The complexity and uniqueness of chronic wasting disease (CWD), a brain disease in some southeast Wyoming and northeast Colorado deer, has led to national reporting that is unnecessarily alarming to hunters, believes Dr. Tom Thorne, wildlife veterinarian and Services Division chief for the Wyoming Game and Fish Department. Thorne cites recent coverage by CBS television news and Field & Stream magazine that distorted the possibility of a link between the disease and humans and may have instilled unnecessary fear in big game hunters.

"There currently is no evidence chronic wasting disease is a threat to human health," said Thorne of the disease which is caused by a "prion" or abnormal protein. Diseases caused by prions are collectively called transmissible spongiform encephalopathies. In addition to CWD, the group includes bovine spongiform encephalopathy or mad cow disease of Europe, a sheep affliction called scrapie and Creutzfeldt-Jakob disease, a very rare human disease.

Thorne adds that some reporting of CWD inappropriately equates it with mad cow disease of Europe, by calling CWD "mad deer disease."

"Although mad cow disease can be transmittable to humans, the symptoms it induces in cattle of erratic behavior that induced its name are entirely different than the symptoms of CWD in deer," Thorne said. "Although both are caused by prions, they are different diseases."

The name chronic wasting disease reflects the impacts it has on infected deer. In its final stages, the disease induces deteriorating body condition, excessive salivation, both increased thirst and urination and eventually death.

There has also been reporting that the deaths of relatively young Utah and Oklahoma men from Creutzfeldt-Jakob disease were possibly linked to CWD because the men were deer hunters, and because Creutzfeldt-Jakob disease typically occurs in older people. "But, the Center for Disease Control and Prevention examined the cases and concluded these unfortunate cases of Creutzfeldt-Jakob disease were not connected with CWD," Thorne said.

The U.S. Food and Drug Administration's advisory panel on transmissible spongiform encephalopathies and a committee of the World Health Organization have both stated there currently is no evidence CWD is a human health threat. "Both organizations stop short of saying it is absolutely conclusive beyond a shadow of a doubt that CWD cannot be transmitted to humans, and I wouldn't say that either," Thorne said. "The reason we can't say that is that, for obvious reasons, experiments of purposely exposing humans to CWD-infected tissue cannot be conducted, nor would we want them to be conducted."

Although CWD was first documented in free-ranging deer and elk in southeast Wyoming and northeast Colorado in the 1980s, it is likely the disease has been present at least 30 to 50 years, Thorne adds. During that time there has been no corresponding incidence of transmissible spongiform encephalopathy diseases in humans in the area.

He also challenged statements that CWD "is found across the West." "Chronic wasting disease has only been found in free-ranging deer and elk in southeast Wyoming, northeast Colorado and one positive case in the very western portion of the Nebraska panhandle," he said. "In the affected area of Wyoming and Colorado, tests show only 4 to 5 percent of deer have the disease and the rate is much lower for elk."

In research facilities antelope and bighorn sheep have been confined in the same pens with CWD-afflicted deer and have not contracted the disease. For those who hunt deer and elk in southeast Wyoming and are concerned about CWD, Thorne has the following common sense recommendations:
• Don't shoot any animal, deer or otherwise, that appears sick, regardless of the suspected cause.
• Wear rubber gloves when field dressing any game.
• Bone out the venison and do not eat or handle the brain, spinal cord, eyes, spleen or lymph nodes.

Chronic wasting disease, a fatal brain disorder of deer and elk, has not been seen in Idaho. Recent reports in national media including Time magazine and National Public Radio linked "mad cow disease" and the occurrence of chronic wasting disease (CWD) in western big game, suggesting that people who eat big game animals might be at risk. However, these reports may lead people to inaccurate conclusions.

Wildlife veterinarian Mark Drew said CWD has been identified in free-ranging deer and elk only in a relatively small area of southeastern Wyoming and northern Colorado. One deer with CWD was recently identified in Nebraska in the same general geographical area as the Colorado and Wyoming cases. The Idaho Department of Fish and Game has had a surveillance program for CWD in deer and elk since 1997 and plans to expand its sampling program this year because of concerns about the disease, he added. No wild deer or elk tested so far in Idaho have been shown to have CWD.

CWD has been confirmed in a few domesticated elk in Canada and the U.S. Dr. Drew noted that domestic game farm operations in Idaho are regulated by the Idaho State Department of Agriculture, not the Department of Fish and Game. State regulations for domestic game farms are currently being revised and strengthened in Idaho to require mandatory testing for CWD in all dead elk and 25% of the animals that are slaughtered for meat or harvested on shooting operations.

Dr. Drew said signs of CWD in deer and elk include weight loss, emaciation and abnormal behavior. Scientists do not yet understand how CWD is transmitted from animal to animal or how to prevent the spread of the disease. However, because no animals with CWD have been found in Idaho, the possibility of the disease being found is small.

Hunters are advised to avoid harvesting animals that are obviously sick or acting strangely. Latex or rubber gloves should be worn when field dressing wild game, Dr. Drew added.

Discovery

"Chronic wasting disease" was first recognized by biologists in the 1960s as a disease syndrome of captive deer held in wildlife research facilities in Ft. Collins, Colorado, but was not recognized as a transmissible spongiform encephalopathy until the late 1970s. This disease was subsequently recognized in captive deer, and later in captive elk, from wildlife research facilities near Ft. Collins, Kremmling, and Meeker, Colorado and Wheatland, Wyoming, as well as at in least two zoological collections. More recently, CWD has been diagnosed in privately-owned elk residing on game ranches in a few western states and provinces. Although CWD was first diagnosed in captive research cervids, the original source (or sources) of CWD in either captive cervids or free-ranging cervids is unknown; whether CWD in research animals really preceded CWD in the wild, or vice versa, is equally uncertain.

