Washington state health officials are investigating after four agricultural workers tested presumptively positive for bird flu, marking the first presumed human cases in the state, the Washington State Department of Health announced Sunday.

During the monitoring period, after the seasonal influenza period had ended, 59 wastewater treatment plants showed increases in the H5 gene in bird flu that coincided with the recorded emergence of highly pathogenic avian influenza H5N1 in dairy cattle.

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