What Causes Chronic Wasting Disease?

September 29, 2025 •

4 min read

Chronic wasting disease (CWD) has been detected in wild and captive cervid populations in over 30 U.S. states and several Canadian provinces. Caused by infectious proteins known as prions, understanding what causes chronic wasting is crucial for managing this fatal neurological condition affecting deer, elk, and moose.

Quick Summary

Chronic wasting disease is caused by infectious misfolded prion proteins that trigger a chain reaction, damaging the brain and nervous system of cervids. It spreads through direct animal contact and indirect environmental contamination via bodily fluids like saliva, urine, and feces.

Key Points

Prion Cause: Chronic wasting disease (CWD) is caused by misfolded proteins called prions, not viruses or bacteria.
Spreading Mechanism: Infectious prions cause normal proteins in the host's nervous system to misfold, leading to cell damage and accumulation.
Dual Transmission: CWD spreads directly through contact with infected animals and indirectly through persistent environmental contamination.
Environmental Persistence: The infectious prions are highly resistant and can remain active in soil and on plants for years.
Fatal Outcome: CWD is invariably fatal to infected cervids, with no known cure or vaccine.
Long Incubation: Infected animals may not show visible symptoms for months or years, but they can still shed infectious prions during this time.
Not for Humans: There is currently no evidence of CWD transmission to humans, but precautions are recommended when handling and consuming potentially infected meat.

The Infectious Agent: Misfolded Prion Proteins

At the core of chronic wasting disease (CWD) is an abnormal, infectious protein known as a prion. Unlike viruses or bacteria, prions are simply misfolded versions of a normal protein, called PrP, found throughout the body, especially in the central nervous system. When an infectious prion enters a cervid's body, it acts as a template, forcing healthy PrP proteins to refold into the same abnormal, infectious shape. This triggers a chain reaction of misfolding, causing the prions to accumulate and form clumps that damage and destroy nerve cells, leading to the sponge-like appearance of the brain seen in advanced stages of the disease.

The Prion's Unique Resilience

One of the most challenging aspects of CWD is the prion's extreme durability. Prions are not easily destroyed by conventional sterilization methods like cooking, freezing, or chemical disinfectants. This remarkable resistance allows them to persist in the environment, primarily in soil, for years, creating a long-term source of infection. Their ability to withstand environmental degradation significantly contributes to the disease's spread and persistence in affected areas.

Transmission Routes: How CWD Spreads

CWD transmission occurs through both direct and indirect routes, complicating efforts to contain the disease.

Direct Transmission

Direct animal-to-animal contact is a primary way CWD spreads. Infected cervids shed prions through various bodily fluids, including:

Saliva: Often during shared feeding or grooming.
Urine: Contaminates the ground and water sources.
Feces: Adds infectious material to the environment.
Blood: Potentially during injuries or processing.

Animals can shed these prions for months or even years before they begin to show any visible symptoms of the illness.

Indirect Transmission

Indirect transmission involves contact with a contaminated environment rather than directly with an infected animal. This happens through several pathways:

Contaminated Soil: Prions adhere to certain soil types and can remain infectious for years. Healthy cervids can ingest these prions while foraging.
Contaminated Plants: Early research suggests plants may be able to take up prions from contaminated soil, though the extent of this transmission route is still being studied.
Contaminated Carcasses: Infected carcasses that are not properly disposed of release prions into the environment as they decompose, creating a significant point source of contamination.

Comparison of Chronic Wasting Disease with Other Prion Diseases

CWD belongs to a family of diseases called Transmissible Spongiform Encephalopathies (TSEs), which are all caused by prions. While they share a common infectious mechanism, they differ in the species they affect and their known transmission to other animals.


