Beneath majestic peaks and along rolling shortgrass and sagebrush prairies, a killer is stalking Rocky Mountain wildlife.

Threatening the very foundation of the region as an outdoor wonderland, the mysterious disease is reducing Colorado's deer and elk to staggering, starving victims of an infectious agent that rots the brain, kills the host and survives to strike again.

The sickness - called chronic wasting disease - has now crossed the Continental Divide, a once-unimaginable leap frightening to sportsmen, biologists and politicians alike. At stake: the storybook wildlife herds and small-town livelihoods sustained by free-spending hunters.

"This is the worst-case scenario," said Chuck Reichert, a district wildlife manager in northwestern Colorado who lately has been forced to gun down the very deer herds he is paid to help manage, all in a desperate effort to slow the spread of CWD. "Wildlife and hunting is the root of our economy, it's what rural people do to make a living."

Colorado is, in fact, the epicenter of the emerging disease, one among a cluster of closely related maladies known as transmissible spongiform encephalopathies, or TSEs. Mad cow disease is a TSE that has killed humans in Britain and elsewhere in Europe. The common denominator is a rogue protein called a prion that attacks healthy brain tissue, riddling it with holes.

Once believed confined to a sparsely populated, 14,873-square-mile region of northeastern Colorado and southern Wyoming, chronic wasting disease's leap west of the Rockies - and its equally stunning jump east, across the Mississippi River into Wisconsin - is sounding national alarms.

Scientists, including a Nobel Prize winner, the meat industry and public health experts are scrambling to learn more about the baffling disease. CWD has not been found to infect cattle or people, but the deadly experience with mad cow disease is driving urgent research on the question.

Regarded for more than two decades as nothing more than an obscure, slow-moving animal sickness - indeed, it has been described as an epidemic in slow motion - troubling new developments are triggering fresh concern over chronic wasting disease:

- Discovery in recent months of what appears to be a rampant expansion of the disease has pushed CWD from its long-festering confinement to wildlands of three additional states.

- Chronic wasting disease is spread in the environment from animal to animal, perhaps via contact with feces, urine and saliva. That's different from the cattle version of TSE, which was passed along when cows were fed infected meal ground from diseased animals.

- There's no known case of CWD infecting a person, but recent research raises new concerns. One set of laboratory experiments demonstrated that the disease could be undetectable in one group of exposed mice, then, when passed to another group, strike with uniform deadliness.

- The disease has prompted a publicly backed wildlife slaughter of historic scale. State biologists in Colorado and three other states have fanned out to kill hundreds of wild deer to both curb the spread, and test for the presence, of CWD.

- The disease appears especially virulent among white-tailed deer, a highly social species that thrives in vast numbers east of Colorado. In one 800-acre pen in Nebraska, half the whitetails contracted CWD, a staggering rate of great concern to biologists.

- CWD has ravaged the domestic elk industry. Agricultural agencies in six states have slaughtered more than 3,800 captive elk exposed to wasting disease, costing taxpayers nearly $15 million, most of it to compensate elk ranchers. In Canada, CWD has struck in two provinces, leading to the slaughter and incineration of nearly 8,000 domestic elk.

- Wildlife agencies and commerce officials say chronic wasting disease could cost states billions of dollars if hunters, scared off by infected herds, stay away.

- In just months, CWD has erupted into a political crisis. In Colorado, with state wildlife and agricultural agencies feuding over the role of wildlife and captive elk in spreading the disease, officials haven't agreed on regulations designed to manage CWD.

At the root of the dread over CWD is the belief that it may be the most virulent and easily transmitted - among the species it infects - of all the TSE strains, which also include scrapie in sheep and the rare but naturally occurring Creutzfeldt-Jakob disease in humans.

In all forms, the disease attacks the brain and central nervous system, destroying healthy tissue. The victim loses basic physical and mental abilities as the disease progresses.

There is no known cure for the fatal illnesses, and there is no way to test for them in their early stages.

If the story of chronic wasting disease were written as a mystery novel, it would be lacking the first chapter. There are strong, plausible theories about its origins in Colorado, but as with so much else about the disease, there is no definitive proof.

Late one night in November 1977, an eager young Colorado State University graduate student named Beth Williams peered into a microscope at tissue of a deer's brain.

Her curiosity fueled by a great interest in wildlife diseases, Williams was poring over slides containing brain tissue from deer that had died of chronic wasting.

"No one had an idea at the time what was causing deer to waste away in our pens, but as I looked at the slides, I noticed lesions that were absolutely striking," Williams said in a recent interview. "They looked exactly like scrapie, the holes that form in sheep brains."

Williams, now a professor of veterinary sciences at the University of Wyoming in Laramie and a leader in the field of CWD research, had discovered the "bullet" in the mystery that is CWD. The disease attacked the brain.

Did scrapie jump the species barrier to become CWD in deer?

"While it's reasonable to assume such a jump occurred from sheep to deer, there is absolutely nothing to prove it," Williams said. "It's equally as plausible that CWD is a naturally occurring disease as well."

It was only a matter of time before the strange illness would be seen outside the pens. But no one was prepared for just how it happened.

In 1981, a magnificent bull elk, one of Rocky Mountain National Park's living icons, was found dead of chronic wasting disease.

It was the first known CWD death outside of the pens at CSU and Sybille.

Exactly how CWD became a killer in the wild remains part of the mystery, and the CWD prion has proved very tough to kill.

