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HELENA — Wildlife officials will let a pneumonia outbreak run its course through a herd of bighorn sheep west of Anaconda after killing dozens of sick animals failed to keep the disease in check.

There are no known vaccines to prevent pneumonia in bighorn sheep, which is usually fatal for the animals, Montana Fish, Wildlife and Parks officials said Tuesday. Instead, wildlife officials kill sick sheep to prevent other animals from being exposed.

The agency has killed 44 bighorn sheep in the herd of about 300 animals west of Anaconda since confirming the pneumonia outbreak in August. But FWP officials say the outbreak has now spread beyond management control.

The whole population appears to be exposed and there is nothing to gain by killing more sheep, FWP Regional Wildlife Manager Mike Thompson said in a statement. Instead, the focus is now on trying to keep alive every animal that has a chance of surviving the outbreak.

"It was worth a try to see if we could eradicate the disease early on, when we still had hope it hadn't spread very far," Thompson said in the statement. "But now we can't tell the difference between sheep that will die and sheep that might survive."

Officials don't yet know how many animals have been killed in the outbreak because the animals are too spread out, FWP spokeswoman Vivica Crowser said. The number of fatalities may be clearer once the rut begins in a few weeks, she said.

Pneumonia killed 60 percent of a different herd in the Upper Rock Creek area earlier this year after officials let the disease run its course. FWP said other herds have lost 90 percent of their populations in other extreme cases.

The Anaconda herd lost more than half its population during a pneumonia outbreak in 1991-1992, and another 19 percent of the population died the next year.

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Wildlife officials suspect the Anaconda herd outbreak may have come from the Upper Rock Creek herd outbreak, Crowser said. The Anaconda herd is otherwise isolated except for a small population of bighorn sheep near Garrison, she said.

The failure to keep the outbreak in check won't cause the state agency to re-examine its methods, Crowser said. Culling sick bands of bighorn sheep has improved survival rates in other herds in the past, she said.

"If we saw the opportunity again we would take it. Unfortunately, it didn't work out in this case," Crowser said.

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