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Chapter 13 – Chronic Wasting Disease Herd Certification Programs
13.1 The disease – January 2020
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1. Chronic wasting disease (CWD) is a fatal disease that affects the central nervous system of cervids, and occurs in both captive and free-ranging populations. CWD is a naturally occurring member of a unique group of fatal neurological diseases called transmissible spongiform encephalopathies (TSEs). Although the precise cause of these diseases has been the subject of significant debate, the abnormal prion protein hypothesis is now widely accepted. This abnormal prion protein converts normal host prion proteins into the abnormal pathological form, and as this process continues to occur it leads to neurodegeneration and death. The accumulation of this abnormal prion protein in the brain is believed to cause the clinical signs of the disease.
2. CWD is the only TSE found in free-ranging animals. Susceptible cervid species that have been found to be naturally infected with CWD include Rocky Mountain elk (Cervus elaphus nelsoni), white-tailed deer (Odocoileus virginianus), mule deer (Odocoileus hemionus), red deer (Cervus elaphus elaphus), moose (Alces alces shirasi), reindeer (Rangifer tarandus tarandus), black-tailed deer (Odocoileus hemionus columbianus),sika deer (Cervus nippon), and Manchurian Sika deer (Cervus nippon manchuricus). Muntjac deer (Muntiacus reevesi) are experimentally susceptible to oral CWD infection. Caribou (Rangifer tarandus caribou) are closely related to reindeer and are likely also susceptible to CWD.
3. CWD is known to be present in free-ranging and/or captive cervids in 26 states and 3 provinces in North America, in captive cervids in the Republic of Korea, and in free-ranging cervids in Norway, Sweden and Finland. In Canada, CWD has been identified in both free-ranging and captive cervids in Alberta and Saskatchewan, and in captive cervids in Quebec.
4. CWD is the most contagious of the 3 major TSEs of animals. The primary routes of transmission are direct animal-to-animal contact, and indirect exposure to contaminated environments, feedstuffs and fomites. The shedding of the CWD agent into the environment in excretions and secretions of an infected animal during the pre-clinical or clinical disease, and by its carcass after death (if not properly disposed of), is an important factor in disease transmission.
In areas where CWD is present in free-ranging cervids, direct fence line contact and contaminated wild environments are of particular concern. Soil and plant cover may also play a role in contamination of the environment through CWD prion binding to certain soil types, followed by plant uptake into roots and leaves.
The CWD prion (and other TSE prions) is highly resistant to both chemical and physical denaturing, and thus can persist in the environment. 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.
5. The pathogenesis of CWD involves the conversion of a normal cellular prion protein PrPC to the abnormal disease-associated isoform (PrPCWD). The lowest dose to cause infection is not definitively known, but has been predicted to be extremely small. The accumulation of PrPCWD within the central nervous system is the primary event leading to neurodegeneration and the manifestation of clinical signs. During the course of disease PrPCWD also disseminates to systemic lymphoid tissues and eventually infects all areas of the body. It has been detected in peripheral nervous tissue, lymphoid tissue, feces, urine, saliva, muscle, blood and many internal organs of infected cervids.
There is species variability in the pathogenesis of the disease in deer and elk. In white-tailed deer, the abnormal prion accumulates in cranial lymphoid tissue (such as the retropharyngeal lymph nodes and tonsils) before it can be detected in the brain. In elk, however, the distribution of PrPCWD in the brain and cranial lymphoid tissue is more variable. Some elk in the preclinical stage of infection will test positive in the obex but do not have detectable levels of PrPCWD in the retropharyngeal lymph nodes. Many elk show deposits in obex and lymphoid tissue and some in lymphoid tissue only. This variability requires submission of both brain and lymphoid tissue for testing regardless of the species under investigation for CWD.
The typical incubation period of CWD in captive cervids has been observed to be 16 to 36 months. Shedding can occur during the preclinical stage, which may last from a few months to years. The clinical period is generally less than 12 months. CWD-infected cervids can be infectious to other cervids for up to 18 months prior to their death.
CWD is invariably fatal.
6. CWD should be considered in any cervids over 12 months of age exhibiting clinical signs such as the following:
- excessive salivation
- unusual behaviour (including decreased interactions with other animals)
- 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
Since many animals do not show overt clinical signs until late in the course of the disease, significant transmission of the CWD prion may occur prior to any visible indications of a disease problem. 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.
7. The clinical signs associated with CWD can mimic other diseases of cervids, and may be very subtle in the early stages of the disease. The differential diagnosis of CWD may include the following:
- brain abscesses
- traumatic injuries
- starvation and nutritional deficiencies
- dental attrition
8. CWD is diagnosed through detection of the abnormal prion protein in brain or lymphoid tissue. Gross pathological lesions are non-specific and our ability to detect CWD in live animals is limited. Therefore, the definitive diagnosis is based on laboratory analysis of post-mortem specimens to detect the abnormal prion protein.
The tissues to submit for diagnosis of CWD 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.
There are 3 tests currently validated by the CFIA for diagnostic use. The CFIA-approved TSE network laboratories are approved to perform the Bio-Rad TeSeE® enzyme-linked immunosorbent assay (ELISA), which is designed as a large-scale screening test. Confirmation of CWD in non-negative samples is performed at the CFIA's National and OIE Reference Laboratory for CWD using a gel electrophoresis immunoassay (Western Blot), and/or immunohistochemistry (IHC). Both Western Blot and IHC confirm ELISA results and are used to differentiate TSE strains.
9. No immune response to the CWD prion protein has been detected.
10. At this time there is no direct scientific evidence that CWD may be transmitted to humans. Health Canada recommends that any tissue which may come from a known CWD-infected animal not be used or consumed by humans.
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