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Introduction and General Information

"Continuous monitoring and accurate diagnosis are essential for assessing the efficacy of a [disease control/management] program." (B127.10.w10)

Diagnosis:

Chronic wasting disease (CWD) is a difficult disease to diagnose because:

  • It may be mistaken for other diseases; the clinical signs of CWD are not pathognomonic.
  • Clinical signs may be difficult to observe in free-ranging deer and elk, particularly early in the course of the disease.
  • Animals showing signs of CWD may be killed and eaten by predators, or killed on roads, before they are observed and diagnosed.
  • Carcasses of animals dying of CWD may be scavenged before they can be acquired for testing..
  • There is a lack of tests currently available for diagnosis in the field in live individuals.

(J64.11.w3, P42.12.w1, D119)

The goal of diagnostic efforts is "to provide reliable information on the disease and infection status in free-ranging and captive cervids for herd certification programs, epidemiological investigations, and control and management activities."  (D110.w3) 

  • Tests selected for diagnostic use in a surveillance programme require laboratory capacity which can process a sufficient number of samples, using accurate, validated, standardised assays, in a sufficiently short time for results to be useful. (D110.w3, P40.1.w19)

Surveillance: 

"Surveillance establishes the prevalence, incidence, and distribution of the disease, and allows the evaluation of management actions." (D110.w3)

  • 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.
    • It is important to recognise that surveillance by itself will not affect the distribution, prevalence or spread of CWD.
  • In order for surveillance to be effective as a tool for management or research it must be based on statistically sound sampling. (D118, D126, D127)
  • Surveillance must be continued over time, with sufficiently large sample sizes to detect changes in prevalence and distribution. (D118, D126)
  • Because CWD appears to occur with a "clumped" distribution, there is a greater risk that disease "hot spots" could be missed, therefore more intensive sampling is required than if the disease was distributed in a more uniform manner. (D118)
  • Surveillance may not detect CWD until it has been present in a population for some time (e.g. a decade or even longer) due to limitations in sensitivity and cost. (J40.66.w1)
  • It is important that management actions are monitored to determine their effectiveness and any environmental impacts. (D110.w3)

(J40.66.w1, D110.w3, D118, D126, D127)

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Clinical signs that indicate CWD

Clinical signs which may be seen in deer and elk with CWD:
  • Loss of weight and body condition, progressing (if the disease runs its full course) to emaciation.
    • This may be mistaken for normal seasonal weight loss.
    • This may be noted as a failure to regain weight following normal seasonal weight loss.
    • This weight loss is not reversible by provision of an improved diet.
  • Behavioural changes, including:
    • alterations in reactions to other members of the herd;
    • alterations in reactions to humans;
    • periods of stupor/somnolence;
    • walking in repetitive patterns;
    • hyperexcitability (rare).
  • The head and ears may be lowered.
  • Polydipsia (excessive drinking) and polyuria (excessive urination): these are common in the terminal stages.
  • Excessive salivation and drooling, leading to wetting of the chin area and hair.
  • Difficulties in swallowing.
  • Reduced eating, although feeding continues at a reduced level.
  • Subtle ataxia and wide-based stance in some animals
  • Fine head tremors in some animals.

In most cases these signs progress slowly over weeks to months, even longer than a year. However, particularly in white-tailed deer, rapid progression of signs (only a few days from first signs to death) may be seen or even acute death without notable clinical signs while the animal was alive.

  • Signs of CWD are progressive and initially subtle, therefore may not be noticed during casual inspection or by individuals unfamiliar with the signs of this disease or with the individual animal.
  • Loss of body condition is the most obvious sign but this is highly non-specific and is more commonly due to other causes such as malnutrition. 
  • Behavioural changes are frequently subtle and detection requires knowledge of the individual animals affected. 
  • Experienced observers may be able to recognise behavioural signs in most Odocoileus spp. deer late in the course of clinical disease, but signs in elk are more subtle. 

The disease is generally seen affecting individuals, rather than groups of animals, at any one time although morbidity in a captive facility over a period of time (years) may be high. 

N.B. No clinical signs are pathognomonic for this disease. 

