Four agricultural workers in Washington state were recently diagnosed with avian influenza A (H5N1). The cases mark the first human infections of the disease under investigation in the state.

Four agricultural workers in Washington state have tested positive for bird flu, the first human cases of H5N1 virus to be reported in the state. Washington is the sixth state to report a human infection of avian flu.

Bird flu is presumed to have infected four poultry workers in Washington state, making it the sixth U.S. state to identify human cases this year, according to the Washington State Department of Health.