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Bison Range bighorns falling to pneumonia

A bighorn ram photographed at the National Bison Range in western Montana. (U.S. Fish and Wildlife Service)
A bighorn ram photographed at the National Bison Range in western Montana. (U.S. Fish and Wildlife Service)

WILDLIFE -- Breeding season is kicking into gear for bighorns at the National Bison Range, but an alarming number of the wild sheep are dying from disease.

An outbreak of pneumonia has killed at least 37 bighorn sheep on the federal wildlife refuge north of Missoula.

The disease has appeared in several other wild bighorn herds in Idaho, Washington and throughout western Montana. Pneumonia has killed at least half or more of the populations in the Bonner, Rock Creek and southern Bitterroot Valley in recent years.

While it is typically transferred from domestic to wild sheep species, the pneumonia strain appears to only be fatal to the wild animals.

Here's more information reported by Rob Chaney of the Missoulian:

“We haven’t ever had a confirmed pneumonia die-off,” U.S. Fish and Wildlife Service wildlife biologist Amy Lisk said on Tuesday. “We had a report of several years of poor lamb recruitment [into adulthood], but that was 60 or 70 years ago. The types of tests we have now weren’t available then.”

In addition to killing all ages of bighorn sheep, western Montana pneumonia outbreaks have also tended to suppress populations by either lowered pregnancy rates or poor lamb survival for several years. The disease does not appear to affect other species of wildlife and is not related to the pneumonia strains that humans catch.

Lisk said there are no known contacts with domestic sheep inside the Bison Range, which has extensive fencing around its 18,800-acre territory. However, Bison Range personnel are working with experts from Montana Department of Fish, Wildlife and Parks and other institutions to develop a fuller picture of the outbreak.

Before the outbreak started killing bighorns this August, the range estimated it had about 175 wild sheep. November is the species’ breeding time. Male bighorns carry distinctive curled horns and may weigh more than 300 pounds. Females have shorter curved horns and average 125 to 150 pounds.

The Bison Range’s mountaintop-crossing Red Sleep Road closed to the public on Oct. 16. Visitors may still travel the lower Winter Drive, weather permitting. The range visitor center is open Monday through Thursdays in winter from 10 a.m. to 2 p.m. and the road gates are open from 7 a.m. to 6:30 p.m.

While state FWP biologists killed many sick bighorns in previous outbreaks, that tactic was intended to prevent the spread of infection to other herds that were still disease-free. Lisk said because the Bison Range herd is essentially a single breeding population, there’s no need or benefit to culling bighorns inside the range.

That isolation might have a silver lining, however. The National Bison Range bighorns have been extensively studied for at least 30 years, and detailed records exist of its breeding and behavior patterns.

“All these animals are individually identifiable,” Lisk said. “That’s what makes it unique. There are so many things we don’t know about this kind of pneumonia die-off. This may lead to learning more about this kind of disease.”

Rich Landers
Rich Landers joined The Spokesman-Review in 1977. He is the Outdoors editor for the Sports Department writing and photographing stories about hiking, hunting, fishing, boating, conservation, nature and wildlife and related topics.

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