Occurrence

Chronic wasting disease is relatively rare, and its geographic distribution is quite limited. Fewer than 300 naturally occurring clinical cases, mostly in captive research and free-ranging mule deer, have been documented. Based on data from a combination of surveillance methods, CWD now appears in 11 contiguous counties in northeastern Colorado (five counties), southeastern Wyoming, and the Nebraska Panhandle (one county). Although the disease doesn't appear to be common, the number of cases detected has increased in recent years. This trend may be explained by increased vigilance by wildlife and animal health officials, the wildlife farming industry, and the public in reporting cases, but it may also reflect increased disease occurrence.

Based on random, preclinical testing of brain or tonsil tissues from animals harvested in specific management units, it appears that on average CWD probably infects about 5-15 percent of the deer in a small, core endemic area of northcentral Colorado and southeastern Wyoming, and 1 percent or fewer of the deer in other surrounding mountain and plains areas. Testing of harvested animals indicates less than 1 percent of the elk in endemic areas are probably infected. To date, with the exception of the recent mule deer in Nebraska, no evidence of CWD has been detected in examinations of over 8,000 deer and elk from outside endemic areas in Colorado and Wyoming, or from other states and Canadian provinces where surveys have been conducted in recent years. Additional surveys are ongoing.

In addition to cases in captive research and free-ranging deer and elk, CWD has been diagnosed in privately-owned elk on a small number of game farms in Colorado, Montana, Nebraska, Oklahoma, South Dakota, and Saskatchewan, Canada since 1996. Infection has been particularly severe in a group of interconnected facilities near Rapid City, South Dakota, that appear to be the original source of infection for other South Dakota game farms as well as the Saskatchewan epidemic. In contrast, infected elk in two of three Nebraska farms originated in Colorado, and infected elk in Oklahoma apparently originated in Montana; CWD has been confirmed in the Montana and Colorado source herds. Epidemiology of the Canadian cases has been under study, and South Dakota appears to be the likely source of CWD in Saskatchewan; it also appears that CWD was imported into Canada prior to 1990, and has spread among at least 18 farms via live animal sales over the last decade. The overall distribution and occurrence of CWD among farmed elk operations should become clearer as industry-wide surveillance programs are developed. There are no apparent epidemiological connections between the Colorado-Nebraska, South Dakota-Saskatchewan, and Montana-Oklahoma foci; moreover, there are no apparent epidemiological connections between any of the cases in farmed elk and cases in free-ranging or captive research deer and elk.

Transmission

Neither the agent causing chronic wasting disease nor its mode of transmission have been definitively identified, but clinical disease is associated with the accumulation of protease-resistant prion protein (PrPres) in brain tissue (as in other transmissible spongiform encephalopathies). Experimental and circumstantial evidence suggests infected deer and elk probably transmit the disease laterally through animal-to-animal contact and/or contamination of feed or water sources with saliva, urine, and/or feces. Chronic wasting disease seems more likely to occur in areas where deer or elk are crowded or where they congregate at man-made feed and water stations. Although CWD does not appear to be transmitted via contaminated feed, artificial feeding of deer and elk may compound the problem. This may in part explain the intensity of infection in some cervid populations housed in farm or research settings.

According to public health (Centers for Disease Control, World Health Organization) and animal health officials, data available to date indicate that chronic wasting disease is not currently known to be naturally transmitted to humans, or to animals other than deer and elk; data from recent molecular studies provide quantitative evidence of the apparent inefficiency of cross-species transmission. As a general precaution, however, public health officials recommend that people avoid contact with deer, elk, or any other wild animal that appears sick. Although there's no evidence that chronic wasting disease can be naturally transmitted to domestic livestock, chronic wasting disease is similar in some respects to two livestock diseases: scrapie, which affects domestic sheep and goats worldwide and has been recognized for over 200 years, and bovine spongiform encephalopathy (BSE), which is a more recent disease of cattle in the United Kingdom and Europe. Despite some similarities, there is no evidence suggesting either scrapie or BSE are caused by contact with wild deer or elk, or that wild deer or elk can contract either scrapie or BSE in countries where these diseases occur.

Clinical Signs

Deer and elk infected with chronic wasting disease show progressive loss of body condition accompanied by behavioral changes. In the later stages of disease, emaciation, excessive salivation, increased drinking and urination, stumbling, trembling, and depression may precede death. As with other TSEs, the clinical course of chronic wasting disease appears to be progressive and irreversible, ultimately leading to the death of affected animals. Because the clinical signs of chronic wasting disease are relatively nonspecific, laboratory examination of clinical suspects is essential for confirming this diagnosis.

Diagnosis

At present, the diagnosis of chronic wasting disease is based on microscopic examination of brain (specifically, the medulla oblongata at the obex) and tonsil tissues from suspected cases. Both histopathologic examination and immunohistochemistry (IHC) are used in routine diagnosis of clinical cases, and may also be used to detect preclinical cases in surveillance and monitoring programs; of these, IHC appears to offer greater sensitivity in detecting early preclinical cases. Western blots and negative-stain electron microscopy have also been used to further confirm diagnoses, and other diagnostic tests are being evaluated. There are currently no validated live-animal tests for diagnosing either clinical or preclinical chronic wasting disease in either deer or elk; however, research is underway to evaluate several promising avenues for ante mortem diagnosis.