NACHES — State and federal biologists plan to euthanize wild bighorn sheep in central Washington to curb the spread of a deadly pneumonia outbreak.

Biologists will spend the next several weeks removing sheep from the Tieton herd, about 10 miles west of Naches and 20 miles northwest of Yakima. The herd is currently estimated at up to 50 animals.

Almost all of the animals will likely need to be euthanized, because most of them are believed to be infected with the disease that causes pneumonia, said Richard Harris, wildlife manager for the Washington Department of Fish and Wildlife.

Recent surveys showed a majority of the live sheep to be in poor condition, he said, with about a third of those animals coughing or showing other signs of the disease.

“We hate to have to take this action, but we believe it’s necessary to stop the spread of a disease that could devastate adjacent herds of wild bighorn sheep in the area,” Harris said in a statement Monday.

The disease is often fatal in wild bighorns and can affect the survival rate of lambs later born to animals that survive the disease. There is no treatment or preventative vaccine.

The Yakima area is home to about a third of the state’s 2,000 wild bighorn sheep, divided into four separate herds.

The disease has already significantly reduced the size of the Tieton herd, which once numbered as many as 200 animals. So far no dead or sick bighorn sheep have been found outside the Tieton herd, the statement said.

Wildlife managers took similar steps in 2010 to stop the spread of the disease in the Yakima River Canyon.

Past outbreaks among bighorn sheep have been linked to contact between wild sheep and domestic sheep or goats that carry, but are unaffected by, the bacteria that cause the disease.

However, Harris said there is no evidence there was contact between domestic and wild sheep in the Naches area.