Tests identify H5N9 avian flu at California duck farm

duck farm

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Highly pathogenic H5N9 avian flu has been identified for the first time in US poultry, on a duck meat farm in California that experienced an outbreak in November 2024, according to a notification today from the World Organization for Animal Health (WOAH).

Both H5N9 and H5N1 were detected at the duck farm in Merced County, according to tests conducted by the US Department of Agriculture (USDA) National Veterinary Services Laboratory. The event began on November 23, with clinical signs that included increased deaths in the ducks.

State officials quarantined the affected farm, and a culling operation of the facility's nearly 119,000 birds was completed on December 2. Scientists with the USDA's Animal and Plant Health Inspection Service (APHIS), state partners, and wildlife officials are conducting a comprehensive epidemiologic investigation.

Angela Rasmussen, PhD, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, said on X today that the H5N9 detection suggests reassortment of circulating H5N1 viruses with avian flu virus that contains the N9 neuraminidase (NA). She added that replication in coinfected hosts can produce unpredictable new reassortant viruses. 

She said ducks don't get very sick from many avian flu, which can make them great hosts for reassortant viruses. Since they can still fly, eat, and mingle while infected, they can transmit the reassortant virus to new hosts, with the virus continuing to adapt along the way.

Though the risk of reassortant flu viruses for people is unclear, they can reassort with human flu viruses, Rasmussen added. She emphasized that its crucial to keep H5N1 out of pigs, given that they are susceptible to human and other flu viruses, including reassortants.

Outbreaks in poultry in 11 states, cows in California

In an update that covers the past few days, APHIS reported several more H5N1 detections in poultry flocks, both commercial and backyard birds, including several in the Northeast.

In events that could worsen egg shortages, more layer farm outbreaks were reported in Indiana, Missouri, Ohio, and Washington state, totaling at least 1.5 million birds. The virus also struck turkey farms in Minnesota and Ohio, as well as broiler farms in Arkansas and Missouri.

The virus also turned up in backyard flocks in Connecticut, Massachusetts, Rhode Island, Vermont, and California. 

Also, APHIS confirmed 6 more H5N1 detections in dairy herds, all in California, raising the national total to 943 and California's total to 726.

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