All of the state's cases have been linked to contact with infected dairy cows, officials said. No human-to-human transmission has been documented.

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 cases, if confirmed, would be the first time people in Washington have contracted the virus since officials began tracking a notable rise of it in birds and other animals