Mad deer disease? Researchers puzzle over brain illness in North American wildlife.
This disease belongs to the same class of maladies as mad cow disease, which appeared in Britain in 1986 and about a decade later, showed up in people who had eaten tainted meat.
Last year, some 11 million people hunted deer and elk in the United States, and many more helped them eat their prizes. So far, the news for hunters looks reassuring. Several weighty groups, including a panel from the World Health Organization, have concluded that there's no evidence so far that people can catch chronic wasting disease.
These reassurances come with plenty of caveats, since scientists know relatively little about transmission of the disease and its relatives. The recent alarm over sick deer and elk has shaken loose new funding for research. Scientists are experimenting with laboratory rodents, setting up controlled experiments in livestock, and scrutinizing cases of brain disease in people.
OUT OF OBSCURITY Elizabeth Williams, the veterinarian who in 1978 discovered the critical brain degeneration in chronic wasting disease, recalls her breakthrough research as "nothing fancy." She's now a wildlife pathologist at the University of Wyoming in Laramie, but she made her discovery when she was still a graduate student in Colorado.
Wildlife-pathology students traditionally dissect animals that perished from various ailments, and one day Williams set about analyzing a mule deer that had died at a research station. With an emaciated frame, the animal seemed to have suffered a mysterious nutritional ailment that had been killing animals there since 1967. To be thorough about her job, Williams examined slices of brain under a microscope. Myriad tiny holes dotted the tissue, a discovery that at the time seemed interesting but not earthshaking.
"I didn't go, `Eureka,'" she says. "I went, `Oh, maybe it's scrapie.'"
Several phenomena can punch an animal brain full of holes, and one of the most widespread is the sheep disease called scrapie. Shepherds and pathologists have recognized it since the 18th century. Afflicted animals twitch, lose weight, and finally die of paralysis. Williams wondered if some relative of scrapie might be saw aging the brains of the mule deer.
Because sheep haven't been known to transmit their brain disease to people despite centuries of opportunity, in the late 1970s and early 1980s--well before mad cow disease--"there wasn't a lot of concern about scrapie," Williams remembers. So, her new mule deer disease didn't spark much alarm.
Scavenging a few thousand dollars here and there for research, she and some interested colleagues established that chronic wasting disease attacks elk and white-tailed deer as well as mule deer. At first, the researchers knew the disease only from captive animals at two western research stations. In the mid-1980s, though, surveys discovered chronic wasting disease in wild herds in northeastern Colorado and southeastern Wyoming. Later, pathologists detected the disease in an adjacent area of Nebraska. Today, the brain malady strikes perhaps 6 percent of deer and less than one percent of elk in this endemic area.
Nobody knows how the disease first arose. The western deer and elk may have somehow caught sheep scrapie, Williams speculates. Or perhaps the brain malfunction started spontaneously in one unlucky wild beast and has been passed around ever since. Also, the deer may have caught the disease from some source that pathologists haven't yet imagined.
"In the realm of wildlife diseases, there's a lot we don't know--believe me," Williams says.
Similar spongy-brain diseases have appeared in at least 10 animal species, so far. Farmed mink, domestic cats and some of their bigger relatives like cougars, and such hoofed stars of zoo displays as bison, kudu, and oryx succumb to this sort of brain disorder.
Williams' troubled wildlife attracted more attention after the discovery of mad cow disease in Britain, and researchers are now exploring the idea that the diseases spread via misfolded proteins, or prions, instead of bacteria or viruses.
When mad cow disease first turned up, its similarities to scrapie made it seem unlikely to be a hazard for people. However, there is a human spongy-brain condition. Called Creutzfeldt-Jakob disease, it pops up randomly in about one in a million people, runs in families, or spreads via cannibalism. A victim can remain outwardly healthy for years then go into a rapid course of confusion, loss of memory, and death.
In 1996, the British government announced evidence that a new variant of Creutzfeldt-Jakob disease might come from eating cattle suffering from mad cow disease. Suddenly, other animals with spongy-brain diseases--especially animals like elk and deer that hunters butcher and eat--seemed threatening.
That same year, chronic wasting disease turned up in a new arena. Elk on a Saskatchewan game ranch became ill. Pathologists worried that other ranches might have instances of the disease, too. Animals carry it for years without showing symptoms, and ranchers sell and ship live elk and deer over great distances.
Since 1996, the disease has struck at least 20 commercial U.S. herds, mostly of elk. Contaminated animals shipped from North America to South Korea spread the disease to game farms there.
This year brought more worrisome news. In February, Wisconsin game managers announced that a routine exam they'd been doing for several years on animals shot by hunters had turned up three white-tailed deer with chronic wasting disease. By the end of the summer, the managers found 40 sick deer, all in an area west of Madison. State wildlife officials declared a special hunting season during the summer to try to eradicate the deer--and stamp out the disease--in the two counties where it had been found.
"Now, we're scaring the stuffing out of hunters," says Scott Wright, branch chief at the U.S. Geological Survey's National Wildlife Disease Center in Madison. The sale of hunting licenses is sagging this season, he says. Some meat processors have stopped carving up venison for hunters, and certain landfills refuse deer carcasses.
