgers New Jersey Medical School, highlighted the adaptability of influenza viruses, stating, "Birds are very susceptible to avian influenza. Some strains are mild, some deadly." Indeed, it's noted by health professionals and researchers alike, the H5N9 strain appears to be different from historical samples, resulting from reassortment—a genetic process wherein viruses share segments of their genetic material when co-infecting the same host.
This genetic reassortment is particularly alarming, as it demonstrates the capacity for influenza strains like H5N9 to adapt potentially toward becoming more pathogenic to humans. Dr. Jacob Glanville, CEO of Centivax, remarked, "Researchers monitor birds constantly, as bird flus have evolved to become human global pandemic strains multiple times in the past." He emphasized the need to watch closely for any mutations stemming from these avian outbreaks.
Additional insights from Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization, echo the same concern as she noted the high probability of reassortment under current circumstances. "With enough H5 among these animals and enough seasonal flu among humans, we have the recipe for potential pandemic strain evolvement," she remarked.
The outbreak has prompted authorities to implement measures such as quarantine protocols and increased surveillance near the affected farms. Notably, the USDA’s Animal and Plant Health Inspection Service confirmed the culling of more than 100,000 birds at the affected farm.
The impact extends beyond just the direct threat to avian and human health; it has sparked economic concerns as the constant threat of bird flu can interfere with food production chains, affecting egg prices and poultry availability. Jennifer Nuzzo from the Pandemic Center at Brown University noted, "The fact these operations are having to cull their flocks is costly. We see this reflected when we look at egg prices at the store. The impact on agriculture is substantial and requires monitoring."
With the recent emergence of H5N9 and its correlation with H5N1, vigilant precautions are advised for those working directly with or near avian populations. The CDC recommends increased caution among individuals who engage with poultry farms, including backyard flock owners and hunters, to mitigate transmission risks.
While currently the H5N9 strain does not pose immediate danger to human health, scientists are maintaining surveillance as the potential for change remains. The mutations observed could allow H5N9 to evolve, thereby raising future risks. Public health officials advise on covering all bases—monitoring flu-like symptoms, ensuring sanitation practices, and staying informed about developments.
The situation remains fluid, and as scientists observe the behavior of the virus, its interactions among various animal species will continue to cause concern and fascination among health experts worldwide. The situation is being monitored, and necessary measures are being enacted to prevent any outbreaks from spilling over from avian populations to humans.