Feature	Chronic Wasting Disease (CWD)	Bovine Spongiform Encephalopathy (BSE)	Scrapie
Primary Host	Cervids (deer, elk, moose)	Cattle (bovines)	Sheep and goats
Human Risk	No confirmed cases in humans, but precaution is advised	Transmissible to humans as Variant Creutzfeldt-Jakob Disease	Not known to be transmissible to humans
Transmission	Direct (animal-to-animal) and Indirect (environmental)	Believed to be caused by contaminated feed	Primarily direct animal-to-animal contact
Symptoms	Progressive weight loss, behavioral changes, excessive salivation	Behavioral changes, lack of coordination, aggression	Intense itching, motor coordination issues

Disease Progression and Clinical Signs

The incubation period for CWD is long, typically lasting 16 months to four years or more, and infected animals show no signs during this time. As the prions damage the nervous system, a number of clinical signs develop during the terminal stages of the disease. These can include:

Severe Weight Loss: The most prominent symptom, giving the disease its name.
Behavioral Changes: Lethargy, listlessness, and a lack of fear of humans.
Neurological Signs: Stumbling, poor coordination, and a blank facial expression.
Excessive Salivation and Urination: Often a result of difficulty swallowing.

Prevention and Management Strategies

Because there is no vaccine or cure for CWD, management focuses on preventing its spread. This is primarily achieved by regulating human activities that can contribute to transmission.

Role of Hunters and Wildlife Enthusiasts

Responsible Carcass Disposal: Carcasses from harvested animals should be disposed of in a licensed landfill or buried at the harvest site to prevent environmental contamination.
Handling Precautions: Hunters should wear rubber gloves when field dressing and bone out the meat to avoid contact with high-risk tissues like the brain and spinal cord.
Reporting Sick Animals: Immediately reporting any sick or abnormally behaving cervids to local wildlife officials is critical for surveillance and management.

Limiting Artificial Congregation

Banning Feeding and Baiting: Regulations in many areas prohibit feeding or baiting deer, as these activities unnaturally congregate animals and increase the risk of direct and indirect transmission.

Carcass and Animal Movement

Transportation Restrictions: Many states and provinces regulate the transportation of cervid carcasses and high-risk parts from CWD-positive areas to prevent the spread of prions.
Minimizing Livestock Movement: For captive cervid populations, strict biosecurity and movement restrictions are key to preventing CWD introduction and spread.

For more detailed information on precautions, visit the Centers for Disease Control and Prevention's CWD page.

Conclusion

The fundamental cause of chronic wasting is the misfolded prion protein, an exceptionally resilient and infectious agent. Its ability to spread through both direct contact and prolonged environmental contamination makes CWD a complex and persistent threat to cervid populations. While there is no cure, understanding the mechanics of this disease allows for informed management strategies focused on preventing its further spread. Public awareness and adherence to precautions are essential in protecting wildlife and mitigating the long-term impact of this devastating disease.

Frequently Asked Questions

A prion is a misfolded version of a normal protein found in mammals. In CWD, the infectious prion comes into contact with a healthy prion, causing it to change its shape. This initiates a chain reaction where more proteins misfold and aggregate, leading to brain damage.

There is no strong evidence to date that CWD can be transmitted to humans. However, health and wildlife agencies recommend taking precautions when handling cervids in affected areas and not consuming meat from known infected animals. CWD does not appear to naturally infect domestic livestock or pets.

Early signs can be subtle, and CWD has a very long incubation period. However, behavioral changes such as lethargy, a loss of fear of humans, or a blank facial expression are often among the first noticeable symptoms.

CWD prions are remarkably durable and can remain infectious in the environment for an extended period, potentially for years. This is because they can bind to soil and are resistant to freezing and other forms of degradation.

Hunters should follow state regulations for carcass disposal, wear protective gloves when field dressing, and avoid cutting through the skull or spinal cord. It is also important to report any deer or elk appearing sick or acting abnormally to wildlife officials.

No, standard cooking and freezing methods are not sufficient to destroy CWD prions. Prions are extremely heat-resistant and can only be reliably destroyed by extremely high temperatures for a sustained period.

Both CWD and Mad Cow Disease (BSE) are prion diseases, but they are caused by different prions and affect different species. CWD affects cervids, while BSE affects cattle. Notably, BSE has been shown to cross the species barrier to humans, unlike CWD where human transmission has not been confirmed.

Feeding and baiting areas force animals to congregate in unnaturally high numbers. This increases the chances of direct, nose-to-nose contact and concentrates prions in the environment, significantly amplifying the disease's transmission.