For the second time in three years, a pneumonia outbreak in a Yakima-area bighorn sheep herd is prompting the state wildlife department to kill some or perhaps even all the diseased sheep to prevent them from infecting neighboring herds.

Dying or dead members of the Tieton bighorn herd began showing up along the U.S. Highway 12 corridor in early March. Subsequent aerial and ground surveys turned up about 25 dead sheep and Department of Fish and Wildlife biologists expect to find many more, since those surveys found only 35 to 50 surviving animals out of an estimated population of more than 150.

“Even if there are 50 animals still surviving, that’s a huge decline,” said Rich Harris, the wildlife department’s section manager for sheep, goats and moose. Many of those surviving animals are also believed to be infected, “coughing or showing other signs of the disease,” he said.

Harris said that biologists would do “some culling” and sending tissue samples to Washington State University’s veterinary lab in Pullman for disease testing. But it’s possible those surviving animals may all have to be killed to prevent their spreading the disease to the neighboring Cleman Mountain herd on the north side of State Route 410.

“It may well get to that,” Harris said. “Our concern is the Cleman herd next door. So far as we know, that (Cleman) herd has stayed healthy and we’re trying to keep it that way.

“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.”

The Tieton herd, believed to number from 50 to 200 animals, was not affected by the 2010 pneumonia outbreak that killed dozens of bighorns in the Umtanum herd west of the Yakima River Canyon. In that case, wildlife officials made the decision to remove as many as 90 more bighorns to prevent the disease’s spread.

Currently none of the Tieton herd animals has radio collars with which the department can track their movements, but some Cleman herd bighorns do, and those tracking devices give biologists no reason to believe the two herd’s members have intermingled in recent months.

To keep it that way, Harris said state wildlife marksmen and others contracted with the U.S. Department of Agriculture Wildlife Services will begin moving from north to south, starting closest to State Route 410 to minimize the possibility of contact between the two herds. This process, which will begin immediately, could take several weeks, Harris said.

The Tieton bighorns are not seen nearly as often as members of the Cleman herd. The latter, in addition to routinely showing up for feeding across from the Oak Creek Wildlife Area headquarters, are often seen along SR 410 on their way down to the Naches River’s edge.

Wildlife officials don’t know what might have caused the current outbreak. As in the case of the 2010 Yakima River Canyon episode, bighorn carcasses tested by Washington State University’s veterinary laboratory were found to have pneumonia apparently caused by — or, at least, accompanied by — the presence of a mycoplasma bacteria. The current strain, though, seems to be a more severe strain.

Because disease takes “only a matter of weeks, not months,” to have its debilitating effect on the sheep, Harris said, the infection is highly unlikely to have been carried through the winter. That would seem to preclude the possibility of the disease having been transmitted from grazing domestic sheep — which can carry the pathogens without being affected — since the grazing season is still more than a month away.

It’s possible there are still healthy Tieton bighorns that may have been missed by the surveys. Should state biologists or Wildlife Services agents find “a pocket of animals that are uninfected,” Harris said, “we might be able to do what we might call a ‘time out,’ and test them live.”

Rapid response is critical, though, with the health of the neighboring herds as the department’s priority, Harris said. “It may well be that we have to take out the entire herd.”

Being forced to remove all or portions of a second herd within three years because of bighorns’ sensitivity to infectious pathogens could have long-ranging impacts on the state’s management of the species, Harris said.

“It’s a management concern,” he said. “That doesn’t mean we’re going to prophylatically get rid of herds that are healthy. But it does affect what we can do.”

• Scott Sandsberry can be reached at 509-577-7689 or ssandsberry@yakimaherald.com.