H5N9 Bird Flu Outbreak: What Are the Symptoms To Look Out For?

In a troubling development, the highly pathogenic H5N9 avian flu has been detected in U.S. poultry for the first time. The outbreak occurred at a duck meat farm in Merced County, California, in November 2024. Initial signs included increased duck mortality, prompting state officials to quarantine the farm and cull nearly 119,000 birds. The World Organization for Animal Health (WOAH) confirmed the H5N9 detection alongside the more common H5N1 strain at the site. Scientists suspect the H5N9 strain emerged through reassortment—an unpredictable genetic mixing of flu viruses within co-infected hosts.

Why It Matters

This discovery is significant because reassortment events have previously led to flu pandemics. Ducks, often asymptomatic carriers, play a key role in spreading these new strains. While H5N9 hasn't infected humans in the U.S., its potential to mix with human flu viruses raises concerns. Experts are particularly wary of the virus spreading to pigs, which can act as hosts for both human and avian flu strains.

What to Know

Bird flu is a global challenge that has already resulted in the culling of hundreds of millions of poultry—and has spread to mammalian species, including dairy cows in the U.S. The H5N1 strain has been the primary culprit, infecting 67 people in the U.S. and killing one person in Louisiana. The newly identified H5N9 strain is rarer, but its emergence in U.S. poultry underscores the need for heightened surveillance and rapid response measures. The U.S. Department of Agriculture (USDA) has initiated enhanced monitoring and epidemiological investigations.

What Are the Symptoms of Bird Flu in People?

Though rare, avian flu can infect humans, with symptoms ranging from mild to severe. Mild symptoms include eye redness and irritation (conjunctivitis), a mild fever, cough, sore throat, stuffy nose, muscle or body aches, fatigue and headaches. Moderate to severe symptoms include a high fever or severe respiratory symptoms, shortness of breath or difficulty breathing, and altered consciousness or seizures. Eye redness has been a common symptom in recent U.S. cases. Symptoms typically begin 2–7 days after exposure, with eye irritation sometimes appearing earlier, according to the Centers For Disease Control and Prevention (CDC).

What People Are Saying

Angela Rasmussen, a virologist from the University of Saskatchewan, posted in X (formerly Twitter): "Reassortment makes pandemics. The last 3/4 flu pandemics (and maybe 1918 too) were reassortant viruses. "Ducks don't get very sick from many avian flu viruses and thus they are great hosts for reassortment. They still fly, eat and mingle while infected. They transmit reassortant viruses to new hosts, with the virus continuing to adapt along the way." The USDA said in the report to the WOAH: "The USDA Animal and Plant Health Inspection Service (APHIS), in conjunction with State Animal Health and Wildlife Officials, are conducting comprehensive epidemiological investigations and enhanced surveillance in response to the HPAI related events."

What Happens Next

While no human cases of H5N9 have been reported in the U.S., infections from bird flu strains are on the rise. According to the CDC, 145,077,200 poultry birds have been affected by avian influenza, alongside outbreaks in 943 dairy herds. Although health authorities emphasize that the risk to the general public remains low, this latest development underscores a growing concern: as infections spread and new strains emerge, the likelihood of a pandemic steadily increases. Do you have a tip on a science story that Newsweek should be covering? Do you have a question about bird flu? Let us know via science@newsweek.com.