(La Grande, OR) -- A pneumonia outbreak is killing bighorn sheep in Hells Canyon. The Oregon Department of Fish and Wildlife along with officials in Washington and Idaho are monitoring the situation. Pneumonia outbreaks often lead to die-offs that can range from five percent to the entire population. Usually, about half of the population will survive. There's no way to treat the animals. They'll monitor the population over the next several months and try to keep the bighorn sheep from spreading it to other populations.