ESTES PARK, Colo. — There is an illness in the wild, leaving a trail of questions as it spreads from state to state.
Along the edge of Rocky Mountain National Park, brown-gray elk graze quietly, wearing the occasional tracking tag, as tourists snap photos nearby. Others walk within breathing distance of well-traveled roadsides. Are these animals healthy, or stricken with a deadly illness known as chronic wasting disease?
CWD, as it is known, is a cousin to bovine spongiform encephalopathy — mad cow disease — which has proven deadly not only for cows but some humans as well.
In northern Colorado, Gary Wolfe keeps a keen eye on the deer he hunts, closely watching for signs they are infected. Wolfe, who runs the Chronic Wasting Disease Alliance, a coalition of hunting and conservation groups, worries that CWD — as the disease is known — could destroy mule deer populations and wreak havoc among herds. "There's some real important reasons for stopping this," he says.
Wildlife officials as far east as Illinois and Wisconsin, where CWD was detected two years ago, are equally worried.
Wisconsin wildlife managers spend up to a third of their budget, as much as $7 million a year, to fight it. "This remains a disease that the country needs to stand up and pay attention to," says Tom Hauge, the state's director of wildlife management.
Mad cow versus CWD
The world has taken note of the spread of mad cow disease. Detected worldwide, it has stricken more than 180,000 cattle in the United Kingdom and is linked to some 140 human deaths. The first U.S. case was found last December.
But unlike mad cow, chronic wasting disease infects species that remain a minor source of food for Americans, and there are notable scientific differences between the diseases.
"There's no such thing as mad deer disease," Wolfe insists.
Yet there are plenty of similarities between mad cow and CWD. And the United States has an estimated 10 million deer hunters and 900,000 elk hunters; in Michigan alone, more than 500,000 deer are killed during hunting season.
Many of these are trophy kills, and even commercial venison remains a specialty item, but hundreds of thousands of Americans eat deer and elk meat. In areas infected with CWD, up to 10 percent or more of deer are found to carry the disease.
More from TODAY.com
Hillary Clinton: Granddaughter led me 'to speed up' political plans
Clinton said she is inspired to keep working to ensure that Charlotte and her generation are provided equal opportunities ...
- Lauren Hill, inspirational college basketball player, dies
- Marathon dad's victories help raise money for son with spina bifida
- Will it work on Vale? Savannah tries tissue sleeping trick at home
- Listen to the chilling 911 call Sandra Bullock made during break-in
- Hillary Clinton: Granddaughter led me 'to speed up' political plans
Lessons learned from CWD may help scientists understand diseases like mad cow.
Both are transmissible spongiform encephalopathies, or TSEs. They can both be passed from animal to animal, though by different means, and are thought to be caused by misshapen forms of proteins known as prions. These deformed prions eat holes in a creature's brain, inevitably leading to death.
"These are especially scary diseases to us people, because they arise from something you can eat, and they affect the brain, and they are uniformly fatal," says Edward Hoover, director of Colorado State University's Retrovirus and Prion Research Laboratory.
'A huge mystery'
Unanswered questions linger about all prion diseases, including scrapie in sheep and Creutzfeldt-Jakob disease in humans. Even more than mad cow, chronic wasting disease remains what Jeffrey Ver Steeg, wildlife programs coordinator for the Colorado Division of Wildlife, calls "a huge mystery."
Can it infect people? Most researchers have yet to see evidence it can. No cases of a human prion disease like variant Creutzfeldt-Jakob have been tied to eating deer or elk. The federal Centers for Disease Control concluded in June that the lack of definitive cases suggests "the risk, if any, of transmission of CWD to humans is low."
At the same time, the CDC acknowledged several puzzling cases of patients who died of neurological diseases after eating wild game. In 2003, doctors at the VA Hospital in Seattle reported Creutzfeldt-Jakob disease in three hunters; the CDC would not investigate, saying there was no evidence the men ate tainted meat. A 2002 study documented three Wisconsin men who regularly ate venison and took part in large "game feasts"; two were diagnosed with CJD and one was diagnosed with Pick's disease, a form of dementia.
The agency concluded "an adequate number of people may not have been exposed to the CWD agent to result in a clinically recognizable human disease."
Similarly, in 2000 a group of well-known scientists wrote in the EMBO Journal, a European biology publication, that while they believed humans had "very low" susceptibility to CWD, the disease could have as much infectivity as mad cow disease, and could potentially cross the species barrier. It was, they said, "premature to draw firm conclusions."
Reassurance and vigilance
With no clear evidence that the disease impacts humans, wildlife agencies have tried to reassure nervous game hunters, who provide crucial revenue, and enlist their help in tracking and eradicating the disease.
Advice to hunters: Try not to hunt in infected areas. If you do, don't hunt apparently ill animals. When handling a carcass, wear gloves.
And officials have sought to educate the public about the disease's telltale signs. As the prions wear holes in an infected animal's brain, the creature slowly loses control of motor functions, behaves ever more erratically and becomes emaciated as it slowly dies.
While wildlife agencies play down any danger, they also provide stark warnings not to eat animal parts considered to be at highest risk, such as the brain and nervous tissue. Many suggest that meat from an infected animal not be eaten at all.
"It's kind of a mixed message," says Jim Woodward, a Wellington, Colo., activist who opposes the state's approach. "They seem to be comfortable in making some extremely definitive statements that humans aren't going get this disease. And that's exactly what happened in England in the 1980s," in the early days of mad cow.
Some researchers share his skepticism. Working with mice, University of Colorado neurologist Patrick Bosque has detected infectious prions not only in neural tissue but in muscle, and believes CWD may be transmissible to other species, including humans. It is a view shared by many who study the disease.
