Scientists look for cause of bighorn sheep deaths

BOISE – Something is killing Rocky Mountain bighorn sheep in Hells Canyon, and for the first time biologists in the three Northwest states that border the rugged chasm have started capturing sick lambs to figure out the cause.

Researchers at Washington State University are examining tissue taken from the lambs. Their findings could have ramifications for wild sheep throughout North America.

“That would be a huge step forward,” said Frances Cassirer, a wildlife research biologist with Idaho Fish and Game. “At least then we’d be dealing with a known enemy. It’s a problem for bighorn sheep almost everywhere they occur.”

Biologists say about 2 million bighorns once inhabited the West, but they disappeared over most of their range in the 1800s and early 1900s due to disease and unregulated hunting. Reintroductions and added protection in the last 50 years have boosted bighorn numbers to about 50,000.

But sweeping epidemics of a mystery illness wiped out thousands of Rocky Mountain bighorns, California bighorns, Sierra Nevada bighorns, and desert bighorns, thwarting attempts by wildlife biologists to fill empty habitat.

Bighorns disappeared from the 8,900-square-mile Hells Canyon area that separates Idaho from Oregon and Washington in the 1940s, but now number about 900 thanks to sheep brought in from other states and Canada.

Biologists say the canyon could support 10,000 sheep, but the illness kills about 100 annually – and in bad years as many as 300 – including every lamb it infects.

To find out why, biologists are capturing and euthanizing as many as 10 sick lambs and taking them to the university’s diagnostic lab in Pullman. By midweek, biologists had captured five lambs.

Tom Besser is a professor in the department of veterinary microbiology and pathology at WSU, and works at the Washington Animal Disease Diagnostic Laboratory on the school’s campus.

Besser said it is clear, from studying lambs found dead in the canyon in past years, that they are dying of bacterial pneumonia. But he said healthy sheep have the bacteria in their upper respiratory tracts and it only leads to pneumonia when the sheep become weakened.

“The confusing thing is that different lambs in the same die-off sometimes have different bacteria in their lungs,” Besser said. “If it’s an epidemic of an agent, you expect to find the same agent that’s killing everything.”

Besser said that means there is something else making the sheep sick – the main suspect being a virus.

“A virus comes through first, knocks down the lungs’ defenses, then bacterial pneumonia gets them,” he said. “A virus is the most likely. But viruses are a lot harder to find than bacteria.”

Besser said the lab will use recently developed techniques to search bighorn tissues for virus DNA or RNA that he said have been identified as typical of a virus.

“It’s not a sure thing to find a virus even if a virus is causing this predisposition to disease,” said Besser. “The less we know about them, the less chance we have of detecting them.”

The research might answer whether bighorns are catching the lethal disease from domestic animals, which some wildlife biologists say is what they have observed.

A U.S. Forest Service study completed in February found Hells Canyon bighorns to be at risk from disease carried by domestic sheep on grazing allotments on the Payette National Forest in Idaho.

Besser said most viruses only affect one species, but there are exceptions.

“If it’s a virus that affects both domestic and bighorn sheep, then it’s possible that the bighorns are getting it from the domestics,” said Besser. “If we find a virus that is bighorn specific, then it will blow that theory out of the water.”

Cassirer said it’s too early in the process to speculate about what the next step might be if a disease agent is isolated.

“The research is extremely important for bighorn sheep everywhere,” said Neil Thagard, director of development with the Foundation for North American Wild Sheep, a hunting and conservation organization. “Hopefully, we’ll be able to save some bighorn sheep.”