Chronic Wasting Disease: A Comprehensive Overview of CWD in Cervids Worldwide

9 min.

Chronic Wasting Disease: A Comprehensive Overview of CWD in Cervids Worldwide

Page Contents

1. CWD What is it? Do We Need to Worry in Australia?

Chronic Wasting Disease (CWD) is a transmissible spongiform encephalopathy (TSE) that affects members of the Cervidae (deer) family. This fatal neurodegenerative disease is caused by misfolded prion proteins. CWD causes a characteristic “wasting” syndrome in infected animals, leading to severe weight loss, neurological deficits, and inevitably death. First identified in the late 1960s in a captive mule deer facility in Colorado, CWD has since become a leading concern for wildlife management agencies, hunters, conservationists, and public health officials worldwide.

2. Disease Mechanism and Pathophysiology

2.1 Prions and TSEs

Prion Protein (PrP):
Prions are abnormal forms of a normal cellular protein (PrP $^C$ ), which, once misfolded into the infectious isoform (PrP $^{Sc}$ ), can induce other prion proteins to misfold.
Neurodegenerative Process:
The accumulation of these misfolded prions primarily occurs in the brain and nervous system, causing spongiform changes (microscopic holes in brain tissue) and clinical signs such as incoordination, behavioral alterations, and progressive weight loss.

2.2 Unique Attributes

Long Incubation Period:
Infected animals may not display clinical signs for 1.5 to 3 years or longer.
Environmental Persistence:
Prions can survive in the environment (soil, vegetation) for years and remain infective.
Universal Fatality:
There is no recovery once clinical signs appear; the disease is invariably fatal in infected cervids.

3. History and Global Distribution

3.1 Early Discovery

Late 1960s: Captive mule deer in Colorado exhibiting neurological symptoms.
1978: Identified as a TSE by veterinary researchers.

3.2 Spread Across North America

United States:
- Now confirmed in at least 30 states. Hotspots include the Midwest, Rocky Mountain regions, and certain areas of the Eastern US.
- Surveillance and mandatory testing of hunter-harvested deer in many endemic states.
Canada:
- Detected primarily in Alberta and Saskatchewan, with recent cases in Manitoba and Quebec.
- Wildlife agencies mandate testing and restrict movement of carcasses from affected zones.

3.3 Scandinavia

Norway:
- First European detection in 2016 among wild reindeer, later in moose.
- Sparked extensive testing programs and significant culling efforts in reindeer herds to contain spread.
Sweden and Finland:
- Additional sporadic cases identified in moose.
- Ongoing large-scale surveillance to clarify prevalence and potential spread.

3.4 Asia

South Korea:
- Cases traced to captive elk imported from North America.
- Authorities have culled exposed herds to prevent further spread.

3.5 Australia

Current Status:
- No known cases of CWD in Australia to date.
- Australia has strict biosecurity and import regulations to prevent introduction of diseases, including TSEs.
Significance of Introduced Deer:
- Several deer species (e.g., red deer, fallow deer, sambar deer) are present in Australia as introduced species.
- Monitoring and maintaining strict controls are key to preventing CWD establishment.

3.6 Other Regions

No confirmed reports of CWD in wild cervid populations in South America or Africa. However, given how difficult the disease can be to detect in early stages, experts urge vigilance in any region with captive or wild cervids.

4. Transmission and Persistence

4.1 Routes of Transmission

Direct Contact:
- Infectious prions shed in saliva, feces, urine, and other bodily fluids can be passed to another animal through direct exposure.
Environmental Contamination:
- Prions in soil and vegetation can be ingested by grazing or browsing cervids.
- These prions remain infective for years, if not decades.

4.2 Incubation Period

Ranges from approximately 18 months to over 3 years.
Cervids often shed prions before showing any clinical signs, facilitating silent spread within herds.

4.3 Environmental Challenges

Prion Resistance:
- Prions are highly resistant to standard disinfection (e.g., bleach solutions, heat, UV light).
Soil Interactions:
- Research indicates certain soil minerals (like clay) can bind to prions, potentially influencing infectivity and environmental persistence.

