Four workers at a poultry farm in Franklin County, Wash., tested positive for bird flu, Washington Department of Health officials said Sunday.

The infected workers had mild symptoms and were treated with antiviral medications. Other workers are being tested, according to the department.

This is the first documented case in Washington of humans contracting avian influenza. Humans have been infected in five other states this year, according to health officials.

The workers were sickened on a farm where bird flu had been detected the same week. According to USDA, about 839,700 chickens were euthanized after the Washington State Department of Agriculture confirmed Oct. 15 the virus was present in the flock.

The Benton-Franklin Health District tested workers exposed to the chickens. Samples have been sent to the Centers of Disease and Prevention for further analysis and to confirm the positive results.

Workers on any poultry farm in Benton or Franklin counties should contact the health district at (509) 460-4550 if they have symptoms such as red eyes or respiratory problems, according to health officials.

Prior to the Washington cases, 27 people in the U.S. had tested positive for avian influenza this year, according to the CDC.

The sickened people had prolonged contact with infected cattle or poultry, or with environments contaminated with the virus, according to health officials.

California has the most cases, with 13 people contracting the virus from cattle. In Colorado, nine were infected by poultry and one by cattle, according to the CDC.

The first U.S. case this year was reported in late March. A worker exposed to infected dairy cattle in Texas tested positive. Humans also have caught bird flu in Michigan, with two cases, and Missouri, one case.

In most cases, 17, the sickened person was in contact with infected cattle.

Poultry infected nine people. The source of the virus for one person is unknown, according to the CDC.

The only previous U.S. case of a human sickened by avian influenza before this year occurred in 2022.

The person was sickened while culling birds at a poultry farm, according to the World Health Organization.

Avian influenza can be fatal. The WHO confirmed fatal cases earlier this year in Cambodia and Vietnam.

The CDC and WHO consider the risk of the virus spreading among the general population to be low.

No human-to-human transmission of the virus has been reported, according to the WHO.