Four people in Washington state are suffering from bird flu, prompting fears about the illness spreading to other agricultural workers in the U.S.

Also known as avian influenza A (H5N1), the disease recently tested presumptively positive in four agricultural workers in Washington state, according to the Washington State Department of Health (WADOH). The press release noted that the workers are employed on an egg farm in Franklin County.

“The individuals experienced mild symptoms and have been provided with antiviral medication,” the statement read. “Testing of additional individuals on the farm is currently pending and the number of cases under investigation may change.”

Washington state officials noted that these diagnoses mark the “first presumed human cases of H5 virus under investigation in Washington state.” In response, more than three quarters of a million birds were killed.

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“About 800,000 birds were euthanized after test results by the Washington State Department of Agriculture on Oct. 15 showed that they were infected with avian influenza,” the statement read.

Washington state is the sixth U.S. state to identify a human infection of H5N1, which is a highly pathogenic avian influenza that has been detected in poultry, dairy cattle and wildlife.

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Jon Arizti Sanz, PhD, Postdoctoral Fellow working in the lab to test bird flu samples in May 2024.

“Washington has monitored the spread of H5N1 closely since it was first detected in poultry in the state in 2022, and our state is prepared with the knowledge, relationships, and tools to minimize its impacts on our community,” Washington Secretary of Health Dr. Umair Shah said in the WADOH’s press release.

According to the WADOH, the disease naturally occurs in wild aquatic birds and can spread to other birds and even mammals.

“On rare occasions, avian influenza viruses infect people and make them sick,” the statement read. “Most instances of people becoming infected with avian influenza have happened after prolonged, close contact with animals infected with avian influenza or environments contaminated with avian influenza.”

Jon Arizti Sanz, PhD, Postdoctoral fellow works in the Broad Institute’s Sabeti Lab testing purchased milk at area grocery stores for the presence of bird flu in Cambridge, Massachusetts, on May 14, 2024. (David L. Ryan/The Boston Globe via Getty Images)

“The CDC currently considers the risk to the general public from this H5N1 avian influenza to be low; however, people with job-related or recreational exposures to infected birds, cattle, or other potentially impacted domestic or wild animals are at higher risk and should take precautions, including wearing personal protective equipment.”