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An Unknown Risk

By Kathy Bunch
WebMD Feature

Jan. 29, 2000 -- A mysterious, brain-destroying condition similar to mad cow disease has been identified in herds of elk and deer in several Western states, but scientists say so far there is no evidence that the fatal illness can be transmitted to humans.

But they caution that there is no proof it can't, either.

"Right now, we have no evidence that humans are susceptible. Obviously, we can't say that that can't happen," says Beth Williams, DVM, PhD, a professor of veterinary science at the University of Wyoming who was among a panel of FDA advisors reporting on the ailment, called chronic wasting disease (CWD), to the agency at a meeting on Jan. 19.

The panel concluded there was no evidence that humans could get CWD from eating elk or venison, or from nutritional supplements made from antlers. Nor was there evidence, they said, that hunters should be prevented from donating blood.

More at risk may be the $150 million elk-breeding industry, which has destroyed 13 of its 3,600 herds so far, and the economies of Western states where hunting is big business. In Colorado, where up to 15% of the mule deer are affected, the sport generates $800 million annually, says Dale Lashnits, a spokesman for the state Division of Wildlife, adding that the state sold 300,000 hunting licenses in 1999, the last year for which figures are available.

"It's a fairly significant recreational activity in this state," Lashnits says.

The impact on the deer population could be devastating. Though mule deer have taken the biggest hit so far, researchers fear it could spread to whitetail deer, whose populations are denser and more widespread, says Mike Miller, DVM, a veterinarian with the Colorado Division of Wildlife, which is tracking the disease.

CWD, nicknamed "mad deer disease," is mysterious in many ways, but this much is known: it's spreading, it's always fatal, and there is no known cure.

It was first identified in the 1960s in captive deer in Colorado and is now showing up in herds to the northeast part of the state, southeast Wyoming and Nebraska, and has been found on commercial elk farms in Colorado, Montana, South Dakota, Nebraska, and Oklahoma, as well as Saskatchewan, Canada, says Williams.

The disease is believed to pass from animal to animal through bodily fluids. The more obvious symptoms include weight loss, excessive salivation, chronic pneumonia, twitching, lethargy, and lack of coordination.

The first captive elk came down with CWD in 1997 in South Dakota, says Paula Southman, a spokeswoman for the North American Elk Breeders Association, which raises 170,000 animals in the U.S. and Canada.

Wildlife officials in the affected states are keeping a close eye on animals afflicted with the illness and are warning hunters to take precautions when handling the animals, such as wearing rubber gloves and avoiding contact with the brains, spinal cords, and lymph nodes. They also are encouraging hunters not to harvest any animals that look sick and to bring in their prey for testing, Williams says. The states offer the information in printed and Internet materials.

In Colorado, authorities have extended the hunting season through February in an effort to cull the number of deer in affected areas by half -- thus limiting the opportunity for the disease to spread -- but that may take up to three years, Miller tells WebMD. He says there is no reason to believe this extension puts hunters at risk. A similar illness called "scrapie" has been observed in sheep and goats for 200 or 300 years and people do not become sick from that, he says.

But when three people who had eaten venison came down with a rare and fatal brain disorder called Creutzfeldt-Jakob disease (CJD) between 1997 and 2000, a lot of sportsmen and their families became unnerved. Mad cow disease -- more properly called bovine spongiform encephalopathy (BSE) or new variant CJD when it occurs in humans -- is closely related to CWD.

The deaths prompted an investigation by the CDC, which concluded that none of the victims was exposed to meat from animals with CWD, says Ermias Belay, MD, a medical epidemiologist. "We did not find any strong evidence to indicate that the CJD illness in the three patients was linked with CWD," Belay says.

CWD and CJD are diseases of the nervous system. New variant CJD has killed about 100 people, mostly in Great Britain, since an outbreak in the mid-1990s. Six years after the first person died from the illness, scientists are still struggling to understand how it spreads to humans, how many more will die from it, and more recently what similarities it may have with CWD.

Current theory holds that agents called prions cause these illnesses. Unlike disease-causing viruses or bacteria, prions are normal cell-surface structures (proteins) found in the tissues of humans and animals. For reasons that are not well understood, these occasionally transform into a deadly version that promotes the destruction of cells, leaving spongy holes in tissue, says Belay.

The most common human form of this malady is called "sporadic" CJD, which arises spontaneously in the brains of about one person per million. An estimated 250-300 Americans, mostly over age 50, die from this each year.

When an abnormal prion comes into contact with a healthy one, it can sometimes force the normal prion to become abnormal -- a process that continues until the brain is destroyed, says Gregory Raymond, MS, a microbiologist at the Rocky Mountain Laboratories, a part of the National Institutes of Health.

Experiments done at the laboratories show that abnormal deer prions can convert normal deer and elk prions into deadly ones, Raymond says. But when put in contact with normal human prions, the chances of conversions "were very low," he says, though they were possible.

"Our data should be reassuring," he says. "Putting it into a broader context, driving down the highway is probably more risky than some of these diseases."

Of course, at first no one thought humans could get "mad cow" either.

"First it was just a cow disease and didn't have any good evidence that it was going to be transmissible to people," says Williams. "Certainly it's similar [to CWD] in that respect."

A big difference in the illnesses is that the millions of people exposed to the cow malady did not know about the disease when then they ate their hamburgers and steaks. For hunters out West, it's a different story.

"These hunters have a choice whether to hunt or not," Williams says. "There's an element of informed consent here that's not present when animals are going into the human food supply in the commercial market."

Kathy Bunch is a freelance writer in Philadelphia.

 

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