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Introduction and General Information

"The development of a disease control programme requires careful consideration of the infectious agent, its mode of spread, survival in the environment, susceptibility to chemotherapy and disinfection, the availability of an effective vaccine, reservoirs of the organism in nature (soil, water, wildlife reservoirs) and the cost of eradication." (B21)

The goals of CWD management are:

  • To prevent the introduction of CWD into new areas or populations. (D110.w3)

    • "There is no substitute for disease prevention as a strategy for managing disease in populations."  (D146)

  • To identify infected areas/populations as quickly as possible. (D119)

  • To eliminate CWD from areas where it occurs, if possible.(J40.66.w1, D110.w3, D118, D119)

    • Elimination is most feasible in captive populations. A proposed USDA program has been designed with the States to accomplish that goal. 

    • In the wild, elimination is most feasible with early detection of new disease foci. With new foci, there may be time to stop disease transmission, reduce movement of infected animals, and minimize environmental contamination.

    • In established endemic areas, based on current understanding of CWD, eradication of the disease is not a realistic option. (D114.III.w3, J40.66.w1, J64.21.w17, D118)

  • In endemic areas where elimination is not practical, to maintain the prevalence of disease at or below a given level. (D110.w3, D114.II.w2, D118)

  • To prevent CWD spreading out of an area in which it is found. (D114.III.w3, D118, D119)

  • To minimise the risk of CWD being transmitted from cervids to domestic animals or humans. There is at present no scientific evidence that CWD can be naturally transmitted to humans or to non-cervid species however it is not possible to state categorically that such transmission could not occur. (J64.21.w17, D114.III.w3, D126)

    • A World Health Organization Report (WHO Consultation on Public Health and Animal Transmissible Spongiform Encephalopathies: Epidemiology, Risk and Research Requirements) noted that "there is currently no evidence that CWD in Cervidae is transmitted to humans" but recommended that "no part or product of any animal with evidence of CWD or other TSEs should be fed to any species (human, or any domestic or captive animal)." (W244.09Apr2002.CWD1)

    • The Food and Drug Administration (FDA) also noted that "there is little scientific evidence to show whether CWD is or is not a hazard to humans or non-cervid animals such as cattle or pigs. Therefore, FDA believes it is prudent that CWD-positive deer and elk not be used in animal feed." (W380.24Mar03.CWD2)

(D109.w1, D110.w3, D114.II.w2, D114.III.w3, D117, D118, D119, D123, D126, D146)

Reasons for managing CWD:

  • The presence of CWD in cervids managed for purposes such as hunting and wildlife viewing is undesirable. (D118)
  • If not controlled, the disease may greatly increase mortality, have a large impact on populations and even cause population extinctions. (D118)
    • Information from captive situations has shown that CWD can reach very high levels and may finally be responsible for practically 100% of mortality in adult animals. (J40.66.w1)
    • Modelling of CWD has indicated that CWD is unlikely to develop a "steady-state" situation in which the disease and the population coexist. Either the disease fails to be transmitted and dies out (in situations with very low transmission rates) or it is transmitted successfully, the proportion of the population infected increases, adult mortality is greatly increased and the population declines. (J40.65.w1)
  • The geographical range of CWD may potentially increase substantially. (D118)
  • The geographical range of CWD has expanded due to human actions, particularly animal translocations. (D118)
  • It cannot be stated definitely that CWD will never be transmitted from cervids to other species, including domestic animals and wild carnivores.
  • Although there is no evidence to suggest that CWD presents a human health risk it is known that at least one other TSE is transmissible to humans and it is not possible to state definitely that CWD will never be a human health risk. (W244.09Apr2002.CWD1, D109.w3, D126)
    • Scrapie, a TSE of sheep and goats, has never been linked to disease in humans.
    • BSE (mad cow disease) in cattle has been linked definitely to the human disease known as "new variant Creutzfeldt-Jakob disease" (nvCDJ) or "variant Creutzfeldt-Jakob disease" (vCJD). 
    • There is some evidence indicating that there is a "transmission barrier" which would decrease the likelihood of CWD being transmitted to humans.

