Wildlife experts are trying to determine what's killing Nevada's largest herd of California bighorn sheep.
Hunters north of Winnemucca began reporting dead or dying sheep to the Nevada Department of Wildlife just before Christmas.
An aerial survey of the southern Santa Rosa Mountains conducted in early January found only 54 bighorns, down from 90 counted in the same area four months earlier, officials said.
Two bighorns that appeared ill were captured and killed by biologists for analysis.
"They were showing signs they were basically dying of pneumonia," said Department of Wildlife spokesman Chris Healy.
"Evidence has started to pile up we could be having some problems. We just don't know the size of it."
Biologists at first suspected domestic sheep spotted in the same area, which is closed to grazing, might have infected the bighorns. That's what nearly killed all native sheep in Nevada's mountains after European settlers arrived.
But tests conducted by the Nevada Department of Agriculture found the two bighorns killed by biologists were infected with a lung worm often found in wild bighorns.
"This is an indigenous parasite for bighorn sheep," said Dr. Anette Rink, a veterinarian with the agriculture department. "This is not an indication there was intimate contact with domestic sheep."
The two bighorns also suffered from mineral deficiencies that likely weakened their immune system, Rink said.
Healy said results of other tests are pending to learn the scope and cause of the apparent die-off.
"We need to find out if there's some kind of disease event that's affecting significant numbers," he said. "We're hearing too many anecdotal reports."
The situation is of particular concern because the Santa Rosa herd represents one of the biggest success stories in Nevada's effort to re-establish bighorn sheep.
Twelve California bighorns were introduced in the Santa Rosa Mountains in 1978. By 2003, the population had swelled to about 300 over the entire mountain range, which extends 50 miles south from the Oregon border.
Since the late 1960s and through fiscal year 2000, Nevada spent $288,000 on efforts to re-establish California bighorns. In 2000 alone, about $178,000 was spent on efforts to help the California, Desert, and Rocky Mountain subspecies of bighorns across the state.
On Jan. 8, biologists were scheduled to capture bighorns from the Santa Rosa Mountains for relocation elsewhere in Nevada and in Idaho. But that project was halted because of the apparent die-off.