Chronic Wasting Disease (CWD), is a fatal prion infection affecting free-ranging and captive cervid herds. Cervids are hooved ruminant mammals that include deer, caribou, elk, and moose.
In North America, CWD has been found in at least 24 states, as well as in Canada, and has devastating effects on herd populations. Wyoming and Wisconsin have the highest concentration of infected herds, with numbers approaching 50% in some areas.
First identified as a clinical disease in captive mule deer in 1967, CWD is caused by an infectious agent called a prion. Not all prions are deadly. However, resistant prions are proteins found in the brain and nervous tissue that, for whatever reason, go rogue. Protein function is entirely dependent upon how they fold, and these rogue proteins fold abnormally.
Ordinarily, mis-folded proteins would be recognized and destroyed by the cell. Instead, these prions resist destruction (i.e. degradation) and, worse, resistant prions can cause healthy proteins to also become mis-folded over time. This recruitment of healthy proteins into prions occurs over many years, leaving tiny holes in brain tissue that can lead to neurodegenerative disease and ultimately death.
Symptoms of CWD include excessive salivation, loss of appetite, progressive weight loss, excessive thirst and urination, listlessness, teeth grinding, lowering of the head, and drooping ears. It is always fatal. Unfortunately, many of these signs can also appear as symptoms of other diseases, making it difficult to distinguish CWD in the field.
There is no cure for CWD and no reliable live animal test to detect its presence in order to stop it before it spreads; testing is normally performed on deceased specimens. Presently, the only effective measure to reduce the spread of CWD is complete depopulation of herds that test positive for the disease. Since resistant prions appear to be somewhat host-specific, it is unknown whether prions found in cervid populations can spread to non-cervid wildlife, other types of livestock, or even humans.
CWD is classified as a TSE (transmissible spongiform encephalopathy), like Scrapie in sheep and goats, Mad Cow disease in cattle, and Creutzfedlt-Jakob disease in humans. One theory suggests that the misfolded prions are triggered by a difficult to culture bacterium that has no cell wall known as Spiroplasma. One researcher found this bacteria in 100% of deer showing symptoms of CWD.
Due to the many unknown characteristics of this disease, public health and wildlife officials caution those who hunt or consume cervid meat to take proper precautions. Precautions include:
- Do not handle or consume any animal acting abnormally or that appears sick. If an animal is found under such conditions, it is recommended to contact state game and fish department personnel if a sick animal is harvested or observed.
- Wear latex or rubber gloves when field dressing game.
- Bone out meat and avoid cutting through brain or spinal tissue; minimize the handling of brain and spinal tissue.
- Wash hands and instruments thoroughly in hot soapy water.
- Avoid consuming brain, spinal cord, eyes, spleen, tonsils and lymph nodes. Cut away fatty tissue to remove remaining lymph nodes.
- Avoid consuming meat from any animal that tests positive for CWD.
- If game is commercially processed, request animals be processed individually, without meat combined from other animals.
Even with these precautions, prions are not easily destroyed, and it is uncertain how long they may lay dormant in the environment. It’s believed soil, water, and plants act as natural reservoirs to accumulate prions deposited from excreta and decaying carcasses. Mounting evidence also suggests that prions can mutate. Therefore, like many biological agents, prions may evolve and become better adapted at jumping hosts. Because of the potential for long-term dormancy and host-to-host transmission, the United States Department of Agriculture’s Animal and Plant Health Inspection Service (USDA APHIS) provides assistance to states through its voluntary Herd Certification Program (HCP) to help minimize the risk of introduction, transmission, and spread.
By Kerry Davies Pierce
Design & Development Microbiologist