    (J64.21.w17)

(J40.65.w1, J40.66.w1, J64.21.w17, D109.w3, D118, D126, W244.09Apr2002.CWD1)

Challenges to the management of CWD:
  • There are no vaccines against nor treatments for the disease.
  • There is a long incubation period after infection prior to the development of clinical signs, during at least part of which time the animal is potentially infectious to other animals.
  • Initial clinical signs are subtle and unlikely to be noticed, particularly in free-ranging cervids.
  • Diagnosis is not possible from clinical signs alone.
  • There is at present no practically applicable (pen side) reliable live-animal test for the disease.
  • Detection in live deer by tonsillar biopsy is most likely to be applicable to captive deer; it is not practical for large-scale use in free-living deer.
  • There is an incomplete understanding of the transmission of the agent.
  • There are no established protocols or proven management solutions for this disease. (D117)
  • Environmental contamination with the agent may play a role in transmission.
  • The agent is extremely resistant to chemical and physical inactivation.
  • Regulatory authority may be divided between agencies such as Departments of Agriculture, Boards of Animal Heath and Departments of Natural Resources. (D121, D124)
  • The market for captive elk and deer involves both intra- and inter-state movement of live cervids, which although apparently healthy, may still be infected with CWD. (D124)
  • Management options may be in conflict with the immediate interests of groups such as cervid farmers, hunters and wildlife watchers.

(B209.17.w17, J40.66.w1, J64.21.w17, N8.18.w4, N8.18.w8, P10.67.w1, D110, D117, D121, D124)

Assumptions for the management of CWD:

  • Much information about this disease is still lacking. 
  • Assumptions which are being accepted in formulating management plans include:
    • The disease is contagious: it is transmitted from infected to noninfected (susceptible) individuals.
    • Shedding of the infectious agent is probably progressive through the course of the disease. 
    • Incidence of infection is increased in locations with high cervid density and high frequency of contact between individuals.
    • The disease is probably transmissible between the cervid species known to be affected but is limited to these species, therefore other species of animals (wild or domestic) do not need to be taken into consideration as either potential sources of the disease or potentially infected by the disease.

    (J40.66.w1, P10.67.w1, D118)

    • While there is no scientific evidence that the disease is transmissible to humans or domestic animals, as it is not possible to state categorically on the basis that CWD is not transmissible to humans or domestic animals, precautions should be taken to minimise the chance that CWD-infected cervid tissues could enter the human or domestic animal food chain. (J64.21.w17, P10.67.w1)
  • In the absence of definite information, it would appear reasonable to extrapolate from information available about the management of other chronic diseases. (D118)

Additional considerations:

  • CWD is an emerging disease. Application of management efforts before or as CWD emerges is more likely to allow effective management than efforts applied after the disease has become established. (D124, D121)
  • CWD does not respect jurisdictional boundaries or the wild-captive boundary; coordination of surveillance programmes and management actions and sharing of results of  research across federal and state agencies is important in the management of this disease. (D114.III.w3, D127)
  • Management of CWD in captive cervids is complicated further by the division of legal responsibilities for managing captive deer and elk with the management of captive cervids sometimes being unclear or divided between governmental agencies. (D117, D121)
  • It is recognised that, with our present state of understanding related to CWD and the many scientific uncertainties that still exist regarding the basic biology and ecology of CWD, any control measures put into operation must be considered experimental. However, based on evidence from the endemic areas of Colorado and Wyoming, if no action is taken the disease would be expected to increase in both its prevalence and its distribution. (D109.w5, D118).
  • Management plans need to be flexible and adaptable to allow for changes as additional information becomes available. (D114.II.w2, D118)
  • Further research is required to enhance CWD management options by increasing our knowledge of the disease including:
    • its present distribution and prevalence;
    • how it is spread between individuals of a given species;
    • how/whether it is transmitted between species;
    • the role of environmental contamination in sustaining CWD in free-ranging cervids;
    • the role of different cervid species (e.g. white-tailed deer) in transmitting the disease from one geographical area to another.
    • how animal movements and behaviour affect transmission and the spatial distribution of CWD.
    • the risk factors associated with "hot spots": areas where clusters of the disease are found.

