Chronic Wasting Disease (CWD) is a neurological illness that affects cervids, a family of animals like deer, elk, moose, and reindeer. This progressive and fatal disease poses challenges for wildlife management and animal health. Diagnosing CWD in live animals presents considerable difficulty because early symptoms are often non-specific or absent. A definitive diagnosis requires laboratory testing performed after an animal has died.
Why Field Observations and Live Animal Tests Are Not Definitive
Observing an animal in the field for signs of CWD is not a definitive diagnostic method. Visual symptoms such as severe weight loss, behavioral changes like listlessness or a blank stare, excessive drooling, and an altered gait can indicate CWD. However, these signs are not unique to CWD and can also be indicative of other illnesses, malnutrition, or normal aging processes, making a precise diagnosis impossible through observation alone.
Current live animal testing methods, while useful for surveillance, do not provide a conclusive diagnosis. Biopsies of rectal lymphoid tissue or tonsils can detect prion proteins in infected animals, but their sensitivity varies depending on the disease stage and cervid species. Blood, urine, or fecal sample analyses are being researched but are not yet widely available or reliable for definitive CWD diagnosis. A negative result from a live test does not definitively rule out CWD, especially in the early stages of infection when prion accumulation may be too low for detection.
The Definitive Method: Post-Mortem Tissue Analysis
The only way to definitively confirm Chronic Wasting Disease is through post-mortem examination of tissues from the deceased animal. CWD is caused by abnormal prion proteins, which are misfolded proteins that accumulate and damage nerve tissue. These prions are most concentrated and reliably detectable in tissues after the animal has died, allowing for accurate detection.
Primary tissues collected for CWD testing are parts of the central nervous system and lymphatic system. The brainstem, specifically the obex region, is a preferred sample due to consistent and early prion accumulation in this area. Certain lymph nodes, such as the retropharyngeal lymph nodes, are collected for analysis. The presence of abnormal prions in these tissues confirms the CWD diagnosis.
Specific Diagnostic Techniques
Immunohistochemistry (IHC) is the gold standard for post-mortem CWD diagnosis. This technique involves preparing thin tissue sections, which are then treated with specific antibodies that bind to the abnormal prion proteins. A subsequent staining process allows visualization of these prions under a microscope, revealing characteristic accumulation patterns.
Another widely used method is the Enzyme-Linked Immunosorbent Assay (ELISA), a quicker screening test for large-scale surveillance. ELISA detects prion proteins in tissue homogenates through a color-change reaction. While useful for initial screening, positive ELISA results are typically confirmed using IHC or other more specific methods to ensure accuracy.
Western blot is another highly sensitive technique used for CWD confirmation and research. This method separates proteins by size and uses antibodies to detect the abnormal prion protein. Proper sample collection and handling, including rapid cooling and appropriate tissue preservation, are important steps for these techniques to ensure sample integrity and accurate diagnostic results.