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Introduction and General Information

  • The Transmissible Spongiform Encephalopathy Diseases (TSEs), of which CWD is one, pose a number of problems for control:
    • There is no known antibody response to the agent in infected animals.
    • There are no vaccines available.
    • There are no known therapeutic agents, either prophylactic (before the individual contacts the disease, to prevent infection) or therapeutic (to control or eliminate the disease in an infected animal).
  • Some genetic factors have been shown to affect the susceptibility of individuals to TSE diseases or the incubation time following exposure to the TSE agents. In domestic animals it has been suggested that it may be possible to breed for resistance to TSEs, removing TSE-susceptible genotypes from the gene pool. However breeding for resistance is not a practical proposition for free-living animals. Based on genetic studies carried out to date, the majority of deer and elk in CWD-positive areas appear to be susceptible to CWD.
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Vaccines

Vaccination is commonly used in the prevention and control of infectious diseases in captive populations. However the use of vaccines to prevent infection in free-living populations of animals is more problematic, due to difficulties in delivering the vaccine to the individuals. (B127.13.w13)
  • Where vaccination is used for the purpose of reducing transmission of a disease within a population (as with vaccination campaigns against rabies in wild carnivores), a minimum number or proportion of the population must be protected by the vaccine if the strategy is to be effective. The proportion which must be protected may be determined by considering the population biology of the disease. (B127.13.w13)
  • In the case of CWD vaccination cannot be used in either captive cervids or free-living populations as there are, at least at the present time, no vaccines available [2002]. (J40.66.w1, P10.67.w1, N8.18.w8, D119).
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Genetic Manipulation

Genetic manipulation of free-ranging deer is extremely difficult and is not a practical management option for the control of CWD.

  • It is recognised that a few individuals in research facilities with endemic CWD did not develop CWD during their lifetime, but the reasons for this, genetic or otherwise, are not known. (J40.66.w1)

  • Work to date in Cervus elaphus nelsoni - Rocky Mountain Elk (Cervus elaphus - Red deer) suggests that genotype may affect the susceptibility of this species to CWD. However the most common genotypes in the population were shown to be susceptible to CWD.

  • Resistant genotypes, if present, will not, over any reasonable time span, become most common in the population, because the disease usually appears well after individuals have reached breeding age. Therefore susceptible genes would already have been passed to the next generation before an individual became clinically ill due to CWD.

    • "If it exists, genetic resistance might spare individuals in wild populations, but it is unclear that it would have a significant protective value at the population level, at least in the short term." (D109.w5)

  • Controlling of breeding in free-ranging deer is not practical.

  • The situation is different from that with scrapie in domestic sheep. In domestic sheep, although as with deer and CWD, reproduction may have occurred before the individual shows clinical signs, it is possible for a farmer to remove from the herd both the clinically affected individual and its offspring (D109.w5). This may be possible in farmed cervids but would not be possible in free-living individuals.

The following text has been taken directly from the Wisconsin Department of Natural Resources Environmental Impact Statement; On Rules to Eradicate Chronic Wasting Disease from Wisconsin's Free-Ranging White-tailed Deer Herd:

It has been suggested that some deer may possess a genetic resistance to CWD and that the fatal brain disease could be controlled by genetically improving the deer herd. Others have suggested that genetically engineering a CWD resistant deer may be possible. Current scientific evidence indicates that the vast majority of white-tailed deer are likely susceptible to CWD, and there is no evidence confirming genetic resistance in this species. While a genetic strategy may appear theoretically viable, there is no practical evidence to suggest that free-ranging deer could be manipulated by genetic engineering, artificial breeding, or propagation and release of animals in order to produced a genetically resistant population. Considering the long course of disease in deer, the high annual reproductive potential of females, and the limited ability to control breeding in free-ranging deer, strategies to manipulate genetic resistance of whitetailed deer to CWD is not currently practical. (D109.w3) Environmental Impact Statement; On Rules to Eradicate Chronic Wasting Disease from Wisconsin's Free-Ranging White-tailed Deer Herd

 

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Further research

For effective management of CWD it will be necessary to develop a better understanding of this disease, including:
  • The role of genetic susceptibility to CWD. (D117)
  • Development of preventative tools such as vaccines (D126)
  • Development of treatments. (D121)

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

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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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