More than half of Idaho's rebounding bighorn sheep herd may have been exposed to a potentially deadly bacterial infection, state Department of Fish and Game officials say.

Department specialists said as many as 1,800 of the 3,300 bighorns roaming the state may be at risk. The race is on to stop the disease before infected sheep wintering along the Salmon River return to summer ranges to spread it further.State big game supervisor Lloyd Oldenburg said that results from diagnostic tests confirmed that four wild sheep and two domestic sheep have been infected with pasturella hemolitica, which causes pneumonia in wild sheep.

Wildlife biologists began flying over five herd areas in the Salmon River Canyon below North Fork and the Middle Fork of the Salmon drainage and in the Morgan Creek area. Some target groups will be netted from helicopters for testing beginning next week, and radio collars will be put on other sheep to monitor movements.

The disease is usually fatal, but Oldenburg said surviving animals become carriers posing a threat to the rest of the herd. It is commonly transmitted by sheep touching noses.

Discovery of the disease came just as the state completed its first auction of a hunting tag for bighorns, which brought over $60,000 to be used in rebuilding the herd.

Oldenburg said the department was halting its transplant program, a key to rebuilding the herd, until it can certify targeted bighorn sheep as disease free.

Tests for the organism were positive on two of three bighorn sheep found dead two weeks ago near Pine Creek, about 20 miles downriver from North Fork in central Idaho. The third sheep had been mostly eaten by predators and could not be tested.

Two other bighorns were trapped and tested positive for the bacteria, Oldenburg said. The two domestic sheep tested were part of a flock pastured near Pine Creek last summer.

There was no way to determine how the disease was introduced, he said, although domestic sheep are suspect.

The organism, working with a naturally occurring lung worm carried by bighorn sheep, causes the animal's lungs to deteriorate.

"The animals appear to be in good shape otherwise," Oldenburg said. "Those that die, die quickly - in one to two weeks - before they have a chance to noticeably decline."

The infection was first noticed about two weeks ago when several sheep were seen coughing, a prime symptom of the disease, Oldenburg said. Department personnel had been watching herds in the area because of the presence of domestic sheep last summer.