"Honestly, CWD was little more than a curiosity in deer we were studying until mad cow came up in Britain and the concerns of the public forced us to look at CWD as a potential health hazard," said Tom Thorne, a respected Wyoming wildlife veterinarian.

When mad cow jumped into humans, it quickly became a natural extension to ask what would happen to hunters who unknowingly ate CWD-infected deer or elk. And might CWD jump into cattle, threatening the state's ranches, great and small?

Studies are ongoing to try to determine if deer and cattle sharing the same habitat might result in CWD jumping to cattle. None of the cattle has shown symptoms. In another experiment in which CWD prions were injected directly into the brains of 13 test cattle, three have become infected to date.

The Colorado Cattlemen's Association points to a Harvard study that discounts the risk of mad cow disease affecting U.S. cattle, and concludes the risk from CWD is minimal.

And while there's no proof that CWD can jump to cattle naturally, the cattle industry supports more research into the risks - if any - posed by CWD, according to Terry Fankhauser, executive vice president of the Colorado Cattlemen's Association.

"Due diligence is what's appropriate," Fankhauser said. "Wherever an unknown lies, we're concerned with that."

In just the past six months, CWD cases have been reported for the first time ever outside the endemic area in Nebraska, South Dakota, Wisconsin, and most recently in Colorado on the western slope of the Continental Divide.

On March 29 of this year, with the Colorado Capitol nearly deserted of both lawmakers and reporters, Gov. Bill Owens hastily assembled directors of three state agencies, called a press conference and dropped a bomb.

The news: Chronic wasting disease had been discovered on the Western Slope, a possibility that scientists and politicians had feared with equal dread. On this day, what had been a relatively obscure wildlife disease affecting mostly the state's wind-swept eastern prairie blew up into a front-burner political, scientific and economic crisis.

As it happened, the two wild deer that tested positive for the disease were found inside the fences of an elk ranch.

Since that time, two more were found inside and six free-ranging deer outside the ranch's fences have tested positive.

The killer is loose in the wild on the Western Slope, which has brought the governor to the fore.

Owens warned that the Western Slope outbreak could cost the state "hundreds of millions of dollars."

His math is well-grounded. There are an estimated 550,000 deer and 270,000 elk in Colorado, making the state a premier area for hunters and watchers of wildlife. Both are big business.

The popularity of elk - and the tourist dollars they draw - is exemplified by the 730,000 visitors to Rocky Mountain National Park during September and October each year when the bulls bugle during rutting season.

One fear: In its zeal to eradicate CWD from local deer, the state Division of Wildlife is throwing the baby out with the bath water. "They're clearing all the deer out - are they going to put the deer back in?" said Al Shepherd, a plumbing and heating man in Craig, Colo.

The state responded to CWD on the Western Slope as it has in other areas, proceeding to thin herds in the vicinity of where infected animals were found. The word "sharpshooter" has re-entered the government lexicon.

CWD has ignited a wildlife slaughter that would have seemed the stuff of fiction a few months ago. It amounts to the premeditated taking of the very wildlife that make Colorado what it is.

Government has sponsored large-scale wildlife kills before. Witness the eradication of buffalo, wolves and "trash fish."

But sharpshooting of creatures adored by the public to manage a disease has sparked protest from those who believe it's an overreaction. Plans to kill herds in Boulder County, believed to be the disease's southern boundary, produced emotional political confrontations.

The theory behind it all is simple: Cutting down on the density of herds slows the spread of disease. The animals must be killed in order to save them.

Hundreds of deer have been culled in disease hot spots, but larger kills are yet to come.

In February, wildlife officials approved plans for the endemic area of northeastern Colorado, calling for culling some 5,000 deer during the next one to three years.

Biologists hope hunters, via loosened restrictions on licenses, can take care of at least half of those.

"We're concerned that allowing those populations to grow will exacerbate the CWD," Rick Kahn, the division's wildlife management supervisor, said of the strategy.

For the first time, herds won't be managed for hunters and wildlife watchers - but for chronic wasting control.

Within the endemic area, the shooters will be looking for hot spots of infected deer, killing all in the small populations where infection rates appear highest, Kahn said. "We're going to try and see if we can snuff out the disease."

Not everyone thinks this is the best approach.

A leading critic is Charles Southwick, a biology professor emeritus at the University of Colorado and formerly a professor of pathobiology at Johns Hopkins University.

Southwick, whose quiet, reasoned eloquence has given him a voice above the crowd of Division of Wildlife-bashers, insists that killing too many deer in areas peripheral to CWD hot zones could have dire effects.

"Natural herds in good habitat show promise of coping with this infection if they are not crowded or unduly stressed," Southwick wrote in a newspaper piece.

But the wildlife division - with the support of the governor - doesn't think it can take the chance that the disease will burn itself out.

In a line repeated by several top state officials, they say something like this: "Do we want to look back someday, when the deer and elk populations are obliterated, and wish we'd taken stronger action?"

Tracking the disease over the last six or seven years shows that "trends are stable or slightly rising," veterinarian Miller said. "The disease doesn't go away, but it doesn't explode."

But it does spread.

And should infection rates rise only slightly, from the current level in deer of 5 percent to 15 percent of wild animals in the endemic area, all bets are off.

Even slight increases in infection rates can set off a ferocious rise in deer mortality. If enough adult deer die off, populations could be at risk, wildlife officials say.

Western states are in a race with a mysterious killer. And the killer, for now at least, has the upper hand.