What Is It?
Chronic wasting disease (CWD) is a progressive, fatal disease of the nervous system of cervids such as mule deer, white-tailed deer and elk. A report from the World Health Organization indicates that there is some evidence of genetic resistance to CWD among elk/wapiti, but not among the other deer species. 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. Examples include:
Scrapie: Sheep Transmissible mink encephalopathy (TME): mink Bovine spongiform encephalopathy (BSE): cows Creutzfeld-Jacob disease (CJD): humans Gerstmann-Straussler-Scheinker syndrome (GSS): humans Fatal familial insomnia (FFI): humans Kuru: humans
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 external and internal 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.
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 Are TSE’s Transmitted?
The mode of transmission of CWD is currently unknown. In a CWD outbreak occurring in 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.
Other TSEs can be inherited, transmitted between individuals, or sporadic. 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.”
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, a human form of spongiform encephalopathy which can be familial, is about 1 case per million per year across the entire human population. The incidence of GSS, which is familial, is about 1 per 15 million per year.
It is suspected that genotype (genetic makeup) may be a susceptibility factor in other TSEs. For example, some genotypes of sheep have consistently been correlated with susceptibility to scrapie infection. On the other hand, one genotype is so resistant to both a scrapie and BSE challenge that only one case of scrapie has been documented within that genotype. Surprisingly, the 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.
Can TSE’s Be Transmitted to Other Species?
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 variant CJD “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 one 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.
Can CWD Transmit to Other Species?
If BSE apparently transmitted to humans, is it possible that CWD can do the same thing? An article in “Wyoming Wildlife” noted: “Of course, from the standpoint of sensational press, the story is in the similarities between the two diseases, not the differences, but the differences are worth considering. It took a massive contamination of feed to establish BSE in British cattle and contamination of British beef products to produce … the new variant of Creutzfeldt-Jakob disease (in an extremely small percentage of the British population).” The article emphasized that very few cervids carry CWD, compared to the BSE infection of over half of the United Kingdom’s dairy herds.
Because of the species barrier, interspecies transmission of CWD is less efficient than within the species. Even in species with closely related proteins, the prions probably will not jump the species barrier unless the prion-infected tissue is injected directly into the brain or there is a huge, continuing exposure such as that experienced in Great Britain. In research published in August of 2000, scientists at the Rocky Mountain Laboratories in Montana reported that they had found evidence of a molecular barrier that limits the susceptibility of humans, cattle and sheep to chronic wasting disease. Strain typing using mice in a laboratory suggests that CWD is unique and does not resemble BSE, CJD, or scrapie.
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 ten-year study found no evidence that CWD can be transmitted from deer and elk to cattle. Diseased tissue was injected directly into the brains of cows or given orally to the animals with no effects.
In the fall of 1998 a geographically targeted survey of adult cattle was initiated to evaluate the possibility of CWD being transmitted from deer to cattle under free-roaming conditions. The brains of slaughtered cattle that had co-mingled with free-roaming deer throughout the years were examined. The Colorado State University Diagnostic Laboratories reported that an analysis of 262 brains showed no indications of CWD or evidence of the prion protein.
Since the differences in the proteins of cervids and the proteins of humans are much greater than the differences between the proteins of cervids and cattle, 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.”
The World Health Organization advises that although there is no evidence to suggest that it can be transmitted to humans, any tissue which may come from deer or elk with CWD should not be used in animal or human food. 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. Contrary to some media reports that suggest that cases of CJD have been caused by consuming venison, NO variant CJD has been identified in North America.
Since August 1997, United States feed companies have been prohibited from feeding ruminant-derived meat and bone meal back to ruminants, including elk. Some scientists believe that this feeding practice in Great Britain set in motion the unusual, intensified cycles of evolution that enabled prions to jump the normally strong species barrier.
Dr. Mike Miller, a wildlife veterinarian with the Colorado Division of Wildlife and one of the world’s leading researchers on CWD, said that his work has led him to believe that velvet is safe and that CWD has a very small chance of leaping to humans. He noted that the abnormal prion protein thought to carry the disease is concentrated in the brain, the spinal cord and a few lymph nodes. “Velvet,” said Dr. Miller, “is a peripheral tissue like a limb, where the prion protein would barely be detectable.” Although there are no studies on the safety of tissues of elk affected by CWD, other species infected with TSEs have been studied. Muscle and skin tissues, and even milk from the animal, were not found to be infectious.
Nevertheless, elk breeders have taken all possible precautions to insure that velvet antler from affected herds does not enter the marketplace. The Canadian Cervid Council has stated that no velvet antler products from herds known to be infected with CWD have been sold in Canada. Elk breeders in the United States have agreed that no antler from quarantined herds is to be sold, and they have requested that the federal program include this prohibition.
Chronic wasting disease is rare and does not spread quickly. CWD occurs in wild deer and elk in northeastern Colorado and southeastern Wyoming (in the areas where CWD first appeared in the department of wildlife research facilities in the late 60s and 70s). CWD has not been reported in wild deer and elk in Canada. The USDA reports that in over 5,000 samples from the free-ranging population examined over the last ten years, approximately 110 clinically affected deer and elk have been identified. The majority of those affected were 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, incidences of CWD have been found in 13 herds of ranched or farmed elk 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, which has probably been infected for ten years. Only 16 of the elk in the eight Canadian herds actually tested positive for the disease.
Control of CWD
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. Since 1998, many states and provinces have instituted mandatory and voluntary testing and monitoring programs, based on the recommendations of the North American Elk Breeders Association to the United States Animal Health Association.
The Elk Research Council and others in the elk industry are funding research to develop a live-animal test for CWD and to increase knowledge of the disease. The study will cost $250,000 over a four- year period.
The presence of CWD in wild deer and elk in Colorado and Wyoming is a continuing potential source of infection in 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. 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 eventually arrived at an elk ranch in South Dakota. It is widely believed that 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 in controlling CWD in the wild cervid population.
Chronic wasting disease is very rare in farmed elk and appears to transmit only to cervids, but the elk breeding community is treating its control very seriously. They understand the importance of protecting the health of the world’s animals and people. They also recognize CWD’s economic ramifications. It is financially devastating to elk farmers who must eliminate all animals in a herd when a case of CWD is found, and government also experiences the costs of eradication.
Although chronic wasting disease first appeared – not on elk farms or ranches – but in department of wildlife research facilities and in wild populations, elk breeders are taking the lead in the fight against the disease. The industry’s associations have been key forces in initiating regulations, testing and other actions that will eliminate this difficult and elusive disease. Further control activities will be developed and implemented as our knowledge of CWD continues to increase.