* Executive Director, Canadian Venison Council, 35D Rayborn
Crescent, St Albert, Alberta T8N 5B6
† Editor/Communications Manager, North American Elk Magazine, 1708 N. Prairie View Rd, Platte City, MO 64079
What is CWD?
Chronic wasting disease (CWD) is a progressive, fatal disease of the nervous system of cervids such as mule deer, white-tailed deer and elk. It is a type of transmissible spongiform encephalopathy (TSE). Although several scientists disagree, the leading theory is that the infectious agent is a prion.
Prions have been defined as "small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids. Prions (pronounced pree-ahns) enter cells and apparently convert normal proteins found within the cells into prions just like themselves. The normal cell proteins have all the same "parts" as the prions--specifically the same amino acid building blocks--but they fold differently. They are like the toy "Transformers" that intrigued children in the 1980s. A car could become a robot; a bug could become a warrior. Nothing was added; nothing was subtracted.
Prion diseases are called spongiform encephalopathies because of the postmortem appearance of the brain, which exhibits large cavities in the cortex and cerebellum (like a sponge). Most mammalian species, including humans, develop prion diseases.
What are the clinical signs of CWD?
The clinical signs of CWD include emaciation, excessive salivation,
behavioral changes (i.e., loss of fear of humans), ataxia, drooping of
head and ears, weakness, bugged-out eyes, and increased thirst and urination.
Clinical signs may last for weeks to months before the animal dies, with
most elk succumbing in less than 12 months. At death, other signs noted
will be generalized absence of subcutaneous and visceral fat, serious atrophy
of the bone marrow, and a dry, rough hair coat.
Can these symptoms indicate a disease other than CWD?
Whenever nervous signs and excessive salivation are seen, rabies must be suspected; however, clinical signs of CWD are less rapid in onset than those of rabies. Bacterial diseases that affect the central nervous system, such as Listeriosis, also cannot be excluded. Johne's disease causes weight loss, debilitation, and eventual death in farmed cervids; but it is also accompanied by progressive diarrhea, which is not a symptom of CWD. Meningeal worm may cause loss of fear of humans and loss of condition.
How is CWD diagnosed?
Although progress has been made toward development of a laboratory assay
that might lead to the validation of a live-animal diagnostic test for
TSEs, there is currently no definitive way to diagnose CWD before death.
The diagnosis is based on clinical signs and can only be positively diagnosed
by post-mortem examination of the brain tissue of the affected animal.
Pathologists look for protease-resistant protein plaques in the brain.
How is CWD transmitted?
The mode of transmission of CWD is currently unknown. It is known that other TSEs can be inherited, sporadic, or transmitted between individuals.
In a CWD outbreak occurring in captive Rocky Mountain elk, it was found that lateral transmission (from animal to animal) seemed the most plausible explanation for the pattern observed. Maternal transmission did not appear necessary to sustain the outbreak. It is thought that the CWD agent is passed in saliva, feces or urine. Once ingested, the disease has an incubation period of 16 to 30 months before the onset of clinical signs.
About 10 percent of human prion diseases are familial, or inherited, and kill half of the members of the affected families. The textbook incidence of CJD, one of the human forms of spongiform encephalopathy, which can be familial, is about 1 case per million human population per year. The incidence of GSS, which is familial, is about 1 per 15 million per year.
It is suspected that genotype may be a susceptibility factor in other TSEs. It has long been known that some genotypes are almost always identified in scrapie infected sheep. In contrast, only one sheep with a particular genotype has been identified with scrapie. These animals are apparently resistant to both a scrapie and BSE challenge. Surprisingly, the same scrapie-susceptible genotypes are common in Australia and New Zealand, but are thought to be free of scrapie. When these sheep are brought to the United Kingdom and maintained in quarantine conditions, they do not develop scrapie. In other words, the genotype does not confer scrapie on the animal but susceptibility to scrapie infection.
