Little concensus in CWD's cause, spread


Mar 11, 2001
Reaction score
Chronic, sure, but why is it widespread?

Little consensus on cause of CWD's explosion in U.S.

By Katy Human, Daily Camera (Boulder) Staff Writer

June 29, 2002

One year ago, chronic wasting disease seemed confined to a wedge of land stretching from northern Colorado to southeastern Wyoming.

Few cases of the devastating disease, which kills deer and elk by destroying their brains, appeared beyond the edges of what biologists called "the endemic area."

Then last summer, chronic wasting appeared to break free of some invisible constraint. Suddenly, white-tailed deer in Wisconsin had it. So did elk in game ranches in Colorado, Kansas, Oklahoma and Nebraska. The fatal disease flared up in northern Boulder County — the southern edge of its known range — attacking one in four mule deer in one herd. Then it appeared even farther south, in New Mexico.

Officials in several states are now planning to kill tens of thousands of deer, an effort to contain what some now call an epidemic.

But experts do not agree about why the disease has advanced into new areas, whether it's a new disease or how to manage it.

"I think we've got a handle on this thing now," said Don Ament, Colorado's agriculture commissioner.

"I think it's going to crop up all over the Western and Midwestern United States," countered Charles Southwick, an emeritus biology professor at the University of Colorado.

"This is a very, very new disease and we just don't know everything about it," said Todd Malmsbury, spokesman for the Colorado Division of Wildlife.

'Very serious threat'

Like related diseases — scrapie in sheep, Creutzfeldt-Jacob in people — chronic wasting disease moves slowly through the brain of an infected animal.

The first thing to go is grace. Deer begin to stumble as their reflexes slow down. As months pass, the animals become stuporous, staring blankly and drooling. Some are taken by predators and hunters, others are struck by cars, others die unnoticed.

In the past nine months, sick animals have turned up for the first time in Kansas, South Dakota, Wisconsin, Nebraska, Oklahoma and New Mexico. In many of those states, the response has been dramatic.

Officials plan to kill up to 25,000 deer in a 461-square-mile area of southwestern Wisconsin, where 18 white-tailed deer are known to have come down with the disease so far.

"We're treating this as a very serious threat to our deer herd," said Bob Manwell, spokesman for Wisconsin's Department of Natural Resources.

The killing strategy, often called "culling" by wildlife managers, is based on researchers' understanding of the dynamics of infectious diseases, said Thompson Hobbs, an ecologist at Colorado State University, who recently received a five-year, $2.2 million federal grant to study the mysterious disease.

Imagine a box of marbles, he said, all of them white but one black one, representing a diseased animal. If there are a lot of marbles in the box, the black one will bump into white ones relatively frequently. Fewer marbles mean fewer disease-spreading collisions, Hobbs said.

"If you think of every contact as the potential for transmission, you can see why higher density means higher infection rates," he said.

Hobbs and his colleagues are trying to nail down how chronic wasting spreads and if there are certain places where it's more likely to reach high rates of infection — in deer herds near housing developments, for example, where animals may bump into one another more often than if they're spread out across a wilder landscape.

It will be several years before he has solid results. In the meantime, state wildlife managers state wildlife managers are attempting to keep the disease from spreading by killing all deer that may have come into contact with sick animals.

After a deer on Sugarloaf Mountain died of the disease, for example, officials killed about 20 nearby animals.

"We don't think we could ever eliminate chronic wasting disease from the endemic area," said Todd Malmsbury, spokesman for the Colorado Division of Wildlife. "But we think we have a reasonable chance of keeping the disease from spreading if we're aggressive in surveillance and in killing deer on the periphery."

Game farms blamed

Wyoming officials are following Colorado's strategy and will try to stamp out new hot spots of chronic wasting disease by culling nearby animals.

Tom Thorne, acting director of the Wyoming Game and Fish Department, says researchers still don't understand a lot about chronic wasting disease, but he's certain about one thing: Game ranchers have moved it around the continent.

"With few exceptions, this increase is following distribution of game farming," Thorne said. Several wild animals that fell sick with the disease in Nebraska, Colorado and Wisconsin were discovered within a few miles of game farms, he said.

Thorne agreed with Colorado's Malmsbury that the disease appears to be a relatively new one. It was first detected in the 1960s and has probably been simmering in the wild since then, quickly in game ranches.

But retired biologist Southwick, who studied epidemiology at Johns Hopkins University before coming to CU, has a different idea.

"Maybe CWD is a natural disease that has been in our deer herds for many decades and it is currently being exacerbated by stressful conditions," he suggested.

When deer and other animals are stressed in captivity or by overpopulation, they may be less able to fight off a disease such as chronic wasting.

Southwick picked back through records from the 1920s and 1930s, when Westerners were killing wolves and mountain lions in great numbers. Deer populations exploded, the animals picked over trees and shrubs and then began dying of starvation.

Or so biologists assumed.

"I found they often used three terms to describe the symptoms: Emaciation, staggering and slobbering," Southwick said. Those are classic symptoms of chronic wasting disease.

"It's made me wonder whether there have been occurrences in the past and then, when environmental conditions get better, the disease drops to low prevalence," Southwick said.

He's quick to say that he may be wrong. Ranchers may have moved the disease around or it may be spreading in animal feed.

Many elk and deer ranchers give their animals pellets that contain ground road-killed deer and other animal parts, Southwick said. If some of those road-killed animals had the disease, that practice could prove deadly.

And, he said, Division of Wildlife biologists might be right, that chronic wasting is a relatively new disease spreading across the continent.

But killing animals that may or may not have chronic wasting disease could backfire, he said. It may cause animals to disperse and carry the disease to new areas, or it might destroy animals with some sort of natural immunity.

New findings?

Wisconsin's Manwell said he doesn't buy the suggestion that the disease has been around for centuries.

Wisconsin biologists have been sampling a few hundred animals every year for the last three and never found it before now, he said. Because wildlife ecologists around the country have been interested enough in the disease for the last five years, someone surely would have seen it earlier.

But Ament said it's easy to imagine missing the disease, because its symptoms resemble those of other ailments, such as internal injuries or starvation, for example.

What's new now is that people are looking for it, and "you don't find it if you don't look for it," he said.

He said he thinks the only explosion the disease has gone through is an explosion into the news. It's a mysterious disease that threatens the hunting industry in the West and chron'ic wasting is related to another mysterious disease, mad cow, which probably killed dozens of people with a disease called variant Creutzfeldt-Jacob disease.

Although scientists say there's no evidence chronic wasting disease has or could infect people, there's no evidence that such a leap is impossible.

Ament said he expects scientists to have a live test for elk within a year, which will mean elk ranches will soon be squeaky clean, and no longer suspects in disease spread. Although he called the state's wild deer culling program a bit excessive, he said it's understandable, given public worries.

"I am more optimistic," he said. "I think we might find some more but I think it won't be an alarming kind of thing."

Contact Katy Human at (303) 473-1364 or
Top Bottom