Table of Contents
History
Etiology
Transmission
Pathogenesis
Clinical Signs
Diagnosis
Treatment and Control
References
History
Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) of deer and elk. To date, this disease has been found only in cervids (members of the deer family) in North America. First recognized as a clinical "wasting" syndrome in 1967 in mule deer in a wildlife research facility in northern Colorado, it was identified as a TSE in 1978. CWD is typified by chronic weight loss leading to death. There is no known relationship between CWD and any other TSE of animals or people.
In the mid-1980s, CWD was detected in free-ranging deer and elk in contiguous portions of northeastern Colorado and southeastern Wyoming. In May 2001, CWD was also found in free-ranging deer in the southwestern corner of Nebraska (adjacent to Colorado and Wyoming). The limited area of northern Colorado, southern Wyoming, and southwestern Nebraska in which free-ranging deer and/or elk positive for CWD have been found is referred to as the endemic area. Soon after diagnosis of the
disease as a TSE, Colorado and Wyoming wildlife management agencies stopped the movement of deer and elk from their research facilities; wild cervids have not been translocated from the endemic area.
CWD also has been diagnosed in farmed elk herds in a number of States and in one Canadian province. The first positive farmed herd in the United States was detected in 1997 in South Dakota. Since then, 16 additional positive herds have been found: South Dakota (6), Nebraska (3), Colorado (5), Oklahoma (1), and Montana (1). As of October 2001, 6 of these 17 positive herds remain under State quarantine. Ten of the herds have been depopulated or have been slaughtered and tested, and the quarantine has been lifted from one herd that underwent rigorous surveillance with no further evidence of disease. CWD also has been found in farmed elk and free-ranging mule deer in the Canadian province of Saskatchewan.
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Etiology
The agent responsible for CWD (and other animal TSEs, such as scrapie and bovine spongiform encephalopathy) has not been completely characterized. There are three main theories on the nature of the agent that causes CWD: (1) the agent is a prion, an abnormal form of a normal protein, known as cellular prion protein, most commonly found in the central nervous system. The abnormal prion protein "infects" the host animal by promoting conversion of normal cellular prion protein to the abnormal form; (2) the agent is an unconventional virus; (3) the agent is a virino, or "incomplete" virus composed of nucleic acid protected by host proteins. The CWD agent is smaller than most viral particles and does not evoke any detectable immune response or inflammatory reaction in the host animal. Based on experience with other TSE agents, the CWD agent is assumed to be resistant to enzymes and chemicals that normally break down proteins, as well as resistant to heat and normal disinfection procedures.
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Transmission
The exact means of transmission of Chronic Wasting Disease is still unknown. Since the exact cause of this disease is proposed to be three distinct agents, different scientists explain the transmission of the disease in different ways. Most evidence, however, suggests a lateral mode of transmission through animal to animal contact or by contaminated feed or water sources by saliva, urine, or feces. Studies have also suggested that the disease may be more likely to occur in areas where animals are crowded or areas where animals congregate at artificial (man-made) feeding or watering sites. The majority of evidence today shows that CWD is only a disease which can be passed between Rocky Mountain elk, mule deer, white-tailed deer, and black-tailed deer. There has been no evidence which suggests that, outside of a lab setting, the disease can be transmitted to either humans or other types of livestock.
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Pathogenesis
The infectious agent of Chronic Wasting Disease is still unknown but it has been proposed to involve a prion, or proteinaceous infectious agent. A prion is a small, proteinaceous infectious agent that resists inactivation by procedures which affect nucleic acids. Prions, such as Scrapie, Bovine Spongiform Encephalopathy (BSE), Transmissible Mink Prionencephalopathy (TME), cause spongiform encephalopathies of animals and humans. It has been proposed that prions are ingested, where they can then be absorbed at Peyer's Patches. Lymphoid cells in the Peyer's Patch then phagocytose the prion and spread to secondary lymphoid organs. The prion then replicates at these secondary sites and enters the nerves which innervate these organs. Through retrograde movement, the prion reaches the spinal cord and eventually, the brain. The clinical signs of CWD include behavioral changes, loss of body weight, emaciation, excessive salivation, increased drinking and urination, stumbling, trembling, and depression. The behavioral changes include decreased interaction with other animals, listlessness, lowering of the head, blank facial expression, and repetitive walking in set patterns. A decreased interest in hay and grinding of the teeth may also be observed. As with the other spongioform encephalopathies, the brain develops vacuolated spaces giving it a spongy appearance. Chronic Wasting Disease is a progressive and irreversible disease which ultimately leads to death of the infected animal.
