Eight Rocky Mountain bighorn sheep died in southwest Colorado last month, and all showed signs of pneumonia, state wildlife officers said Wednesday.
The Division of Wildlife said seven bighorns were found dead and the eighth was so sick it had to be euthanized. Five were rams and three were ewes.
The bighorns were part of the small Fossil Ridge herd near Gunnison, about 130 miles southwest of Denver. Because the herd had only about 50 bighorns before the deaths, the loss of eight animals is significant, officials said.
Field examinations suggested pneumonia was the likely cause of death for at least some of them. Tests at the Division of Wildlife laboratory in Fort Collins found evidence of the disease in all eight.
Further tests are planned, with results expected by mid-January, division spokesman Joe Lewandowski said.
J Wenum, the division's Gunnison area manager, said wildlife officers are monitoring the survivors and evaluating treatment options, but they are limited.
"It's not like cattle where you can run them into a pen and make sure they get the proper dose," he said.
Wenum said antibiotics could be placed in feed or in darts fired from rifles, but it would be difficult to ensure the right animals were treated and that they got the correct dose.
Trapping and treating the animals would be risky because they are already ill and physically stressed, he said.
The Rocky Mountain bighorn sheep is the state animal and has become an iconic image of the Colorado mountains. Rams carry a distinctive curled horn.
About 7,500 live in the state, compared with an estimated 275,000 elk and 600,000 deer. The state permits limited hunting of bighorns, with about 300 licenses issued each year.
A rancher east of Gunnison reported the deaths of three of the sheep on Dec. 23. Wildlife agents found the four other dead sheep and the sick sheep that they euthanized.
Lewandowski said winter is tough on bighorns and other wildlife and advised residents to keep their distance to avoid worsening the animals' stress.