Prions are misfolded proteins, distinct from viruses or bacteria. These unusual infectious agents cause Chronic Wasting Disease (CWD), a neurological illness affecting deer and other cervids like white-tailed deer, mule deer, elk, moose, and reindeer. CWD is a significant concern in wildlife populations across North America and other countries, posing challenges for management and raising questions about broader impacts.
Understanding Prions and CWD
Prions differ from conventional pathogens as they lack genetic material like DNA or RNA. They are abnormally folded versions of normal proteins found in the body. A misfolded prion can induce normal proteins to also misfold into the abnormal, disease-associated shape. These misfolded proteins are stable and resist degradation, accumulating in tissues, especially the brain and spinal cord.
This accumulation damages nerve cells, causing a fatal neurological illness. CWD is a transmissible spongiform encephalopathy (TSE), a group of progressive, fatal neurodegenerative disorders that includes bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. Once infected, the disease progresses slowly, typically over 18 to 24 months.
How CWD Spreads
Chronic Wasting Disease spreads through direct and indirect pathways. Direct transmission occurs via animal-to-animal contact, like nose-to-nose interaction, especially in high-density deer populations. This allows for the exchange of infectious prions in bodily fluids.
Indirect transmission involves environmental contamination. Infected deer shed prions through saliva, urine, and feces, contaminating soil, water, and plants. Decaying infected carcasses also release prions. These prions can remain infectious in the soil for years, posing a long-term risk to healthy deer that contact or ingest contaminated materials.
Recognizing Infected Deer
Observing symptoms is an initial step, but clinical signs alone are not definitive for CWD, as other illnesses can appear similar. The disease has a prolonged incubation period; animals may appear healthy but can still shed prions. The most common symptom is progressive weight loss, leading to a “wasting” appearance.
Infected deer may also exhibit behavioral changes, including decreased social interaction, an altered gait or stumbling, and a loss of fear of humans. Other symptoms include excessive drooling, increased thirst, and frequent urination. For a confirmed diagnosis, specific tissues from a deceased animal, such as the brain stem and lymph nodes, are tested for abnormal prions using methods like immunohistochemistry (IHC) or enzyme-linked immunosorbent assay (ELISA).
Potential Impact on Humans and Animals
The potential for CWD to affect humans is a concern, though no confirmed cases exist. CWD is considered a theoretical risk due to its relation to other prion diseases like bovine spongiform encephalopathy (BSE), which has transmitted to humans. Studies in non-human primates, such as squirrel monkeys, show they can contract CWD by consuming infected tissue.
Research on the species barrier suggests a substantial barrier prevents CWD transmission to humans. In vitro studies with human brain cell models showed no infection even with direct exposure to CWD prions. However, due to long incubation periods of prion diseases, ongoing surveillance and research are important. CWD also poses a potential risk to other animal species, including livestock, if exposed to contaminated environments or infected materials, though natural transmission to cattle or sheep has not been demonstrated.
Controlling CWD Spread
Controlling CWD spread involves a multi-faceted approach by wildlife management agencies, hunters, and landowners. Surveillance and testing programs monitor the disease’s presence and prevalence, often collecting samples from hunter-harvested and road-killed deer. These programs identify affected areas and inform management strategies.
To limit transmission, restrictions on baiting and feeding deer are implemented, as these practices concentrate animals and increase direct contact. Hunters are advised on proper carcass disposal, including disposal in approved landfills and discouraging movement of high-risk parts (brain, spinal cord, lymph nodes) across management zones. Selective culling may be used in localized areas to reduce deer densities and stabilize CWD prevalence. Hunter education, emphasizing precautions like wearing gloves during field dressing and avoiding consumption of meat from sick or CWD-positive animals, also aids mitigation.