Chronic Wasting Disease (CWD) is a progressive and fatal neurological disease affecting members of the cervid family, which includes deer, elk, moose, and reindeer. First identified in 1967 in captive mule deer in Colorado, CWD has since spread to wild and captive populations across North America, affecting at least 33 U.S. states and five Canadian provinces, with cases also reported in Norway, Finland, Sweden, and South Korea. This disease represents a significant concern for wildlife health, given its progression and containment challenges.
Understanding Prions and Chronic Wasting Disease
Prions are infectious agents composed solely of misfolded proteins, lacking genetic material. All mammals naturally produce a normal cellular prion protein (PrPC) which has an uncertain physiological function. In prion diseases, however, this normal protein undergoes a conformational change, misfolding into an abnormal, disease-associated form known as PrPSc. This misfolded PrPSc then acts as a template, inducing other normal PrPC proteins to misfold into the abnormal form, leading to a chain reaction.
As these misfolded prions accumulate, they form aggregates in the brain and neural tissues. This accumulation disrupts normal tissue structure, causing neuronal vacuolation, creating a spongy appearance in the brain. This damage results in neurodegeneration and severe neurological symptoms.
CWD is a type of transmissible spongiform encephalopathy (TSE), a group of such diseases that includes scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle. The incubation period for CWD can be prolonged, often averaging 18 to 24 months, during which animals may appear healthy despite being infected. Once clinical signs appear, the disease progresses relentlessly, leading to death.
How Chronic Wasting Disease Spreads
Chronic Wasting Disease is contagious among cervids, spreading through both direct animal-to-animal contact and indirect environmental contamination. Direct transmission can occur through close contact between infected and healthy animals, such as during social interactions or through bodily fluids. Infected animals can shed prions into their environment even before showing symptoms, sometimes for up to two years.
Environmental contamination plays a significant role in the spread of CWD because prions are resistant and can persist in the environment for years, even decades. Prions shed through bodily fluids or from decomposing carcasses can bind to soil and plants, creating long-term reservoirs of infectivity. Healthy animals can then become infected by ingesting contaminated soil, food, or water. The movement of infected animals, either naturally or through human activity, also contributes to the geographical spread of the disease across regions and states.
Effects on Wildlife and Zoonotic Potential
CWD manifests with a range of clinical signs in affected cervids after a long incubation period. The most consistent symptom is progressive weight loss, known as “wasting,” even while eating.
Neurological symptoms include stumbling, lack of coordination, tremors, and a wide stance. Behavioral changes are also common, such as listlessness, decreased interaction with other animals, repetitive walking patterns, and loss of fear of humans. Excessive salivation, increased drinking and urination, and teeth grinding are also observed. These symptoms worsen over time, leading to the death of the infected animal.
No direct confirmed cases of CWD infection in humans have been reported. However, research and vigilance continue regarding this possibility. Studies in non-human primates, such as squirrel monkeys, have shown CWD transmission through consumption of infected meat or brain tissue. While some studies using human cerebral organoids suggest a strong species barrier to CWD transmission, a low theoretical risk persists.
As a precautionary measure, public health organizations like the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend against consuming meat from CWD-positive animals or those appearing sick. Hunters are also advised to avoid consuming tissues that can harbor CWD prions, such as brain, spinal cord, spleen, tonsils, and lymph nodes, and to bone out meat before transport in affected areas.
Detection and Control Efforts
Diagnosing CWD relies on post-mortem testing of tissues from deceased animals. The primary diagnostic methods involve examining brain tissue, specifically the obex, and lymph nodes (e.g., retropharyngeal lymph nodes), for the presence of abnormal prion proteins. Techniques like enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry (IHC) are commonly used for this purpose. While live animal testing presents challenges, research is ongoing to develop more accessible methods, such as detection in ear tissue.
Control efforts for CWD involve a multi-faceted approach aimed at managing its spread. Surveillance and monitoring programs are in place to detect the disease early for containment.
Regulations are implemented to restrict the movement of live cervids and carcass parts from CWD-affected areas to prevent further spread. Population management strategies, such as targeted culling in high-prevalence areas, are employed to reduce disease incidence. Public education and awareness campaigns also play a role, informing hunters and the general public about safe practices, including carcass disposal and avoiding artificial concentration of cervids. Research continues into long-term solutions, including genetic resistance in cervids and environmental decontamination.