Saturday, September 10, 2005
Copyright © Las Vegas Review-Journal

Bighorn sheep deaths in July still unsolved

Graphic by Mike Johnson.

Biologists still are baffled by what caused the deaths of 22 desert bighorn sheep in July, after tests on the animals were negative for disease, toxins and bacteria.

Steven Kimble, supervising biologist for the Nevada Department of Wildlife's Las Vegas office, said Friday that the bighorn sheep deaths near a watering hole in the McCullough Range during a July heat wave might remain a mystery.

"We don't really know quite ... what to look at. We haven't found anything positive," he said.

A statement Thursday from the department cites an Aug. 31 report by state veterinarian Dan Crowell, who said, "We did all the testing we reasonably could for 13 different factors," including metals, poisons and bacteria.

Crowell said three factors were not ruled out: blue-green algae, dehydration and botulism, or some combination of the three.

"Although water samples were negative for the toxins associated with blue-green algae and botulism toxicity, it is possible that the toxins could have been present and were subsequently flushed from the system with the rain," Crowell stated.

Two men riding all-terrain vehicles discovered the dead bighorns July 24 near a developed watering hole, known as a guzzler or drinker, that is maintained by the wildlife staff in the McCullough Range south of Henderson.

Kimble said there was no evidence that a lightning strike killed the bighorns. Game wardens thought lightning was to blame for the deaths of 13 bighorns in 1990 in the Mormon Mountains, 65 miles northeast of Las Vegas.

Examination of the bighorn carcasses from the McCullough Range showed they had died at least 24 to 48 hours prior to a lightning storm early July 24.

Researchers found no reports of lightning strikes in the area the previous week and there was "no observable evidence of lightning on the carcasses, or in the environment. In addition, some of the animals were as far as 200 yards apart," according to the Department of Wildlife statement.

A rainstorm that occurred the night of July 23 and lasted into the early morning hours of July 24 could have filled the guzzler, which might have been dry prior to the rain.

The rainstorm followed a string of record-setting high temperature during the previous week, including 117 degrees on July 19, which tied the highest mark ever recorded in the Las Vegas Valley on July 24, 1942.

However, Kimble said the sheep carcasses contained no evidence of severe dehydration. "It's hard to imagine bighorn sheep that evolved in the area would succumb to that," he said.

The dead sheep showed no signs of trauma, such as being shot.

Biologists also ruled out anthrax, heavy metal and antifreeze contamination, toxicity, strychnine, respiratory viruses, fertilizers and a compound used in predator control.

Asked about the possibility that the West Nile virus could have killed the sheep because two Gambel's quail and a pair of mourning doves also were found dead in the area, Kimble said there are no previous accounts of West Nile virus affecting bighorn sheep.

Also, he said, "We would not have expected to have all of them die in such a short period of time so close to each other."

West Nile virus, which is carried by mosquitoes, affects people, horses and birds and can be fatal.

Kimble said wildlife officials have retained some of the sheep tissue samples and they will continue "to watch the literature to see if there are any other ideas" that could solve the mystery.