Chronic Wasting Disease in Cervids
by Jenny Alonge
Chronic wasting disease (CWD) is a prion disease that affects cervids, such as deer, elk, reindeer, sika deer, and moose, that was first identified in captive deer in Colorado. By November 2023, CWD had been reported in free-ranging deer, elk, and moose in 31 states across all four United States regions and three Canadian provinces. The disease has also been identified in farmed deer and elk. States with the highest incidence include Wyoming, South Dakota, Colorado, Nebraska, and Wisconsin. The disease spread is concerning for numerous reasons, including the potential risks to human health.
Signs and pathophysiology
CWD is a member of the transmissible spongiform encephalopathy (TSE) disease family, which includes prion diseases, such as bovine spongiform encephalopathy (BSE), scrapie, transmissible mink encephalopathy, kuru, and Creutzfeldt-Jakob disease. CWD is a fatal, neurodegenerative disease whose signs include significant weight loss, apparent ruminal atony, ataxia, and hypersalivation. In later stages, affected cervids may lose their fear of people and exhibit polydipsia/polyuria and generalized incoordination. The incubation period ranges from five months to five years in experimental settings. Once signs manifest, the disease slowly progresses, and the animal typically dies in one to four months.
After infection, prions migrate to the lymphoid centers, including the tonsils and retropharyngeal lymph nodes. The CWD prions cross the blood-brain barrier and travel through the central nervous system (CNS) via the autonomic nervous system’s ascending fibers, resulting in prion deposits and spongiform degeneration in the vagus nerve and medulla oblongata.
Transmission
CWD is highly contagious—one study demonstrated that the disease infected 90% of mule deer in a captive herd. CWD can be isolated in saliva, blood, urine, feces, and antler velvet, and scientists believe that the disease is transmitted through direct contact with these secretions and excretions or contact with contaminated food, soil, or water. Prions are extremely resistant to degradation and can persist for extended periods in the environment.
Potential human health risk
CWD’s rapid spread has caused concern about the potential for human exposure, because BSE food-borne transmission to humans suggests the species barrier may not protect people from animal prion diseases. A 2022 study using mice expressing human prion protein as an infection model demonstrated CWD’s zoonotic potential.
Cooking doesn’t kill prions but concentrates them, so someone eating infected deer meat will more likely ingest the prion. However, although tens of thousands of infected animals have been eaten in recent years, no human cases have been reported, but concern about a mutation that could lead to human infection still exists.
Ecological and economic impact
In addition to the potential for human infection, other CWD-affected areas include:
- Ecology — If CWD continues to spread, a decrease in cervid numbers could negatively impact plant and predator species involved in the cervids’ life cycle.
- Financial cost — According to the U.S. Geological Survey, CWD costs the country tens of millions of dollars per year. Stakeholders who are financially impacted include state and federal agencies that regulate wild or farmed cervids, Tribal Nations, cervid farmers, and the hunting industry (e.g., outfitters, land leasers, taxidermists, meat processors). From 2000 to 2021, the federal government spent more than $280 million on CWD, including more than $16 million in indemnity payments to cervid farmers. The money was mostly from the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. CWD may also affect local and regional economies—for example, hunters may not want to hunt in areas where CWD has been detected, impacting the revenue from permits and travel expenses.
- Tribal Nations — Cervids are important to Indigenous people, and CWD may influence the cultural and spiritual connection of these populations with cervids. The federal government funds some Tribal Nations to address the problem.
Management
Treatment, prophylaxis, or supportive care are not available for any TSE, including CWD. Management in farmed cervid herds is by depopulation with indemnity and CWD herd certification programs, which are voluntary cooperative programs between the industry and federal or state governments, administered by the USDA Animal and Plant Health Inspection Service. The program involves CWD testing in all herd animals that die over a certain age and limiting new herd additions to animals from herds of comparable or higher CWD status. Five years of monitoring is typically needed to achieve the highest CWD status.
Controlling CWD in free-ranging herds has proven extremely difficult. Measures include population reduction, testing and removal, and intensified surveillance, all with limited success.
Decreasing potential risk
Recommendations to reduce human CWD exposure include:
- Do not shoot, handle, or eat meat from cervids that look sick, act strangely, or are found dead.
- Wear latex gloves when field dressing a deer and handling the meat, minimize handling the organs, especially the brain and spinal cord, and do not use household knives.
- Check your state’s wildlife and public health guidance website to see whether they recommend or require animal testing.
- Test the cervid for CWD before eating the meat and eat only meat that tests negative.
- If your meat is commercially processed, request that your animal be processed separately to avoid mixing meat from multiple animals.
CWD is concerning, but researchers, including those at the Minnesota Center for Prion Research and Outreach (MNPRO), are studying the disease to help mitigate human risk and improve cervid health and welfare. So far, the research has led to the development of a faster, easier CWD test that hunters can use in the field.
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