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.

Four agricultural workers in southeast Washington have preliminarily tested positive for bird flu after working around an infected poultry flock at a commercial egg farm.

The Washington State Department of Health is investigating after four agricultural workers tested presumptively positive for the first human infections of bird flu.