(D114.III.w3, D114.IV.w4, D118, D121, D124, D127)

  • A long-term commitment is required if CWD is to be eliminated or prevalence reduced. (J40.65.w1)
  • It is recognised that management techniques may need to be adjusted or replaced as new information becomes available, including information from the assessment of the results of management actions. (D118)
  • It is important that states "work cooperatively with landowners, recognize and respect private property rights and provide the appropriate level of information to landowners during development and implementation of any management or research plan." (D126)
  • In the long term, CWD management must include plans to restore the affected species and their environment. (D110.w3)
  • The APHIS Proposed Rule "Chronic Wasting Disease Herd Certification Program and Interstate Movement of Captive Deer and Elk" sets out in detail the intended herd certification program and interstate movement requirements for captive deer and elk. This aims "to eliminate CWD from the captive deer and elk herds in the United States." (W30.12Jan04.CWD1)
Published Guidelines linked in Wildpro
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Actions available for the management of CWD

The following are possible actions which may be used, in appropriate combinations, in order to achieve the desired management goals.

  • It is important to remember that a combination of methods may be required within a disease management program and that the approach used should be sufficiently flexible to allow changes in methods as the situation or knowledge of the disease varies. (B127.10.w10)

No action:

  • All actions have consequences. Doing nothing in response to a disease outbreak would have consequences in the same way as any other action. 
  • Management actions for the control of CWD must be considered to be experimental, because there are still many scientific uncertainties regarding CWD. However this should not be taken as an argument to do nothing. (D109.w5)
  • In the absence of any action it would be expected that transmission of the disease would continue, the number/proportion of cervids infected with CWD would increase, the geographical area containing infected cervids would enlarge and the level of (potential) environmental contamination would increase. In the long term the increased prevalence of infection may be expected to "significantly impact" the population. (D109.w5)
    • N.B. "It is difficult to determine circumstances in which CWD might "burn itself out" because there are no documented instances in which the disease [CWD] was introduced but failed to establish itself and died out." (D109.w5)
    • "Evidence from endemic areas of Colorado and Wyoming indicates that the result of no intervention is increased prevalence and
      distribution of the disease.
      " (D109.w5)
  • In an area in which the disease has recently become established, doing nothing may be expected to allow the disease to become endemic. (D109.w5)
  • A full analysis of the expected consequences of no action in an area into which CWD has recently been introduced is available in Wisconsin Case Study for Chronic Wasting Disease
Research:
  • At this time there is still much that it not known or understood about CWD.
    • Further research is required to provide a greater understanding of the disease, its natural host range and ecology, how it is transmitted and how it is likely to affect infected populations.
    • Contingency plans need to be sufficiently flexible to allow for alteration as scientific knowledge progresses.(D114.IV.w4)
    • Research will not in itself control this disease.

    (D110.w3, D114.IV.w4, D117, D118, D124, D126, J40.66.w1)

Monitoring (surveillance):

  • Surveillance is an essential part of CWD management, required to determine the distribution and prevalence of the disease and to monitor changes in distribution and prevalence, including changes due to management actions. (P10.67.w1, D110.w3, D114.III.w3, D118, D124)
    • It is important to recognise that surveillance by itself will not affect the distribution, prevalence or spread of CWD.
  •  It is important that management actions are monitored to determine their effectiveness and any environmental impacts. (D110.w3)
  • See: CWD CONTROL: CWD Diagnosis and Surveillance (Overview of Techniques)

Quarantine:

Import and transport restrictions:

  • Due to the difficulties in controlling and eliminating CWD once it has been introduced into an area or captive facility, preventing the transportation of infected animals out of affected areas facilities / importation of the disease into areas/facilities which are presently CWD-free is an important component of CWD management.(J40.66.w1, D118)
  • Restrictions on transport of carcasses or parts of carcasses most likely to contain CWD-infectivity is a prudent measure to minimise risk of spreading this disease. (D117, D126)
  • See: CWD CONTROL: CWD Import and Transport Restrictions (Overview of Techniques)

Culling:

  • Culling is a well-recognised management tool which has been used in both captive and free-ranging population and disease management for many years.
  • For CWD control culling may be used for:
    • Targeted removal of sick animals
    • Targeted removal of animals considered likely to spread the disease by natural movements.
    • Population reduction in specified areas
    • Depopulation in specified limited areas

    (D110.w3, D114.III.w3)

  • See: CWD CONTROL: CWD of Deer and Elk Culling and Carcass Disposal (Overview of Techniques)

Carcass disposal:

  • Appropriate disposal of potentially infectious carcasses is an important part of disease control. 
  • In the case of CWD this important not only to reduce potential sources of infection for cervids but to minimise any potential risks to human and domestic animal health. (W244.09Apr2002.CWD1)
  • See: CWD CONTROL: CWD of Deer and Elk Culling and Carcass Disposal (Overview of Techniques) Specific sections:
    • Recommendations for processing of deer and elk carcasses; 
    • Disposal of non-edible portions of individual deer and elk carcasses; 
    • Large-scale Carcass Disposal and Implications; 
    • FDA Regulations regarding use of material from animals with CWD
Habitat modification:
  • Environmental manipulation may be used to "limit animal use of areas and potential exposure" This could be useful for dealing with environmental contamination. (D110.w3).
  • Maintaining a healthy environment may assist in avoiding unnecessary crowding of cervids within an area and thereby reduce the potential for disease transmission. (D114.III.w3)
    • Ensuring that food and cover is available in a dispersed manner reduces the requirement for congregation of animals at scare resources and the attendant close contact which may increase the ease of disease transmission. (B126)
    • Healthy habitats "should result in more dispersed populations, less emigration [from the CWD-endemic area], and reduced transmission of CWD". (D114.III.w3)

Banning or discouraging feeding or baiting of free-living cervids:

Genetic manipulation:

Vaccination:

Education and communication:

(D109.w5, D110, D114.III.w3, D118, N8.18.w8)

Associated techniques linked from Wildpro

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Factors to be considered in choosing the management goals for a particular situation

Factors to be considered in deciding on the appropriate management goals and actions include:
  • Whether or not CWD is believed to be present;
  • The size of the area and population in which the disease is thought to be present;
  • The prevalence of the disease in the infected area;
  • The length of time for which the disease is thought to have been present;
  • The feasibility of carrying out proposed management actions.
N.B. Surveillance is an essential part of all CWD management plans, required to determine the distribution and prevalence of the disease, to monitor changes in distribution and prevalence, and to monitor the effectiveness of management actions.

(J40.66.w1, D109.w3, D110.w3, D118, D126)

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Options for CWD Management in Captive Facilities / Game Ranches

In general a wider range of actions may be taken in captive than in free-living situations and therefore the options for disease management are greater.

Surveillance:

Prevention of importation of CWD:

Elimination of CWD in an infected herd/premises:

Prevention of spread of CWD from an infected herd/premises:

Summaries of the regulations and activities for each state are provided in D110.w3: Document Ref. 110 - Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids: Appendix I - State Regulations and Activities

The APHIS  Proposed Rule "Chronic Wasting Disease Herd Certification Program and Interstate Movement of Captive Deer and Elk" sets out in detail the intended herd certification program and interstate movement requirements for captive deer and elk. This aims "to eliminate CWD from the captive deer and elk herds in the United States." (W30.12Jan04.CWD1)

Associated techniques linked from Wildpro

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Options for CWD Management in Free-living Populations believed to be free of the disease

Prevention of importation of CWD:
  • "Prevention is the most efficient strategy to be employed in combating diseases in free-ranging wildlife." (D146)
  • Due to the difficulties in controlling and eliminating CWD once it has been introduced into an area, preventing the importation of the disease into an area which is presently CWD-free is an important component of CWD management. (B127.10.w10)

Surveillance:

Banning or discouraging feeding or baiting of free-living cervids:

Culling:

Education and communication:

  • Dissemination of information is an important part of all disease management plans, including those in areas which are presently free of CWD. (D110.w3)
Associated techniques linked from Wildpro

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Options for CWD Management in New Foci in Free-living Populations

Of all the wild situations this is the one in which elimination in the area is most feasible. There may be time to stop disease transmission, reduce movement of infected animals, and minimize environmental contamination. (D110.w3)
  • In an epidemiological model of CWD, the spread of the disease within a simulated population was highly sensitive to the transmission rate. Simulated test and slaughter programs "revealed the importance of initiating control while CWD prevalence was low (<0.05). Low rates of test and slaughter (e.g., <20% of the infected population) effectively eliminated wasting disease in simulated populations if control measures were initiated while prevalence was low (i.e., 0.01), but the likelihood of control diminished rapidly as disease prevalence increased. Test and slaughter programs will require an effort sustained over many decades to ensure elimination of chronic wasting disease." (N9.1999.w1)

