BOISE, Idaho - Biologists at Washington State University say they have isolated a bacterium that may have contributed to the deaths of thousands of bighorn sheep in the West over the past five decades.

The biologists say they found mycoplasma ovipneumoniae in tissue taken from dying lambs captured in Hells Canyon - a chasm that borders Idaho, Oregon and Washington. They believe it inhibits the ability of hairlike structures in the sheeps' airways to eliminate bacteria that lead to pneumonia - a leading cause of death in bighorns.

"This is the first problem I've worked on where there is quite a bit of evidence piling up where the agent is a mycoplasma," said Tom Besser, a professor in WSU's department of veterinary microbiology and pathology.

In herds known to be infected with mycoplasma, more than half the sheep die each year from pneumonia. Lambs are the most susceptible, mainly because their immune systems are not fully developed, said Frances Cassirer, a wildlife research biologist with Idaho Fish and Game.

Pneumonia is the leading killer of bighorns infected with mycoplasma, she said. In herds not infected, the leading cause of death is predators, she said.

In blood samples from herds in several Western states and Alberta, Canada, WSU researchers found antibodies to the mycoplasma in herds that experienced deaths due to pneumonia, but not in herds that had not.

"We found some really promising patterns, and things seemed to fit together really well," Cassirer said.

Further testing on the bacteria's effects are being conducted.

North Dakota has about 300 bighorn sheep - mostly in the Badlands in the western part of the state. Biologists say the animals were reintroduced in North Dakota in 1956, and have rebounded after the die-off of about 150 bighorns in 1998.

About 2 million bighorns once inhabited the West, but they disappeared over most of their range in the 1800s and early 1900s due to unregulated hunting and disease believed to have been carried by domestic animals, biologists say.

Repopulating projects and added protection in the last 50 years have now boosted bighorn numbers to about 50,000, Cassirer said.

But sweeping epidemics of a mystery illness have wiped out thousands of Western bighorns, and biologists say the herds are not growing as fast as they should be.

Vic Coggins, a biologist with the Oregon Department of Fish and Wildlife, said pneumonia likely was more to blame than even unregulated hunting for the species' decline. He said habitat loss also factored in, but there is enough habitat available now across the West to support far more than the current population.

"We estimate that, in Hells Canyon, we could have over 10,000," Coggins said. The area is currently home to about 900, he said.

If mycoplasma bacteria are to blame, treating infected sheep may not be possible, said Cassirer. Attempts to develop vaccines for the bacteria for domestic sheep have failed, and even if a working vaccine existed, administering it to wild bighorns would be difficult, she said.

Furthermore, biologists have not found that infected herds can build a resistance to the bacteria over successive generations, Cassirer said.

"If it's happening, it's not obvious to us. That's why we're looking for another solution, because the sheep might not be able to deal with it on their own," she said.