Possible Origins of Chronic Wasting Disease

Fort Collins, Colorado – A state Division of Wildlife biologist believes a nutritional study he conducted with deer, sheep and goats in the late 1960s might have been the genesis of chronic wasting disease. Gene Schoonveld suspects some of the sheep in his study had scrapie, a relative of chronic wasting disease. Some of the deer might have become infected with scrapie, which then mutated into CWD and spread to other deer.

For more than 25 years, scientists have searched without success for the starting point for CWD, which has spread into the wild and to domestic herds of elk on game ranches.

The state is in the process of killing more than 1,500 wild deer north and east of Fort Collins and more than 1,000 elk infected with CWD on game ranches throughout the state.

Schoonveld admits he doesn’t have conclusive proof, but he said if the sheep had scrapie, it might have “jumped” from the sheep and mutated in deer as CWD. The deer and sheep were penned together from 1968 to 1971 during his master’s degree project at Colorado State University.

Schoonveld was attempting to determine why mule deer didn’t digest alfalfa and natural hay supplied during extremely harsh winters. Over the course of study, about three dozen deer died of what later would be identified as classic CWD symptoms.

It wasn’t until 1977 that CWD was positively identified in the family of transmissible spongiform encephalopathies, or TSE, which includes scrapie in sheep, Creutzfeldt-Jakob disease in humans, and bovine spongiform encephalopathy or mad cow disease in domestic cattle.

“I learned after the study that some of the sheep may have had scrapie, and I think they infected deer, which in turn infected wild deer that came around the pens during rutting season, in particular,” Schoonveld said.

He said that during the study, Colorado State University left sheep in pens where his spare deer were kept. Schoonveld said he was told to use any sheep he wanted as a comparison for the deer in his study.

“During that time, two or three dozen of my deer died, and when I sent them to CSU for a necropsy, it always came back that they died of enteritis, an inflammation or infection of the intestinal tract,” Schoonveld said.

Many of the doe deer came from the wild where they were bred, gave birth, and were turned back into the wild after having been in the pens.

“The sheep didn’t show evidence of scrapie at the time,” Schoonveld said. “I know because I went into pens where I had 40 or 50 deer in reserve and 30 to 40 sheep. I got what I needed from that pen, but it wasn’t until later I was told those sheep came from a scrapie project. Since scrapie and CWD is so closely linked, what else could it be?”

Schoonveld said after CSU graduate student Beth Williams positively identified chronic wasting disease in deer and elk in 1977, “it became crystal clear it was CWD that had killed my deer.”

But the exact cause of CWD in deer or how they contract it remained a mystery.

“I think Gene’s hypothesis is very reasonable. I, too, would lean toward scrapie, but there’s nothing to prove it is how this disease first began,” said Williams, now a professor at the veterinary science lab at the University of Wyoming in Laramie.

Williams said she asked around about sheep with scrapie being on the campus in the late 1960s and couldn’t find anyone who was aware of any such project.

“However, they didn’t always diagnose scrapie so I wouldn’t rule it out,” she added.

Wildlife division veterinarian Mike Miller, who has done extensive study on CWD, doubts Schoonveld’s theory.

“In all the literature we have searched there never has been a mention of scrapie in sheep in those pens during that period. And even if there was, there is nothing to prove CWD is the result of a transfer of scrapie from infected sheep to deer or elk,” Miller said.

Cleon Kimberling, a professor of clinical sciences at CSU, said he also heard there was scrapie in the pens at the time of Schoonveld’s study, but had no proof.

Steve Kerr, a work-study student at CSU at the time of the experiment who now is a veterinarian in Torrington, Wyo., said, “I remember at the time there were sheep that had surgeries, but they weren’t kept as long as the deer. They could have had scrapie, but we didn’t know it.”

Going back through slides from the pens, Kerr said he doesn’t see sick sheep but does see sick deer.

“There have been sheep with scrapie on grazing allotments in the wild, but the disease doesn’t seem to have spread to deer. The only thing different was the proximity of their being in the pens,” he said.

That’s what led Schoonveld to his conclusion.

“I’m guessing it was prolonged nose-to-nose exposure between infected sheep and deer that may have led to the jump,” he said.

Wild deer would come around the pens during the rut and because it was a single fence, Schoonveld said, they could have come nose to nose with the deer in the pens and may have contracted CWD, which spread into wild herds.

Transmissible spongiform encephalopathies attack the brain and central nervous system, destroying healthy tissue. The victim loses basic physical and mental abilities as the disease progresses. The word spongiform describes the spongelike condition of brain tissue, which has microscopic holes in it.

To date, there is no cure for the fatal disease.

Since proteins differ from one species to another, pathogens — proteinaceous infectious particles, or prions — are far less likely to survive when transferred, or “jumped,” from one species to another. This “species barrier” is extremely difficult to breach.

If a breach happens, however, scientists say the prion could mutate and be rather easily passed to other members of the same species.

“We don’t know how it jumps, but we do know it’s possible for diseases of one species to cross the barrier to other species,” said Gregory Raymond, a scientist with Rocky Mountain Laboratories, Laboratory of Persistent Viral Diseases, in Hamilton, Mt.

“Sheep that grazed in an area may have left a disease on the ground that can’t be killed by disinfectant, and it could pass on to other species,” Raymond said.

Extended nose-to-nose contact, or a deer licking the placenta of an infected sheep might be a factor in a disease crossing the species barrier, said Raymond, who added that hasn’t been proved.

Reprinted with permission. By Gary Gerhardt, Staff Writer, Rocky Mountain News, Colorado USA November 5, 2001.