Elimination of CWD in a newly infected area/ free-living population:

Eradication is the stated goal of CWD management in areas in which CWD may not yet be endemic (Nebraska, Wisconsin, Saskatchewan and the Western Slope of Colorado). (N8.18.w8, P10.67.w1)

Surveillance:

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Options for CWD  Management in Free-living Populations in Endemic areas

Based on current understanding of CWD, eradication of the disease in endemic areas is not a realistic option. (D114.II.w2, D114.III.w3, N8.18.w8, P40.1.w34)
  • "It currently has not been established that it is possible to reduce the prevalence of CWD in an endemic area, and large-scale efforts to reduce prevalence of CWD could have more severe effects on deer and elk populations than does CWD." (D114.III.w3)
  • In the CWD-endemic area of Colorado and Wyoming eradication of CWD appears to be unobtainable. (N8.18.w8)

In this situation control and limitation of the disease’s spread is most appropriate, plus a reduction in prevalence in localised areas if possible (P10.67.w1, D110.w3)

Prevention of spread of CWD from an infected area/free-living population:

  • Culling is the main management tool available for elimination of the disease in free-living populations. 
  • Where the goals are maintaining the disease below a certain level and/or preventing geographical spread of the disease, targeted removal of sick animals, targeted removal of animals considered likely to spread the disease by natural movements and population reduction may be required.
  • See: CWD CONTROL: CWD of Deer and Elk Culling and Carcass Disposal (Overview of Techniques)
    • Targeted removal of sick animals
    • Targeted removal of animals considered likely to spread the disease by natural movements.
    • Population reduction
  • Provision of feed for cervids, whether for the purposes of watching them (feeding) or to facilitate hunting (baiting) encourages concentration of cervids and may increase disease transmission. Reducing this may therefore be very important in reducing CWD transmission. 
  • (B209.17.w17, J64.21.w17)
  • It is recommended that this should be a part of all plans for CWD management. (D126)
  • A ban on translocation of cervids from endemic areas is important to reduce expansion of the disease outside these areas. (J40.66.w1, P10.67.w1)
Habitat modification:
  • Maintaining a healthy environment may assist in avoiding unnecessary crowding of cervids within an area and thereby reduce the potential for disease transmission. (D114.III.w3)
    • Healthy habitats "should result in more dispersed populations, less emigration [from the CWD-endemic area], and reduced transmission of CWD". (D114.III.w3)

Surveillance:

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Further research

For effective management of CWD it will be necessary to develop a better understanding of this disease, including:
  • How CWD spreads between cervids within a species and between cervid species; (D114.V.w5, D117, D126)
  • The relationship between CWD and other TSE diseases such as BSE; (D114.V.w5, D117, D126)
  • The natural host range: whether CWD has the capability to spread to other species such as cattle, sheep, goats, carnivores and humans; (D114.V.w5, D117, D126)
  • The present distribution and prevalence of CWD; (D114.V.w5)
  • The dynamics of this disease in free-ranging white-tailed deer; (D114.V.w5)
  • The effect of animal density on CWD prevalence and transmission. (D117)
  • The effects of animal movement and behaviour on CWD transmission and spatial distribution. (D117)
  • The interaction of demographic factors such as age and sex with CWD; (D117)
  • The role of environmental contamination in maintaining CWD in free-ranging populations; (D114.V.w5, D117)
  • Genetic susceptibility; (D117)
  • The risk factors associated with "hot spots" where clusters of CWD cases are found; (D117)
  • The effects of intensive culling on dispersal of cervids; (D118)
  • The economic and social impacts of CWD and of CWD management programmes. (D118)

N.B. Results of research must be distributed effectively if they are to be of benefit to agencies responsible for disease management.

Management plans must be adaptable as new information becomes available. (D109.w3, D110.w3, D114.IV.w4)

Associated techniques linked from Wildpro
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Authors & Referees

Authors Dr Debra Bourne MA VetMB PhD MRCVS (V.w5)
Referee Suzanne I Boardman BVMS MRCVS (V.w6), Chris Brand (V.w52), Dr Terry Kreeger (V.w49), Dr Julie Langenberg (V.w50), Bruce Morrison (V.w48), Michael Samuel (V.w53), Scott Wright (V.w54)

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