A recent outbreak at a California duck farm has brought attention to H5N9, a strain of bird flu that was detected alongside the more commonly known H5N1 virus. The discovery marks the first time H5N9 has caused severe disease in American poultry.
Understanding the basics
Bird flu viruses are classified based on two key proteins: hemagglutinin (H) and neuraminidase (N). While both H5N9 and H5N1 share the H5 component, they differ in their N proteins, which affects how they behave and spread.
Key differences between H5N9 and H5N1
H5N1 has been the main strain driving recent outbreaks across the U.S., affecting wild birds, poultry, dairy cattle, and pets. It has infected at least 67 people.
H5N9 historically causes milder symptoms in birds. However, the California outbreak revealed it can evolve to cause severe disease.
The California outbreak
The outbreak occurred at a commercial duck farm in Merced County in November 2024, leading to the culling of nearly 119,000 birds. Scientists believe the H5N9 strain emerged through "reassortment" - when H5N1 mixed with other influenza viruses inside ducks.
Human health risks
Currently, neither strain spreads easily between humans. Most infections occur through direct contact with infected animals. The CDC reports no evidence of human-to-human transmission in the U.S.
However, experts note that if H5N9 were to adapt to humans, people may have less natural immunity against it compared to H5N1.
Prevention measures
Health officials recommend:
- Avoiding contact with sick birds
- Practicing good hygiene
- Following local health authority guidelines
- Cooking poultry thoroughly
Looking ahead
Scientists continue monitoring both strains for mutations that could increase their ability to spread among humans. While H5N9's appearance is concerning, experts say its detection alone doesn't necessarily signal increased danger.
The USDA and state officials maintain surveillance of bird populations and investigate new outbreaks to protect both animal and human health.