UC Davis School of Veterinary Medicine
Highly Pathogenic Avian Influenza (Bird Flu) H5N9 Strain
Detected for the First Time in California, A Different Strain than H5N1
A commercial duck flock in Merced County, confirmed to have H5N1 bird flu on December 14, 2024, received secondary testing and the final report on January 13, 2025 confirmed the presence of a different, additional strain of bird flu—H5N9. This was reported to the World Organization for Animal Health (WOAH) per protocol and published on January 24, 2025.
This finding is not unexpected or alarming since ducks serve as a reservoir host for influenza A viruses and there is evidence that the H5N9 virus is a North American N9 reassortant of the H5N1 virus that has been seen in wild birds since 2022 with periodic spillover into domestic flocks. The California Department of Food and Agriculture (CDFA) and USDA remain vigilant for any H5 viruses and will continue to monitor for new or unusual viruses as our on-going testing strategy.
Currently, there is no additional information to indicate that this reassortant/strain H5N9 presents any increased risk to public health. There is still no known human-to-human transmission of the bird flu virus. Public health agencies and the Center for Disease Control (CDC) continue to monitor for any impacts from the bird flu strains.