Last November, on the Sunday before Thanksgiving, a hunter at the Darby game check station told wildlife officials about a dead bighorn sheep that appeared to have been hit by a car. Turned out, the sheep had been hit by pneumonia.
"For all we knew," recalls Montana Fish, Wildlife and Parks (FWP) wildlife manager Mike Thompson, "maybe we found the first sick sheep."
It was the first of many.
Five months later, wildlife biologists in Montana and four other western states find themselves scrambling to contain nine bighorn die-offs, a pneumonia epidemic the Western Association of Fish and Wildlife Agencies describes as "unprecedented."
Before the outbreak, wildlife officials estimated there were about 1,000 bighorn sheep in the four wild herds surrounding Missoula—in Bonner, the East Fork of the Bitterroot, and upper and lower Rock Creek. To prevent the spread of the pneumonia, known to kill up to 90 percent of a herd, FWP culled about 225 infected animals. Scores more have been found dead.
"By the time this is over," Thompson predicts, "I would say that we might have lost upwards of 500 sheep"—half of the regional population.
This past winter and spring, Wyoming, Utah, Washington and Nevada have also seen deadly pneumonia outbreaks, but none as severe as in Montana, home to an estimated 5,700 wild sheep, excluding the populations in Yellowstone and Glacier parks. Biologists are puzzling over how nine geographically distinct herds could all suffer from the same infection.
"It's certainly possible that, just as a matter of coincidence and probability, herds in a wide scattering of areas encountered the source of the pneumonia," says Thompson. Still, he adds, "It's remarkable that it's happened in so many places."
Wild sheep risk getting pneumonia if they have contact with domestic sheep or goats carrying the pathogens. Stress can be a culprit, too, Thompson says.
One factor contributing to the spread of infection, he suspects, is that the beginning of the epidemic coincided with the rut, a time when rams move great distances to breed, making contact with a great many sheep along the way.
Whatever the cause, the epidemic is not only affecting wild sheep—it's also affecting sheep hunting. FWP is not offering any 2010 tags for hunting bighorns in Bonner, lower Rock Creek or upper Rock Creek (hunting districts 283, 210 and 216, respectively). The agency is offering only one either-sex license for district 270, near Darby.