GREEN RIVER - Scientists remain unsure what caused a mysterious illness that has led to the death of 250 elk southwest of Rawlins, wildlife officials say.

Researchers have ruled out bacterial diseases as the most likely cause of the illness, according to the Wyoming Game and Fish Department.

Scientists are currently testing tissue samples from infected elk and ground, water and forage samples from the area in an effort to determine the source of the pathogen.

"It's a real head-scratcher," Game and Fish Public Information Officer Tom Reed said.

"We're narrowing in on toxins … the lab folks have eliminated all of the major bacterial and well-known diseases such as chronic wasting disease, mad cow disease, brucellosis, that sort of thing," Reed said Wednesday in a phone interview.

"We've still got guys in the field finding more elk … the count is up to 250 now," he said. "When we find them, we're putting them down because these critters are pretty much stressed out."

Researchers from the Wyoming State Veterinary Laboratory and the department are continuing to search for the source of the toxin, which will be much easier once the toxin is identified.

"The first step will be to identify the toxin and then the source … and then we'll see what we can do to keep elk from getting into whatever this is," Reed said.

"We've taken samples from the elk … we're also basically looking for anything that they might have eaten or consumed, we've taken samples from nearby water sources … we keep looking at all those type of things."

The discovery of dead or dying elk was initially reported to the department Feb. 8 when a coyote hunter found two elk near Rawlins. The elk were alive, but unable to move.

Since the initial discovery of about 80 dead or dying elk, officials on foot and using four wheelers and airplanes have found a total of 250 diseased elk over the past two weeks.

Reed said all of the elk exhibit similar symptoms. The elk were in good condition and showed no evidence they had been harassed or run until exhaustion.

"It looks like what they do is come in and the herd beds down, but when the herd gets up and moves off, some of them can't get up," he said. "From the amount of scat that's piled up … that indicates they've been there quite a while."

University of Wyoming officials said students are helping scientists with sampling and some lab work.

Tissues from 10 elk are now being analyzed in the state vet lab to determine the cause of death. Officials noted the animals exhibited muscle lesions, excessive fluid within skin tissues and that most of the affected elk have been cows.

Reed said the disease is particularly unique because it doesn't seem to have affected in any way the other wildlife in the area including pronghorn antelope, mule deer, birds and wild horses.

The dying or dead elk were scattered over a 10-15 square mile area within the department's Elk Hunt Area 108. People are being asked to stay out of the hunting unit while the investigation is ongoing.