YAKIMA, Wash. — The last of the Tieton bighorn sheep herd was shot and killed Monday morning, seven months after members of the herd began showing up dead or dying along the Highway 12 corridor, victims of a particularly virulent strain of pneumonia.

But now, having contained one potentially disastrous disease outbreak that might have spread to neighboring herds, wildlife officials in this part of the state have another outbreak to worry about: a recurrence of the same disease that in 2010 killed dozens of bighorns in the Umtanum herd on the west side of the Yakima River Canyon.

Following that pneumonia outbreak, Washington Department of Fish and Wildlife officials lethally removed as many as 90 more bighorns to keep the disease from spreading to the bighorns on the east side of the Yakima River.

But this year, after what regional wildlife manager Ted Clausing called “two good lamb years” in 2011 and 2012 that had helped the herd’s population rebound to nearly its pre-outbreak numbers, this year’s lambs have experienced a significant dieoff.

The wildlife department’s Yakima office began receiving reports of sick lambs in early summer, roughly about when the lambs are being weaned from their mothers and learning to fend for themselves.

“In the last couple of years (the Umtanum herd has) produced 50 to 60 lambs each year,” said state wildlife biologist Jeff Bernatowicz. “This year it’s probably going to be 20 or fewer.”

Some ailing lambs were sent for testing to Washington State University, where Thomas Besser, an expert in veterinary pathology, has done extensive studies on bighorn dieoffs throughout Western states. Besser verified that the strain infecting the Umtanum lambs was the same one responsible for the 2009-2010 outbreak.

Still, having the previous two disease-free seasons was probably “a best-case scenario” for the Umtanum herd, Besser said.

“The worst-case scenario,” he said, was Hells Canyon on the Snake River in Washington’s southeast corner and Idaho, where most lambs continue to perish from the disease every year, 18 years following a massive dieoff in 1995.

“Within these individual populations, it’s not uncommon for there to be a year or two of good lamb survival,” Besser said. But (the disease) always comes back, so right now I’d say that’s what happened at Umtanum: It was controlled within these animals for those two years, but it’s still there.”

The fact that the current outbreak has apparently been confined only to the lambs, with no adults apparently yet affected, gives biologists some hope.

“I don’t think anybody can predict (whether it will spread to the adults),” Bernatowicz said. “Given that the herd’s already been through it once and we saw some mortality but not a total wipeout, there’s some hope that the adults would be fine and that it will be strictly lamb mortality.”

If the mortality continues at a rate of nearly 80 percent of each season’s new lambs, though, the population will ultimately dwindle until its adults are too old to produce young. That’s the case of one of the Hells Canyon sub-populations in a place called Sheep Canyon, where only seven aging bighorns remain. Said Besser, “It currently just looks like it’s going to survive as a population until those die off.”

So far, biologists believe the Cleman Mountain bighorn herd has managed to avoid disease infections from either of its neighboring herds, the Umtanum to the east or the Tieton to the south.

Part of that is probably the result of the state wildlife department’s quick decision to remove the surviving members of the Tieton herd.

Because rams during the autumn mating season had been known to travel between the Cleman and Tieton herds looking for ewes, biologists and contract hunters had been scouring the hillsides overlooking Highway 12. By the end of May, well before the rut, they believed they’d gotten all but three of the herd’s surviving adults. Two of the three were found and killed last week.

The final ram killed on Monday marked the 55th Tieton bighorn lethally removed on behalf of the wildlife department. Biologists believe the remaining members of the herd, which had numbered between 150 and 200 prior to the outbreak, all died from the disease.

State wildlife officials will probably begin the process of reestablishing the Tieton herd with animals from another herd in “a year or two,” Clausing said.

The reasoning for bringing the herd back, he said, was simple to “not having all your eggs in one basket. If you only have a few herds and one gets sick, you’ve lost a big chunk of the population.” Having more distinct herds, he said, becomes “sort of an insurance policy.”