Chapter 13 – Chronic Wasting Disease Voluntary Herd Certification Program
13.1 The Disease
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Chronic wasting disease (CWD) is a naturally occurring member of an unusual group of fatal neurological diseases called transmissible spongiform encephalopathies (TSEs) and occurs in both captive and free-ranging North American cervids. Abnormal prion protein is associated with the presence of disease.
Susceptible cervid species that have been identified with CWD include mule deer (Odocoileus hemionus), Rocky Mountain elk (Cervus elaphus nelsoni), red deer (Cervus elaphus), white-tailed deer (Odocoileus virginianus), black-tailed deer (Odocoileus hemionus), moose (Alces alces shirasi), Sika deer (Cervus nippon), and Manchurian Sika deer (Cervus nippon).
CWD is known to be present in free-ranging and/or captive cervids in 22 states and two provinces in North America, and in the Republic of Korea. In Canada, CWD exists in free-ranging cervid populations in Alberta and Saskatchewan, and has been identified in captive cervid premises in those two provinces.
1. Following an unknown infectious dose (which has been estimated to be as small as 5-25 μg of PrPCWD), the incubation period of CWD, in captive cervids, has been observed to be 16 to 36 months, with an average incubation period of 22 months. This is followed by a clinical period of less than 12 months. CWD-infected cervids can be infectious to other cervids for up to 18 months prior to their death.
2. The primary route of transmission of CWD is believed to be lateral, with infection acquired by oral exposure to secretions or excretions of animals in the infectious phase. Animals in captivity have a greater opportunity for exposure through indirect and direct transmission. Shared water sources and feed contaminated with CWD-infected feces or saliva are major environmental risks on farm. Infection can also occur when animals are exposed to the CWD prion in highly contaminated environments alone.
3. The CWD prion is highly resistant to both chemical and physical denaturing, and thus can persist in the environment. Decontamination efforts must keep this in mind. The agent can survive in soil for extended periods of time and is resistant to dry heat up to 600°C, sunlight, freezing, and desiccation.
CWD should be considered in any cervids over 12 months of age. Since clinical signs can vary, can be similar to other cervid diseases, and may not be overt until late in the disease course, significant transmission of the CWD prion may occur prior to any indication of a disease problem.
Clinical signs can present as follows:
- excessive salivation;
- unusual behaviour (including decreased interactions with other animals), listlessness, depression;
- aggressive or violent behaviour;
- neurological signs (including paralysis, difficulty in swallowing, head pressing, ataxia, polydipsia/polyuria, proprioceptive deficits, and recumbency);
- weight loss and/or poor body condition;
- retention of winter hair coat; and
- aspiration pneumonia.
Signs usually last for weeks or months before the animal dies; however, some animals may not show clinical signs, except for acute pneumonia, or may succumb to a fatal injury. As there is no treatment for CWD, clinical signs progress until the animal dies.
The clinical signs associated with CWD can mimic other diseases of cervids, and may, in fact, be very subtle in the early stages of the disease. The differential diagnosis of CWD will depend on which clinical signs the animal exhibits, but may include the following:
- brain abscesses;
- traumatic injuries;
- starvation and nutritional deficiencies; and
- dental attrition.
There is currently no validated live animal test for CWD, and gross pathological lesions are non-specific. CWD is diagnosed through detection of the abnormal prion protein in brain or lymphoid tissue. The tissues to submit for diagnosis in every eligible animal on the CWD Voluntary Herd Certification Program are the obex of the medulla oblongata and the retropharyngeal lymph node (RPLN). The primary target tissue tested for all members of the family Cervidae (with the exception of members of the genus Odocoileus), such as elk, red deer, reindeer, sika deer, and fallow deer, is the obex of the medulla. The primary target tissue tested for Odocoileus species (including white-tailed deer and mule deer) is the RPLN.
The screening test used in this program is the standard Bio-Rad enzyme-linked immunosorbent assay (ELISA) test. The confirmatory test used in suspect cases is either immunohistochemistry (IHC) or Western Blot (or both).
No immune response to the CWD prion protein has been detected.
CWD is not known to be a human health hazard; however, experimental primate challenge studies are ongoing. As such, Health Canada recommends that any tissue which may come from any CWD-infected cervid should not be used or consumed by humans.
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