First case of highly pathogenic H5N9 avian flu detected in U.S.

Highly pathogenic avian influenza (HPAI) H5N9 has been identified in U.S. poultry for the first time, following an outbreak at a commercial duck farm in Merced County, California, in November 2024.

The World Organization for Animal Health confirmed the detection, stating, “Highly pathogenic avian influenza H5N9, Eurasian lineage goose/Guangdong clade 2.3.4.4b and HPAI H5N1 clade 2.3.4.4b were confirmed in a commercial duck premises in Merced County, CA. This is the first confirmed case of HPAI H5N9 in poultry in the United States.”

Tests conducted by the U.S. Department of Agriculture National Veterinary Services Laboratory confirmed the presence of both H5N9 and H5N1 at the farm.

The outbreak, which began on November 23, led to increased mortality among the ducks. In response, state officials quarantined the premises, and nearly 119,000 birds were culled by December 2. USDA’s Animal and Plant Health Inspection Service, along with state and wildlife officials, is conducting an in-depth epidemiological investigation.

Angela Rasmussen, Ph.D., a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, suggested the presence of H5N9 indicates a reassortment between circulating H5N1 strains and another avian flu virus carrying the N9 neuraminidase. She warned that coinfected hosts can facilitate the emergence of unpredictable new viral strains.

Ducks don’t get very sick from many avian flu viruses, making them ideal hosts for reassortant viruses, Rasmussen explained. Since they can continue flying, eating, and mingling while infected, they can spread the virus to new hosts, allowing further adaptation.

While the potential risk to humans remains uncertain, Rasmussen noted that reassortment between avian and human flu viruses is a concern. She also emphasized the importance of preventing H5N1 from infecting pigs, as they can serve as mixing vessels for flu viruses, including reassortant strains.

The outbreak in Merced County is part of a broader spread of H5N1 in the U.S. APHIS has recently confirmed additional poultry infections in 11 states, affecting both commercial and backyard flocks.

In Indiana, Missouri, Ohio, and Washington, outbreaks at layer farms have impacted at least 1.5 million birds, raising concerns about potential egg shortages. Turkey farms in Minnesota and Ohio, as well as broiler farms in Arkansas and Missouri, have also been affected.

Backyard flocks in Connecticut, Massachusetts, Rhode Island, Vermont, and California have tested positive for the virus. Additionally, APHIS reported six more H5N1 detections in dairy herds, all in California, bringing the national total to 943 cases, with California accounting for 726 of them.

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