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Deer disease hasn't infected humans -- yet

April 30, 2001 Rocky Mountain News (Denver) by Dick Foster

Deer and elk hunters in northeastern Colorado and southeastern Wyoming may be bagging a mysterious and deadly traveler along with their quarry -- a close cousin of Britain's "mad cow" disease that could flare up and kill them years down the road.

All who eat the infected animals are potentially exposed to chronic wasting disease, or CWD, always fatal in deer and elk.

The question -- and it remains a major medical mystery -- is whether CWD can spread to the human population. If so, the disease will have jumped the species barrier, just as the closely related mad cow disease infected humans in England in the 1990s.

The families of mad cow victims describe a terrifyingly rapid deterioration in their loved ones, in which memory lapses became dementia and bedridden immobility within a few months.

This is the situation Colorado wildlife officials admit could happen with CWD. A tiny percentage of diseased deer are taken by hunters every year. Can CWD spread to people who eat sick animals as happened with mad cow disease?

At the point, no one knows.

"In a sense, those people are doing their own experiment," said Dr. Larry Schonberger of the National Centers for Disease Control and Prevention in Atlanta. "I'm sure they've been told that there's a lot of unknowns with this disease."

So far the disease is contained in relatively small numbers of animals in the limited geographical area of the two states, with wildlife experts uncertain as to how it might spread to other deer populations.

And there is no scientific evidence that chronic wasting disease has ever passed from deer or elk to humans, so the species barrier -- the biological firewall that protects one species from another's diseases -- appears intact.

But similar confidence was voiced when mad cow disease appeared in English cattle during the 1980s. People continued to consume English beef. Then, 10 years later, the disease erupted in its human victims, ravaging their mental and physical capacities as they died.

To date, at least 94 people who ate diseased beef in England have died of a human form of the disease known as "new variant" Creutzfeldt-Jakob Disease, or nvCJD, which is invariably fatal.

The potential threat of chronic wasting disease to humans places Colorado in an uncertain position.

Wildlife officials are concerned about protecting hunters and the public, but lacking any hard scientific evidence that the disease actually threatens people, options such as hunting bans or required testing of killed animals have been deemed unwarranted.

Chronic wasting disease in deer and elk, CJD in humans and mad cow disease in cattle belong to a family of diseases called Transmissible Spongiform Encephalopathies, or TSEs. The term "spongiform" describes the perforated, spongelike condition of the brain found in victims during autopsy.

Doctors know one thing -- spongiform diseases kill without exception. Those infected will die. It is a fatal illness with no recovery and no cure.

Origin is uncertain

Doctors and scientists are uncertain how CWD originated or how it spreads among deer and elk.

Scientists saw the earliest version of spongiform disease in a sheep and goat illness called scrapie in the 1700s. Now, besides deer, elk, cattle and humans, they see it in minks, zoo monkeys, and domestic and wild cats.

The human form, CJD, is named after two German physicians Dr. Hans Gerhard Creutzfeldt and Dr. Alfons Jakob, who in 1921 linked the wasting and dementia of its human victims to millions of microscopic cavities discovered in victims' brains at autopsy.

CJD remains rare. It infects about two to five people annually in Colorado, about one person per million worldwide.

A few cases of CJD are apparently inherited. Others have been caused by transplanting tissue from a diseased person. A cornea transplant and a few cases of human growth hormone injections have each led to fatal CJD in their recipients.

The cause remains unknown for most human cases, which seem to erupt spontaneously in middle-age adults, killing them within eight to 10 months.

Now scientific evidence is strongly linking the human strain -- new variant CJD -- to mad cow disease.

The human victims in Britain most likely ate meat that had been contaminated with fragments of brain, central nervous system and lymphatic tissue when the animals were mechanically deboned and their meat ground into hamburger or sausage.

The human victims were found to have patterns of damaged brain tissue similar to those of the cattle that died of mad cow disease. Those patterns of damaged brain tissue were significantly different that those in traditional human CJD.

This gave scientists the first indication that they were dealing with an insidious new desease capable of jumping from one species to the next and infecting humans through the food they ate.

English cattle are believed to have contracted the disease by eating the remains of dead, scrapie-infected sheep that were mixed into cattle feed to enhance protein content.

In the United States, the Food and Drug Administration banned the use of dead sheep and other mammals in cattle and other ruminant feeds in 1997. But a nationwide FDA inspection last year found up to 28 percent of rendering plants and 9 percent of FDA licensed feed mills had no system to prevent the mingling of such materials in feed.

Most of the errant companies were unaware of the regulation and have made corrections, FDA spokeswoman Gloria Dunnavan said.

"By the time we leave the plant, firms are already under way with ways to remedy the problems," she told the Rocky Mountain News.

No cases of mad cow disease in cattle or the new variant CJD in humans have appeared in the United States, health officials emphasize.

However, last month, federal agriculture officials seized two flocks of European-reared sheep in Vermont for study to determine if they'd been infected with mad cow disease, not the traditional scrapie. It was suspected that they or their parents might have been fed protein derived from diseased cattle. If they test positive, it would be the first known case of mad cow disease in the United States.

But test results won't be known for two years. Although a microscopic exam of the sheep's brain tissue will quickly show if the animals have a spongiform disease, the brain tissue must then be incubated in mice to determine its strain, whether scrapie or mad cow disease.

Hunters take risks

In Colorado, the big question is this: Can people contract a variation of CJD from diseased deer and elk as they have from diseased cattle in England?

Again, nobody knows.

