Four new cases of bird flu were detected in farmworkers in Washington state this week. Human infections have cropping in six states as the virus spreads among farm animals.

Four farm workers who helped cull poultry on an commercial egg farm in Washington are presumed to have been infected with bird flu, making that state the sixth in the country to report human H5N1 infections this year.

The worker are the first presumed human cases of highly pathogenic avian influenza, H5N1, under investigation in Washington state, the Washington state Department of Health said Sunday. The workers have had mild symptoms and have been offered antiviral medication by the Benton Franklin Health District. None have required hospitalization.