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New Bird Flu Strain Detected in the U.S.

Medically reviewed by Carmen Pope, BPharm. Last updated on Jan 29, 2025.

By India Edwards HealthDay Reporter

WEDNESDAY, Jan. 29, 2025 -- A new strain of bird flu, H5N9, has been detected for the first time in the United States, raising concerns about how it continues to spread.

The strain was discovered at a commercial duck farm in California's Merced County, and was reported to the World Organization for Animal Health, which maintains a database of animal disease threats.

The detection comes as the nation continues to battle a growing outbreak of another, more common bird flu strain, H5N1, which is rapidly spreading across poultry farms and infected dairy cows.

While H5N9 is not currently considered a serious threat to humans, experts warn that more mutations could increase the risk of bird flu spreading to people in the future.

Scientists are particularly concerned about a process called "reassortment," The Washington Post reported, where different flu strains mix in infected animals, creating new versions of the virus.

The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) confirmed that the H5N9 strain detected in California is the result of reassortment involving H5N1, the strain that's already spreading rapidly in the U.S.

“It does suggest there’s enough virus around that reassortment might become more frequent,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, told The Post. “With enough H5 in these animals and enough seasonal flu in humans, you get them together, and you have a recipe for a potential pandemic virus.”

Past bird flu outbreaks have started due to reassortment, making ongoing monitoring critical.

In late November, both H5N1 and H5N9 cases were discovered in a Merced County commercial duck meat farm, which led to the quarantine of the facility and the culling of more than 100,000 birds.

Genetic sequencing confirmed the presence of H5N9 in January, triggering a formal report to health authorities.

According to The Post, Steve Lyle, director of public affairs for the California Department of Food and Agriculture, said no further action is being taken because the flock has already been euthanized. The farm will undergo cleaning and testing before resuming operations.

While H5N9 is not new, with similar strains detected in other countries, its outbreak in the U.S. highlights ongoing risks posed by the bird flu.

“The fact that these [agricultural] operations are having to cull their flocks is costly. We see that when we look at the cost of eggs in the store,” Jennifer Nuzzo, director of the Pandemic Center at Brown University, told The Post. “The fact that cows can continue to get infected is ultimately going to prove costly.”

Disclaimer: Statistical data in medical articles provide general trends and do not pertain to individuals. Individual factors can vary greatly. Always seek personalized medical advice for individual healthcare decisions.

© 2025 HealthDay. All rights reserved.

Posted January 2025

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