There is no known case of a human contracting the illness from eating diseased deer or elk. And humans do not appear to contract CJD from eating lamb and sheep even though scrapie has been around for centuries.

Three hunters -- one each from Oklahoma, Utah and Nevada -- died of CJD between 1997 and 2000. But all had traditional CJD, not a variant resembling that of the diseased animals, said Dr. Ermias Belay of the Centers for Disease Control and Prevention.

But that is not entirely reassuring to medical experts.

"Nobody knows for sure that it couldn't spread to people and, therefore, there is every reason to be cautious," said the CDC's Schonberger.

The trail of the disease is narrow and difficult for researchers to track. Compared with the millions who eat domestic beef, the number of people who eat wild deer and elk each year is exceedingly small. Smaller still is the number of animals eaten that have chronic wasting disease.

Only 5 to 15 percent of the deer in northeastern Colorado and 2 to 18 percent in southeastern Wyoming are infected. It's been seen in only two elk, about one-tenth of 1 percent of those tested in the past four years.

Hunters killed 2,146 deer and 1,262 elk last season in northeastern Colorado, where chronic wasting disease has been found. In southeastern Wyoming they killed 5,497 deer and 798 elk. Based on wildlife experts' estimates, between 107 and 321 diseased deer might have been killed in Colorado, and between 109 and 989 diseased deer in Wyoming.

The chances of killing a diseased elk are about 1 in 1,000.

During a special hunt to thin the diseased Colorado herds in January and February, 19 of 205 deer killed by hunters were infected with CWD, the state Division of Wildlife said.

Another difficulty in tracking spongiform diseases is their long, silent incubation periods. It may take 10 years or more before symptoms appear, a few months before the victim's death. There are no medical tests, as in other illnesses, to determine whether a person has contracted the disease.

"The problem is that it's difficult to get the information we need. Our technology to deal with this agent is not great. You can't tell from any kind of lab test on a normal person whether they're incubating this disease," Schonberger said.

The specter of CWD spreading into human hosts concerns experts enough to urge caution among hunters and those who eat game animals.

"If an animal tests positive (for chronic wasting disease), it's not a good idea to eat it, and it should be disposed of," said Dr. Beth Williams, a Laramie, Wyo., veterinarian and university professor who identified CWD as a spongiform disease in 1977.

Williams thinks that experts' advice on CWD is not always heeded.

"I think some people probably do go ahead and eat an animal that tests positive," she said.

Disease a mystery

There are other deeply troubling aspects about chronic wasting disease and all of the spongiform diseases, medical scientists say.

The cause of all spongiform diseases, including CJD in humans, is not a virus or bacterium. Rather, its a kind of mutant protein particle called a prion that spreads through healthy brain and nerve tissue forming cavities and lesions. The prion is not even a life form in the traditional sense; it contains no nucleic acid, DNA or RNA.

The prion stubbornly resists traditional methods of killing infectious agents, including extreme heat used in sterilizing surgical instruments.

At Tulane University last year, surgical instruments used on a patient with a neurologic disorder were then used on eight other patients before the first patient was diagnosed with CJD and later died.

Now, say Tulane officials, all eight patients "may have been exposed to CJD" and "might have some risk of contracting it."

A similar case was revealed at Denver's St. Joseph Hospital last month. Hospital officials said six brain surgery patients there may have been exposed to CJD through the use of surgical instruments that had been used earlier on a CJD patient before she was diagnosed with the disease.

Officials at both hospitals said the CJD risk is "not eliminated by normal sterilization protocols.

That's why cooking the diseased beef in England did not eradicate the disease, and it may not protect those who eat diseased deer or elk here.

"The prion is only rendered inactive by bleach. That's the only thing that deactivates the prion. Cooking won't do it, unless you baste your venison in Clorox," said Dr. Randall J. Bjork, a Colorado Springs neurologist who has treated CJD patients.

Perhaps the most important discovery about the spread of CJD came from three physicians who studied a CJD-type illness called "kuru" in a New Guinea tribe during the 1950s.

Members of the tribe who engaged in cannibalism, eating their own dead tribesmen for lack of other food, were afflicted with the fatal wasting disease. Microscopic brain examinations of the dead tribesmen showed the perforated brain tissue described three decades earlier by Creutzfeldt and Jakob.

The illness was passed on, they concluded, by eating the diseased tribesmen.

The disease was halted when the cannibalism was stopped.

Scientists' faith in the species barrier to protect humans from other varieties of spongiform disease had seemed intact. Then mad cow disease rewrote the book.

For centuries humans did not appear to contract the disease from scrapie-infected sheep. But cattle got the disease when they ate the infected sheep, and humans contracted it from cattle.

Even accomplished physicians like Bjork profess some apprehension over the spongiform diseases. They are concerned about the ability to jump species, to withstand traditional methods of sterilization and disinfection, and to avoid detection in creatures for years until it erupts in a final destructive phase.

"It's fascinating, but it's very weird, and actually a little scary," Bjork said.

Knowing as little as medicine knows about them, Bjork says, it is unwise to diminish their potential threat to humans.

He and Williams issued similar cautions to hunters: Hunt in areas where CWD has not been found. Don't harvest a sick animal, or take an animal that seems too easy to shoot. Wear gloves and protect open wounds when dressing the animal. Bone the meat, and don't eat meat that contains brain, spinal column or lymph nodes. Send the head to Colorado State University for testing before eating any of the animal.

"This is serious stuff," Bjork said. "Any attempts to make light of the safety issues or to downplay it is not in the best interests of the public."


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