A California duck farm has tested positive for H5N9, marking the first confirmed case in the U.S. The outbreak began on November 23, 2024, after a rise in duck deaths.
H5N9 is a low-pathogenic virus that usually causes mild symptoms in birds. Officials have not reported any human infections linked to the outbreak. The virus has been detected in farmers in other countries, but no cases have been confirmed in the U.S.
What Do We Know About the Outbreak So Far?
The outbreak occurred in November 2024 at a meat farm in Merced County. State officials quarantined the farm and culled 119,000 birds by December 2.
More than 3 million birds have been culled in this county over the past year. California has faced record bird flu outbreaks recently, especially in egg farms.
Officials have not confirmed how the flock became infected. The USDA has not identified the source of the outbreak.
So far, there are no reports of H5N9 spreading to other farms. No wild birds have tested positive for this strain.
There are also no confirmed human infections from H5N9. However, H5N1, a widely circulating bird flu strain, was also found at the farm.
How Did the H5N9 Strain Emerge?
The H5N9 strain likely emerged through reassortment, a process where viruses mix genes. This happens when H5N1 combines with another avian flu strain. Co-infected ducks can spread unpredictable new flu variants.
Ducks often carry avian flu without showing symptoms. This makes them good hosts for reassortant viruses. Since they fly and interact with other birds, they spread flu easily.
Reassorted flu strains can mix with human flu viruses. This raises concerns about human-to-human bird flu transmission. Pigs can also catch human and avian flu, increasing reassortment risks.
Scientists classify bird flu by two key proteins: hemagglutinin (H) and neuraminidase (N). H5N1 is the dominant strain in recent U.S. outbreaks. It has infected poultry, dairy cattle, pets, and at least 67 people.
H5N1 frequently reassorts with other viruses. The H5 protein infecting U.S. livestock has already reassorted. Accordingly, reassortment in poultry is not surprising.
H5N9 was first recorded in Wisconsin turkeys in the 1960s. Those birds had mild respiratory symptoms and reduced egg production.
The new California strain is different because it caused severe disease in ducks. It is linked to the H5N1 lineage that devastated poultry flocks.
How Dangerous Is H5N9?
The combination of H5 and N9 is not new. Other strains, like H5N5, also exist. A potentially concerning outbreak of H5N5 has spread from Europe to mammals in Canada.
H5N9 would need key mutations to spread easily among humans. The CDC says no human-to-human bird flu transmission has been found in the U.S. Most infections come from direct contact with sick animals.
Three human cases remain unexplained with no clear source of infection. It is unclear if health officials tested workers from the Merced County farm.
Experts say H5N9 is not a bigger risk to humans than H5N1. Richard Webby, a World Health Organization scientist, said H5N9 is unlikely to cause a human pandemic.
Still, if H5N9 spreads to humans, people may have less immunity to it. Some have partial immunity to H5N1 from past flu exposure. This may not apply to H5N9.
However, if H5N1 reassorts with a human flu virus, it could become more contagious. Scientists have warned about bird flu’s pandemic potential. The virus is mutating as it spreads among cows and infects people.
Experts have urged U.S. authorities to increase surveillance and share more data. Like pigs, a person infected with both bird flu and seasonal flu could act as a mixing vessel. This could create a highly contagious strain, possibly leading to a pandemic.
Officials state that the risk to the general public is low. However, concerns remain high as zoos take precautions and parks close.
Conclusion
The H5N9 outbreak in California marks a new development in U.S. bird flu cases. While the strain poses little risk to humans, experts remain cautious. Surveillance and biosecurity measures will be key in preventing further spread.
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Logan Hamilton is a health and wellness freelance writer for hire. He’s passionate about crafting crystal-clear, captivating, and credible content that elevates brands and establishes trust. When not writing, Logan can be found hiking, sticking his nose in bizarre books, or playing drums in a local rock band. Find him at loganjameshamilton.com.