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Clinical Signs
Chronic Wasting Disease is ultimately a fatal disease, but animals with the disease may not show any symptoms for a number of years. Once clinical signs are observed the animal may live for a number of weeks or months before death.
Animals with the disease most consistently exhibit gradual weight loss accompanied by decreased consumption of roughages.
Most deer will also exhibit increased water consumption and urination.
Social behavior is generally decreased and effected animals may appear listless and depressed with drooping ears, a lowered head, and blank facial expression.
Other clinical signs include: increased salivation, difficulty swallowing, grinding of teeth, pneumonia, lack of coordination and paralysis.
Because symptoms are not specific enough to distinguish between Chronic Wasting disease and other diseases the only accurate detection method is to do a post-mortem examination of brain tissue.
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Diagnosis
Diagnosis of Chronic Wasting Disease is based on abnormal clinical signs and postmortem samples of neural tissues of the brain and spinal cord.
To distinguish CWD from other diseases with similar clinical signs, the brain tissue and spinal cord are examined histopathologically for definitive results.
CWD is characterized histologically by neuronal vacuolation or lesions within the nervous system, resembling other forms of Transmissible Spongiform Encephalopathy diseases such as scrapie and BSE.
CWD can be diagnosed via immunohistochemistry where brain tissues are tested for the presence of protease-resistant prion protein.
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Treatment and Control
Agricultural Research Service has ongoing research programs in conjunction with United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS), academia, and the animal industry which is resulting in a new live-animal and post-slaughter assays for Chronic Wasting Disease.
An Agriculture Research Service scientist has developed a laboratory assay (which needs further validation) that might lead to the development of a diagnostic test for TSE's which would in turn, help with the control of Chronic Wasting Disease. Agriculture Research Service and Animal and Plant Health Inspection Service help state agricultural agencies manage disease outbreaks by prohibiting transport of deer and elk, keeping positive herds under quarantine or depopulating or slaughtering positive herds.
Individual state wildlife management agencies and APHIS have intensive surveillance programs for CWD in free-ranging deer and elk in endemic areas.
Public health officials suggest precautions to hunters including: do not shoot, handle or consume any animal that appears sick; contact the Division of Wildlife for your area if you see any animal that appears sick; wear rubber gloves when field dressing carcasses; bone out the meat from your animal; minimize the handling of brain and spinal cord tissues; wash hands thoroughly after field dressing is completed; avoid consuming brain, spinal cord, eyes, spleen, tonsils and lymph nodes; request that your animal be processed individually without meat from other animals being added to meat from your animal.
As part of an ongoing monitoring process, the Division of Wildlife samples deer in certain game management units to track disease prevalence and trends (hunters can submit heads of harvested deer for testing).
Veterinarians advise that nothing can be done to save deer and elk affected by Chronic Wasting Disease.
Destroying affected animals may help prevent the spread of infection to other deer or elk.
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References
United States Department of Agriculture
South Dakota Fish and Game Department
Missouri Fish and Game Department
CBS News, here you can view a video on Chronic Wasting Disease
Wyoming State Veterinary Laboratory
www.agric.gov.ab.ca/agdex/600/663-43.html
www.aphis.usda.gov/
www.inspection.gc.ca/english/anima/heasan/disemala/cwdmdce.shtml
www.uwyo.edu/CES/PUBS/Chronic.htm
http://www.agric.gov.ab.ca/agdex/600/663-43.htm
http://www.oie.int/eng/normes/mmanual/a_00119.htm
http://www.ars.usda.gov/speeches/testimony/index.html
http://www.ars.usda.gov/is/pr/1999/991021.htm
http://www.ars.usda.gov/is/np/ha/han9.htm
http://wildlife.state.co.us/hunt/HunterEducation/chronic.asp
http://www.ngpc.state.ne.us/wildlife/cwd.html
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