"We have to be careful about concluding that it hasn't happened," says Hoover.
State officials acknowledge the difficulty in crafting a clear message from often conflicting research. Perhaps most confounding: Unlike most animal prion diseases, which appear almost exclusively in captive animals, chronic wasting disease moves both through wild and captive deer and elk herds.
This has created a huge challenge for Colorado's Jeff Ver Steeg and his counterparts, who must control a disease they barely understand. "There's no manual on this," he says.
All the while, researchers, game farmers and hunters often point fingers, trying to assign blame for the disease's spread. In the end, there is little clear guidance for people like Gary Wolfe.
As a wildlife biologist, he has spent decades trying to preserve wild deer and elk populations in Wyoming and Colorado now threatened by CWD. As the former head of the Rocky Mountain Elk Foundation, an influential wildlife conservation and hunting association, he sympathizes with other hunters' worries: "They want to know that the venison is safe to eat."
Tracking CWD's origins
How did this disease even get here? There are only theories. Researchers speculate it might be a transformed type of scrapie, mutated from infected sheep. Others believe it may have occurred spontaneously in cervids, the family of mammals that includes deer and elk.
First detected in 1967 among captive mule deer at a Fort Collins, Colo., research facility, scientists initially blamed it on the stress of captivity, or problems with food, and took more than a decade to recognize it as a TSE. Later, they determined the cause was most likely the same malformed proteins that cause diseases like mad cow.
While similar diseases are found almost solely in captive animals, in 1981 CWD was found in a wild elk in Colorado and subsequently in other cervids. And it began spreading from its Colorado epicenter to nearby states.
It has spread throughout the West and the Great Plains — to Wyoming, South Dakota, Montana and Nebraska, reaching as far east as southern Illinois and north to Saskatchewan.
New infection clusters often baffle epidemiologists. At least four cases have turned up on the White Sands Missile Range in southern New Mexico, a huge leap beyond what had been the southernmost disease site near Colorado Springs.
With a U.S. wild deer population of more than 21 million, according to Petersen's Hunting magazine, state agencies are trying to get a grasp on CWD's scope. Illinois has recorded at least 30 cases; but only 4,000 of 104,000 deer hunted last year were tested. Even with tags and radio bracelets, wildlife officials have limited resources to track herd movements. And cervid populations are generally growing.
Colorado the epicenter of research
Colorado, with more than 800,000 wild deer and elk, remains a focal point for detection and research. In infected areas, as much as 6 percent of deer and 4 percent of elk test positive for chronic wasting disease, and it is nearly impossible for the state to track each case. Instead, the Division of Wildlife encourages hunters to send in brain samples from carcasses, some 40,000 thus far.
However, the program is voluntary, which prompted criticism, as have the few occasions state officials organized widespread deer kills.
An enlightening, if unsettling, bit of research came earlier this year, when a team led by Mike Miller, a Colorado Division of Wildlife veterinarian, and University of Wyoming pathologist Beth Williams tested exposure methods on mule deer. They concluded the animals could be infected not only by the carcasses of other deer, but merely by traveling through places where infected deer had been.
Their finding lent credence to one measure taken by an increasing number of states: to ban the transport of deer and elk carcasses. Epidemiologists have long speculated about the spread of cases when captive animals were hauled across state lines, or when carcasses tied to hunters' trucks were driven hundreds of miles.
These concerns even prompted states from Oregon to South Carolina to ban the import of live cervids. The USDA has devised rules that would require game farms to prove their animals are disease-free and limit interstate transport of the animals, though final approval is not expected this year.
Even states without a single case of chronic wasting disease are on the lookout. In Washington, a statewide surveillance program monitors for the disease's possible arrival. One Kentucky brochure warns the disease could "be devastating" in rural areas, chasing away sportsmen and a $400 million hunting industry.
Animal rights groups have applauded transportation bans as an effective way to limit the disease's spread. Like many wildlife officials, they worry about the proximity of infectable animals and lobby to keep herds separated. "Every single state should be closing their borders," says Heidi Prescott of the Fund for Animals.
But deer and elk can still wander across state lines. And the bans don't merely affect hunters: They also have had a devastating impact on many of the nation's 10,000 captive-deer farmers, who continue to tend herds they often cannot ship or sell. Phyllis Menden, executive director of the North American Deer Farmers Association, believes "billions" of dollars have been lost.
While game hunters face voluntary tests, most farmers must test every animal at slaughter. Yet state bans remain in place.
"New Zealand can sell all the venison they want over here," Menden says. "It's mind-boggling."
Limiting CWD's spread
Infected states are struggling to limit the spread of CWD, and working to keep stricken herds from infecting others. Some have enlisted help from hunters to cull animals from infected areas. Wisconsin, which suffers from chronic deer overpopulation, hopes to reduce deer density to just five per square mile, down from 75.
The best way to prevent CWD's spread may be to keep deer and elk away from infected hotspots, no matter whether that happens to be on a farm or in the wilderness. "We view the protection of both wild and captive herds as intertwined," Hauge says.
Colorado State's Hoover, with help from an $8 million National Institutes of Health grant, hopes to unlock the mysteries of the disease's transmission, and ultimately intends to develop a vaccine.
Even with a vaccine, chronic wasting disease remains a disease tied to the rules of nature. "It's very, very difficult to vaccinate wildlife," says Wayne Cunningham, Colorado's state veterinarian.
Many hunters remain undeterred. So long as the public danger remains unclear, it will be difficult to convince people like Gary Wolfe that they should rack their rifles and stay home.
"Am I going to stop hunting in my favorite area of Colorado?" he says. "No."
© 2013 msnbc.com Reprints