Mad deer disease? Researchers puzzle over brain illness in North American wildlife

Science News, Nov 30, 2002 by Susan Milius

This autumn, the nation's big-game hunters are lifting their guns and bows in the service of science. They're collecting the biggest sample ever of deer and elk brains--predicted to total 200,000--to test for a once-obscure wildlife disease that's become the stuff of headlines and headaches coast-to-coast. So-called chronic wasting disease strikes mule deer, white-tailed deer, and elk. It riddles the brain with tiny holes as the victim slowly withers and dies. Once found in the wild only in an area intersecting Colorado, Wyoming, and Nebraska, the disease appears to be spreading. This year, it turned up in wild herds in South Dakota, New Mexico, and Canada and jumped all the way to Wisconsin and Illinois.

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.