NR ASMS
AU Leighton,T.
TI Chronic Wasting Disease of Wild Deer
QU Canadian Cooperative Wildlife Health Center Newsletter 1997 Summer; 4(3)
PT Feature Article
VT
Chronic Wasting Disease is a fatal disease known to occur among wild deer in only one small geographic area of the United States. It belongs to the group of diseases known as Transmissible Spongiform Encephalopathies (TSE's). Considerable public interest and anxiety have arisen about TSE's over the past few years, stimulated largely by the emergence of Bovine Spongiform Encephalopathy (BSE), also sometimes referred to as "mad cow disease" - another TSE. Creutzfeld-Jacob Disease (CJD) of people, and Scrapie of sheep and goats also are TSE's. Public anxiety about these diseases is understandable and justified. These are terrible diseases in which people and animals suffer mental and physical deterioration over long periods of time and then inevitably die. Compared to many other diseases, very little is known about the TSE's. Their cause(es) is/are enigmatic. It is not known with certainty whether or not people can acquire CJD from animals with TSE's, but there is considerable suspicion that this may be the case with BSE. Scientists issue contradictory statements on this issue because the evidence available can be interpreted in different ways. Hard proof of anything about the TSE's is elusive. Into this climate of anxiety and uncertainty have tumbled popular books depicting the TSE's as the major human health threat now and in the future, and government advisories admonishing the public to eat, or not to eat, meat according to the politics and culture of the particular ministry. There is no easy resolution of this confusing situation. Wildlife personnel should be as informed as possible about Chronic Wasting Disease, but also must live with the fact that all aspects of this and the other TSE's currently are plagued with uncertainty.
Chronic Wasting Disease (CWD) is known to affect Mule Deer, Elk (Wapiti) and White-tailed Deer. It was first recognized as a disease in a herd of captive Mule Deer and Mule Deer hybrids at Fort Collins, Colorado in 1967 and was determined to be a form of TSE in 1977. It subsequently was diagnosed in other captive deer and elk in the Colorado facility and in a research facility in neighbouring Wyoming. CWD has been recognized in free-ranging wild deer since 1981. From that year to June 1995, the disease was diagnosed in 41 Mule Deer, 6 Elk and 2 White-tailed Deer, all from free-ranging populations in north-central Colorado within a 100 km radius of Fort Collins. A total of 11 diseased wild animals of the same three species have been found in an adjacent area of southeastern Wyoming. In Canada, the disease has never been identified in wild deer, but two cases have occurred in captive deer: in 1978 in a Mule Deer at the Metro Toronto Zoo and in 1996 in an elk imported from the United States and held on farms in Saskatchewan. No further cases have occurred at the zoo and a comprehensive eradication program was undertaken by Agriculture and Agri-Food Canada in cooperation with the game farming industry with respect to the case in Saskatchewan.
CWD gets its name from the progressive emaciation, or wasting away, that occurs in affected animals. Abnormal behaviour is the other central feature of the disease. Affected animals are weak and have an odd gait, reduced fear of humans, drooped heads and ears, and excess salivation. There are no gross abnormalities except emaciation. Diagnosis is based on microscopic changes in the brain. Nerve cells and the brain substance around them develop large, clear vacuoles that give the tissue the appearance of a sponge with its many holes - hence the name "spongiform" encephalopathy - literally a spongy abnormality of the brain. These vacuoles develop in association with accumulations of a special protein found only in TSE-affected brains. This protein can be identified by special techniques, and its presence together with the vacuoles is generally accepted as the ultimate criterion for making a diagnosis.