5. Species Affected and Potential Cross-Species Risks

5.1 Primary Cervid Hosts

Mule deer, white-tailed deer, elk (wapiti), moose, reindeer, and sika deer are confirmed hosts.
Farmed cervids are also susceptible and play a significant role in pathogen dissemination if biosecurity is lax.

5.2 Domestic Livestock

Cattle & Sheep:
- Despite similarities with bovine spongiform encephalopathy (BSE, “mad cow disease”) and scrapie in sheep, natural field transmission of CWD to livestock has not been documented.
- Experimental studies have shown low-to-negligible susceptibility under normal field conditions, though research continues.

5.3 Zoonotic Potential

Humans:
- To date, no confirmed human cases of CWD infection.
- Leading health organizations (CDC, WHO) nonetheless recommend caution:
  - Hunters are advised to test deer harvested in known CWD areas.
  - Consumption of CWD-positive or sick-looking deer is discouraged.
Laboratory Findings:
- Certain animal models (e.g., ferrets, transgenic mice) have shown varying susceptibility under experimental conditions, raising questions about prion strain adaptability.
- Ongoing research aims to determine if any prion strain mutations could facilitate human or non-cervid infection in the future.

6. Clinical Signs and Diagnosis

6.1 Clinical Presentation

Early Stage:
- Often asymptomatic; subtle signs of decreased socialization or occasional confusion.
Late Stage:
- Severe weight loss (“wasting”), drooping head and ears, excess salivation/drooling.
- Erratic behavior (lack of coordination, repetitive movement), difficulty swallowing.
- Loss of fear of humans in wild populations.

6.2 Diagnostic Methods

Post-Mortem Tests:
- Examination of the obex region of the brainstem and the retropharyngeal lymph nodes for misfolded prions.
- Immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA) are standard.
Live-Animal Tests:
- Biopsy of tonsil or rectal lymphoid tissue in captive settings can help identify infection but are less practical for large-scale testing of wild populations.

6.3 Surveillance

Many wildlife agencies have established or mandated testing for hunter-harvested cervids, especially in known or suspected endemic regions. This data helps map the disease’s spread and prevalence.

7. Management and Control Measures

7.1 Surveillance and Testing

Mandatory Check Stations:
- Hunters in CWD-endemic areas often must submit deer heads or lymph nodes for testing.
Targeted Removal:
- Agencies sometimes cull symptomatic animals or entire high-risk herds to slow or contain an outbreak.

7.2 Movement Restrictions

Live Animal Transport:
- Many jurisdictions prohibit moving captive cervids out of CWD-positive facilities or zones.
Carcass Transportation:
- Hunters may be restricted from bringing whole carcasses across state or provincial lines.
- Deboned meat, hides, or skull caps (cleaned of brain and spinal tissues) are typically allowed.

7.3 Habitat Management

Feeding and Baiting Bans:
- Reduces unnatural congregation and direct contact among wild cervids, which can lower transmission rates.
Contaminated Sites:
- Decontamination is extremely challenging. Some agencies consider long-term quarantines or changes in land use for severely affected areas.

7.4 Captive Cervid Regulations

Regular Testing:
- Farmed deer and elk herds undergo routine testing; any positive detection usually triggers quarantines or depopulation.
Biosecurity Protocols:
- Double-fencing or “no-contact” fencing can prevent interactions between captive and wild cervids.
- Restricting shared water sources or feed troughs can reduce transmission.

7.5 Public Education

Hunter Outreach:
- “Know before you eat” campaigns encourage testing of meat from CWD-endemic areas.
- Proper field dressing and disposal of high-risk materials (e.g., brain, spinal cord) are emphasized.
Community Awareness:
- Wildlife agencies engage with local communities to report sightings of sick or strange-acting deer.

8. Australia’s Approach and Recommendations

8.1 Current Status

Australia is currently CWD-free and highly vigilant regarding prion diseases. This is achieved through stringent import controls on animals, animal products, and feed materials.