Dr. Stanley Prusiner won a Nobel Prize for his work in the study of prions. In a 1995 article in "Scientific American," he wrote, "Prions are indeed responsible for transmissible and inherited disorders of protein conformation. They can also cause sporadic disease, in which neither transmission between individuals nor inheritance is evident."
Before a strain of BSE prion apparently infected humans in Europe, researchers believed that a phenomenon known as the "species barrier" would make it virtually impossible for prions made by one species to cause disease in another species. Researchers who intentionally attempted to transmit scrapie to other species found it very difficult. Although scrapie in sheep has been recognized for hundreds of years, it has never crossed the species barrier to humans.
Prion diseases do not move easily between species. Scientists at NIAID's Rocky Mountain Laboratories (RML) in Montana and their colleagues, for example, demonstrated that abnormal protein from a mouse cannot convert normally folded protein from a hamster. The "molecular dance" that converts normal proteins to prions is most effective when the protein and infecting prion have the same amino acid sequence and are from a single species. (The known prions all have about 250 amino acids. Cow and sheep prions differ by only seven amino acids. Human and cow prions differ by 30.) If the two proteins are not exactly the same, if the prion is from a cow or sheep, for example, and the normal protein is from a person, the transformation takes more time.
A different type of CJD has been linked to BSE in Europe recently. Some call the disease "Human BSE" because the strain is very much like the BSE agent and is very different from "classical" CJD. Scientists have concluded that the most likely cause is ingestion or handling of infected beef. The meat found to be infected was from the brain, spinal cord, eye, and parts of the gut.
BSE in cattle was identified in Great Britain in 1986. It is estimated that a total of 1 million cattle were affected. The source is believed to be a food supplement that included meat and bone meal from dead sheep. (The method for processing sheep carcasses had been changed in the late 1970s, and the method apparently did not kill the infectious agent.) Scientists speculate that years of exposure to scrapie in cattle feed caused the disease to cross over the barrier to cause BSE. One scientist reasons that the fact that the TSE jumped species from sheep to cattle and from cattle to humans is a result of intensified cycles of evolution due to the recycling of carcasses.
The British government banned the use of animal-derived feed supplements in 1988. To date, there have been 84 CJD deaths in the United Kingdom.
Because of the species barrier, interspecies transmission of CWD is less efficient than within the species. Based on its recent research, the USDA suggests that the probability of transmission of CWD from cervids to cattle is low. Beth Williams, a University of Wyoming professor of veterinary science and leading expert on CWD, said that researchers involved in the early stages of a 10-year study have found no evidence that CWD can be transmitted from deer and elk to cattle. Diseased tissue has been injected directly into the brains of cows or given orally to the animals with no effects so far, Williams said.
Since the differences in the proteins of cervids and the proteins of humans are much greater, the probability of transmission to humans would be much lower. John Pape, an epidemiologist with the Colorado Department of Health, stated, "There is no indication that chronic wasting disease is a threat to human health."
Although no cases of variant CJD have been identified in North America
and there is no scientific evidence that CWD affects humans, caution should
be exercised. Offal, brain, and spinal cord tissue, as well as all meat
from affected animals, should not be used as food or as a protein source
in animal food.
Where in North America is CWD found?
Chronic wasting disease is relatively rare. CWD occurs in wild deer and elk in northeastern Colorado and southeastern Wyoming (around the areas where CWD first appeared in wildlife research facilities). CWD has not been reported in wild deer and elk in Canada. Since 1981, fewer than 200 cases have been documented in the wild, mostly in mule deer. It is estimated that the incidence in the affected counties of Wyoming and Colorado ranges from LESS THAN 1 percent of elk to 5 percent of mule deer.
Incidence in domestic herds is even more rare. So far, 13 herds of domestic
elk with incidences of CWD have been found in the United States. Seven
have been depopulated or released from quarantine; six remain under quarantine.
Canada has found eight herds with cases of CWD, including the Canadian
source herd, 16 elk have tested positive for CWD to date. Currently, over
180,000 domestic elk are being raised in North America.