The causal agents of the TSE's are transmissible from infected to uninfected individuals. In general, there is a very long interval, usually a large proportion of the normal lifespan of the animal species involved, between exposure to the causal agent and development of disease. Thus, it is very difficult to study transmission of these diseases, and relatively little is known about it. Eating TSE-infected tissue is one way to acquire the disease. The most famous form of CJD, the human version of these diseases, was studied in the Fore people in New Guinea, among whom the disease, called Kuru, was transmitted by ritual cannibalism. The sudden emergence of BSE in Europe has been linked to feeding cattle with meat meal, presumably contaminated by a TSE-causing agent from an animal source. Various of the TSE's have been transmitted to species in which they do not regularly occur by feeding the brains of affected animals. Thus, quite aside from the loss of the animals themselves, the TSE's, and particularly BSE, have become a major issue of public health and food safety. Has CJD been caused in people from eating parts of cattle affected with BSE ? Is it safe to eat the flesh of deer that might carry the agent of CWD ? Unfortunately, the answers to these questions are not clear at all. CJD occurs spontaneously in people world-wide at the rate of about one case per million people per year. A considerable increase in this low rate of occurrence would be necessary in order to be certain, statistically, that the incidence is indeed rising. And, because there can be decades between exposure to the causal agent and development of the disease, exposure of people to the agent in the 1980's might not result in disease until the 1990's or 2000's. How should society respond to this situation? Some feel the health risk to people who eat TSE-affected animals is high and that drastic and immediate actions are called for to prevent any possibility of this occurring. Others consider this an alarmist view based on unfounded assumptions and insufficient information.
What causes CWD and the other TSE's ? The current answer to this question is in the grey zone between knowledge and theory. It appears too early to be sure whether we now have the entire story or only a partial answer. Current understanding of the cause of the TSE's rests on the concept of the prion ("pree-on") protein. As disease-causing agents, prion proteins fall somewhere between our usual notions of infectious agents and of toxins. They are abnormal versions of a normal body protein that accumulate in cells because of two properties: they propagate themselves by changing the normal proteins into abnormal forms whenever they encounter them, and they can not be broken down and removed by the body. Accumulation of these abnormal proteins in nerve cells appears to account for the brain dysfunction that characterizes the TSE's. These abnormal prion proteins are remarkably hardy. They are not destroyed by cooking, formaldehyde, alcohol or ultraviolet light. They can be inactivated by strong sodium hydroxide (lye) , household chlorine bleach (undiluted) and autoclaving at 132C for 4.5 hours. The spontaneous cases of CJD that affect one in a million people each year are thought to arise by chance spontaneous conversion of some normal prion protein to the abnormal form, which then propagates itself and accumulates. Ingestion of a quantity of abnormal prion protein, as found in the brains of affected animals, is thought to result in absorption of a sufficient amount to initiate the slow process of self-propagation and accumulation, culminating in disease.
Wildlife personnel must respond to the legitimate public concerns about CWD without the benefit of much firm information. No one knows whether or not CWD poses any risk to human health or to domestic animal health, or if it is a significant threat to wild deer populations. Its mode and rate of transmission are unknown, as is its relationship to other TSE's such as Scrapie and CJD (BSE does not occur in North America). In Colorado and Wyoming, major surveillance programs for CWD are underway; the results may help define the geographic extent of the disease and other aspects of its biology. Even in the affected area of north-central Colorado, the prevalence of affected animals appears relatively low, being highest among mature male Mule Deer with about 6% of animals affected. Both Saskatchewan and Alberta have initiated limited surveys for CWD based on available specimens, but quite large samples will be needed to determine the prevalence of a disease that probably does not occur or at least is very rare. Dr. Beth Williams, University of Wyoming, has established a battery of tests for detection of CWD and the application of these tests to large samples of animals will help determine the best methods for detection and their limitations. We are at a very early stage in our understanding of this disease. We must communicate this honestly to the public and enlist its support for the research necessary to learn enough about this disease to respond to it wisely. (For more detailed information, readers are directed to the following publication and references cited therein: Spraker, T.R. et al. 1997. Spongiform encephalopathy in mule deer, white-tailed deer and Rocky Mountain elk in north central Colorado. Journal of Wildlife Diseases 33: 1-6.) (Ted Leighton, CCWHC Headquarters Office).
SP englisch