8.2 Biosecurity Measures

Import Restrictions:
- Strict protocols on importing live cervids, semen, or embryos.
- Quarantine and certification processes to ensure no introduction of TSEs.
Surveillance of Introduced Deer:
- Monitoring free-ranging populations of introduced deer species for unusual morbidity or mortality events.
Public Awareness:
- Education programs, especially for game hunters who may travel abroad.
- Guidance on how to identify symptomatic animals and whom to report suspicious cases to.

8.3 Future Considerations

Continuous Vigilance:
- Maintain up-to-date detection capabilities, including readiness to test suspicious neurological or wasting cases in deer.
Research and Collaboration:
- Collaborate with international experts to stay informed of new diagnostics, prion inactivation methods, and findings on TSEs.
Emergency Response Plans:
- Have robust response strategies in place should CWD ever be detected, including quarantine, culling, and tracing movements of potentially exposed animals.

9. Economic, Ecological, and Social Impacts

9.1 Hunting and Wildlife Economy

Revenue from deer hunting (licenses, tourism, and associated businesses) can be significant, particularly in North America. CWD fears may reduce hunter participation or lead to heightened management costs.

9.2 Ecological Balance

Deer and other cervids are critical parts of many ecosystems, influencing vegetation growth and serving as prey for large carnivores.
Major die-offs or culling operations have ecological ripple effects, potentially altering predator–prey dynamics and habitat composition.

9.3 Public Health and Perception

Even without confirmed human cases, diseases labeled as “prion diseases” evoke public concern due to parallels with BSE (“mad cow”) crises of the past.
Maintaining public trust through transparent surveillance and clear communication is crucial.

10. Ongoing Research and Unanswered Questions

10.1 Zoonotic Potential

Researchers continue long-term studies to check for any association between CWD exposure and human neurological conditions.

10.2 Prion Strains and Mutation

Prions can exist in multiple “strains,” each potentially having differing virulence and host range. Ongoing studies seek to understand how these strains evolve.

10.3 Environmental Decontamination

Scientists explore chemical treatments (e.g., strong alkali, specialized enzymes) to break down prions in soil or on surfaces.
Large-scale, field-applicable solutions remain elusive.

10.4 Genetically Resistant Cervids

Certain genotypes in deer appear to have longer incubation periods or decreased susceptibility. Research on selective breeding in captive populations—or the natural selection that might occur in the wild—could inform future management.

10.5 Vaccine Development

Multiple laboratories have experimented with potential vaccines that might delay onset of CWD or reduce infection rates.
To date, no proven effective vaccine exists for large-scale use in wild populations.

11. Conclusion and Outlook

Chronic Wasting Disease remains a major wildlife health issue, with considerable ecological, economic, and social ramifications. Its continued geographic spread, coupled with the prion’s remarkable environmental persistence, poses challenges for wildlife managers and policymakers across continents. Although no human cases have been confirmed, health and wildlife agencies worldwide recommend a precautionary approach to protect both human and animal health.

For Affected Regions:
Collaborative and science-based policies—focused on monitoring, research, restrictive transport measures, and public education—provide the best chance of slowing CWD’s progress.
For Regions Without CWD (like Australia):
Vigilance, strong biosecurity protocols, and contingency planning are essential to maintaining a CWD-free status.
Long-Term Research:
Work continues to develop new diagnostic tools, explore genetic resistance, investigate potential vaccines, and clarify any unknown risks to other species, including humans.

Ultimately, while CWD’s future course is uncertain, ongoing scientific efforts and responsible management can help mitigate its impact on the world’s cervid populations and, by extension, our broader ecosystems.

References and Further Reading

U.S. Centers for Disease Control and Prevention (CDC) CWD Resources
World Organisation for Animal Health (WOAH) – Prion Disease Guidelines
State/Provincial Wildlife Agency Websites (e.g., Colorado Parks & Wildlife, Saskatchewan Ministry of Environment)
Scientific Journals such as Prion, Journal of Wildlife Diseases, and Emerging Infectious Diseases

Note: This article is for informational purposes. For region-specific regulations and guidelines, consult local wildlife agencies and public health authorities.