Galvanized by the outbreaks in captive herds and Wisconsin wildlings, game managers in other states are encouraging, or even ordering, hunters to submit the heads of killed deer and elk for disease testing.
Looking so much more intensively for the disease will probably reveal more pockets of it, predicts Gary J. Wolfe, a wildlife biologist in Missoula, Mont., who serves as project leader at the sportsman's coalition called the Chronic Wasting Disease Alliance.
RISKY QUESTIONS In all the furor, what people most want to know is whether the disease can cross from meat to meat eater or handler. Patrick Bosque, a neurologist and prion researcher at the Denver Health Medical Center says that he doesn't yet know of a reliable experiment to see if a prion disease of animals will infect people. New molecular evidence convinces him that scientists don't yet fully understand the basics of prion transmission (see box below).
When Proteins Go Bad How does a mysterious agent cause chronic wasting disease? The basic infectious entity causing mad cow disease, chronic wasting disease, and related brain destruction has puzzled scientists for decades. Researchers couldn't pin the disease on a bacterium or virus. The disease-causing agent didn't even seem to have genetic material of any sort. Stanley Prusiner of the University of California, San Francisco proposed an entirely different villain--a misfolded protein. First met with incredulity, his idea won him a 1998 Nobel prize. Misfolded forms of normal brain proteins, or prions, trigger the brain-eating diseases, contends Prusiner, a biochemist and neurologist. Warped prions corrupt normal ones into going haywire, too, and folding into the wrong shape. Thus, the disease spreads without any virus or other DNA-bearing pathogen. Once scientists began to look for misbehaving proteins that could spoil others of their kind, a similar phenomenon even turned up in yeast. Mercifully, there's no indication that anyone has to worry about mad beer disease. As researchers venture into the world of prions, the territory gets stranger and stranger. For example, there's evidence that a prion can have strains, says Patrick Bosque of the Denver Health Medical Center. Because all the prion proteins made by one host animal have the same sequence of amino acids, scientists had expected these prions to show the same behavior. Yet in various experiments, Bosque says, prions from the same animal appear to misfold in several ways, sometimes creating different effects. Also, Bosque notes, some species survive as apparently unfazed carriers of prions that kill another species. Prions from a diseased hamster, for example, didn't make a mouse sick during a 2-year experiment. However, an inoculate of brain tissue taken from mice at the end of the experiment induced fatal prion disease in hamsters. The original prions probably didn't last 2 years, so the mouse must have created the agent that was transferred to the hamsters. "The hamster is more sensitive to the mouse prions than the mouse is," Bosque marvels.--S.M.
Yet, other scientists have plunged into experiments looking for animal-to-animal transmission of chronic wasting disease. Tests with such a worrisome disease require keeping animals inside secure facilities where workers take showers and change clothes each time they leave the animals. Workers also heat-treat the waste from all their test animals.
At the National Animal Disease Center in Ames, Iowa, researchers have taken liquefied brain tissue from diseased mule deer and injected it into the brains of raccoons, sheep, and cattle. Amir N. Hamir reports that so far, 4 of 13 cattle and 1 of 4 sheep of a type prone to scrapie have developed the prion disease. The raccoons, and scrapie-resistant sheep have shown no sign of prion disease, now some 3 years after exposure. Hamir plans to continue the cattle-and-sheep experiment for another 5 years.
Williams, however, is exploring more realistic scenarios. She fed cattle one dose of liquefied tissue from a diseased mule deer. Afar 2 years, she sees no signs of disease. Also, she's monitoring cattle penned in a paddock with sick deer. Here, too, she reports no signs of transmission after 3 years.
Both Hamir and Williams warn that prion brain diseases can take many years to show up. Good news so far from their experiments means no more than that there's no bad news yet.
"There's no strong evidence" that any person has contracted chronic wasting disease, says Ermias Belay of the Centers for Disease Control and Prevention in Atlanta. He and others are examining any suspicious cases of Creutzfeldt-Jakob disease that turn up.
Chronic wasting disease has afflicted wildlife in Colorado and Wyoming for at least 20 years and probably longer. So far, neither federal nor state health officials have found more-frequent prion-related deaths of people in areas known to host the wildlife disease than in areas untouched by chronic wasting disease.
Of course, the records may have missed prion deaths. Conservative voices also debate whether the numbers of people exposed so far are great enough to detect a disease that affects people infrequently.
Three provocative cases turned up during the late 1990s, Belay says. Two 28-year-olds and a 30-year-old who had eaten venison or elk meat died after rapid mental deterioration.
Belay and his colleagues researched these peoples' lives, reviewed their medical records, and examined what tissue samples remained, However, no link showed up between eating game meat and the disease, the researchers reported. The "most important evidence," according to Belay, was that he couldn't find any indication that the game meat had come from areas where chronic wasting disease occurred.
Scrutiny of brain-disease cases continues, Belay says.
All the uncertainty though, isn't going to stop Wolfe from hunting elk, he says. He's even traveling from Montana to the corner of Wyoming with endemic disease. "I hope people are going to put this in perspective," he says. "I figure my greatest risk is driving to get there."
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|Date:||Nov 30, 2002|
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