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.

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.

Four agricultural workers in Washington state tested presumptively positive for avian influenza while working at a farm with an outbreak.

However, there is urgent concern over dairy cows. In California’s Central Valley, the carcasses of dead cattle have been piling up, a phenomenon documented in videos taken by a local veterinarian. According to the CDC, there have been confirmed infections in more than 300 herds of cattle in the U.S. in 14 states since March.

Washington state health officials said four agricultural workers tested presumptively positive for avian influenza, marking the first presumed human cases in the state.

Four workers at a commercial egg farm near Pasco in Eastern Washington have tested positive for avian influenza, according to preliminary test results. They work at a commercial poultry operation that reported many chickens suddenly dying on Oct.

Four people have reportedly developed symptoms of avian influenza after working with infected poultry in Franklin County.

Seven cases of H5N1 bird flu were detected in California this week. No confirmed cases have occurred in people or livestock in Arizona.