How is the spread of CWD being controlled?
With input from the elk farming industry, both the United States Department of Agriculture (USDA) and the Canadian Food Inspection Agency (CFIA) have developed similar programs for the control and eventual eradication of chronic wasting disease. The programs include surveillance, monitoring, and indemnification. The elk industry is also funding research to develop a live-animal test for CWD.
The presence of CWD in wild deer and elk in Colorado and Wyoming is a continuing potential source of CWD for farmed elk. Wild deer jumping the fence into a hunting preserve may be the source of one infected herd located in the area of the most heavily infected wild herd in Colorado. This operation will be closed after all the bulls are hunted. Most of the CWD in farmed elk appears to trace back to the Colorado Division of Wildlife research pens where CWD first appeared. Mule deer from this facility were given to the Denver Zoo. The Denver Zoo gave some mule deer to the Toronto Zoo and also sold some animals, which wound up on an elk ranch in South Dakota. Most of the CWD herds in the United States can be traced to this South Dakota herd. Some unfilled gaps remain, and federal and state epidemiologists are working to find these connections.
The Colorado Division of Wildlife has announced plans to reduce by 50%
the core CWD deer herd north of Fort Collins. They will issue large numbers
of hunting licenses to landowners in this area. Hunters, not licenses,
will be the limiting factor. This is a good first step. Further control
and eradication activities will be developed and implemented as our knowledge
of this difficult and elusive disease increases.
References and Further Reading
1.Haigh J.C. and Hudson R.J. "Farming Wapiti and Red Deer." St. Louis: Mosby-Year Book, Inc., 1993: pp. 314-315.
2.Schreuder B.E.C. Animal spongiform encephalopathies--An update. Part I. Scrapie and lesser known animal spongiform encephalopathies. Vet Quart 1994; 16(3): 174-181.
3.Schreuder B.E.C. General aspects of transmissible spongiform encephalopathies and hypotheses about the agents. Vet Quart 1993; 15: 167-174.
4.Miller M.W., Wild M.A., and Williams E.S. Epidemiology of chronic wasting disease in captive Rocky Mountain elk. J of Wildl Dis 1998; 34(3): 532-538.
7.George, L.W. Diseases of the Nervous System. In: Smith BP, ed. Large Animal Internal Medicine 2nd edition St Louis: Mosby - Year Book, 1996: pp. 1011-1015
8. Personal communication. 1999 Dr. George Luterbach, Operations Directorate, CFIA, Winnipeg.
9.Mackintosh C.G. Deer health and disease. Acta Vet Hungar 1998; 46(3): 381-394. Spraker T.R., Miller M.W., Williams E.S., Getzy D.M., Adrian W.J., Schoonveld G.G., Spowart R.A., O'Rourke K.I., Miller J.M., and Merz P.A. Spongiform encephalopathy in free-ranging mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus) and Rocky Mountain elk (Cervus elaphus nelsoni) in north central Colorado. J of Wildl Dis 1997; 33(1): 1-6.
10.Williams E.S. and Young S. Neuropathology of Chronic Wasting Disease of Mule Deer (Odocoileus hemionus) and Elk (Cervus elaphus nelsoni). Vet Pathol 1993; 30: 36-45.
11.Guiroy D.C., Williams E.S., Yanagihara R.Y., and Gajdusek D.C. Immunolocalization of scrapie amyloid (PrP27-30) in chronic wasting disease of Rocky Mountain elk and hybrids of captive mule deer and white-tailed deer. Neurosc Let 1991; 126: 195-198.
12.Fraser C.M. editor. The Merck Veterinary Manual, 7th ed. New Jersey: Merck and Co., Inc., 1991: p. 399.
14.Blazeiko, Jennifer and Murray Woodbury, DVM. "Chronic Wasting Disease." Western College of Veterinary Medicine, University of Saskatchewan. 2000.
22. Associated Press Release, "Search for CWD from Montana extends to Missouri, Oklahoma." Billings Gazette, Jan. 27, 00