Additional indications:

  • Affected free-ranging individuals may be found near water sources or in riparian areas.
  • Individual cases in farmed deer may present as animals of prime age which lose condition, are unresponsive to symptomatic treatment, and die of pneumonia. (J40.66.w1)

(J40.66.w1, J64.11.w3, B294.10.w10, P10.67.w1, N8.18.w6)

It is important to realise that the presentation of an individual case may vary; examples include (J40.66.w1):

  • Aspiration pneumonia;
  • "Sudden death" following handling;
  • An elk getting its head caught under a fence.

ANY DEER OR ELK [in North America] WITH SIGNS COMPATIBLE WITH CWD SHOULD BE TESTED FOR THIS DISEASE. FOR FREE-RANGING ANIMALS THIS MEANS THAT THE ANIMAL SHOULD NORMALLY BE REPORTED TO THE APPROPRIATE AUTHORITY OR BE CULLED AND THE CARCASS OR HEAD PRESENTED TO AN APPROPRIATE AUTHORITY.

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Other diseases that look like CWD

  • Differential diagnoses include all ill-thrift and nervous disorders, including for example (P41.14.w1):
    • Ryegrass staggers;
    • Chronic form of malignant catarrhal fever;
    • Fading elk syndrome;
    • Polioencephalomalacia;
    • Copper deficiency;
    • Abomasal/omasal ulceration in adult deer;
    • Nutritional deficiencies;
    • Toxicities;
    • Stress due to confinement;
    • Pneumonia;
    • Injury.
    • Epizootic hemorrhagic disease (EHD) (also known as blue tongue or black tongue (BT))

    (J40.66.w1, J64.11.w3, P41.14.w1, D119, V.w49)

Aspiration pneumonia may be the proximate cause of death in a deer or elk with CWD and CWD must be considered as a differential diagnosis in juvenile or adult animals which apparently die from aspiration pneumonia. (J40.66.w1) 

N.B.

  • ANY DEER OR ELK WITH SIGNS COMPATIBLE WITH CWD SHOULD BE TESTED FOR THIS DISEASE. FOR FREE-RANGING ANIMALS THIS MEANS THAT THE ANIMAL SHOULD NORMALLY BE REPORTED TO THE APPROPRIATE AUTHORITY OR BE CULLED AND THE CARCASS OR HEAD PRESENTED TO AN APPROPRIATE AUTHORITY.
  • Testing for CWD does not preclude also examining the animal for the presence of other diseases, for example by performing a necropsy on the culled animal. It is prudent always to take appropriate hygienic precautions when performing a necropsy. When performing a necropsy on a CWD suspect additional precautions may be appropriate, including not sawing through the spinal column or skull and either discarding instruments used (into an appropriate "sharps - for incineration" container or thorough disinfection of instruments using strong bleach.
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How to make a definite diagnosis (testing methods)

"For any disease, diagnostic and surveillance methods need to be sensitive, specific, efficient for large scale throughput of samples and, crucially, suitable for use on typical field material." (J21.72.w1)

Samples to be taken:

Laboratory tests:

Summary:
  • Confirmed diagnosis is based on the presence of spongiform lesions in the brain and/or the detection by immunohistochemistry of PrPCWD in the brain and/or lymphoid tissues [2002]. (J40.66.w1, P10.67.w1)
    • The most important site to be examined is the parasympathetic vagal nucleus at the obex, in the dorsal portion of the medulla oblongata. (P10.67.w1)
      • Immunohistochemistry of the obex has the advantage that it allows visualisation of PrP in association with specific tissue architecture for high confidence of diagnosis.
    • In many cervids it may be possible to detect PrPCWD by immunohistochemistry in the retropharyngeal lymph nodes before lesions or PrPCWD are detectable in the CNS. In Odocoileus spp. deer PrPCWD may also be detected in samples from the tonsils early in the course of infection.
  • In addition to immunohistochemistry, PrPCWD may be detected using other laboratory tests. To date [January 2004] ELISA-based test kits from three different companies have been approved by USDA Center for Veterinary Biologicals for use as screening tests for the detection of CWD in free-ranging cervid populations, but none has been approved for use for testing of farmed cervids in regulatory programmes. 
  • Standardisation of tissue collection and trimming techniques, to ensure that the correct tissues are collected and tested, is critical if high-throughput tests are to be accurate. (J212.15.w3)

Histopathology:

  • In CWD the spongiform lesions characteristic of the TSE diseases are most prominent and striking in the medulla oblongata at the obex, particularly the parasympathetic vagal nucleus, the thalamus, hypothalamus and olfactory cortex. (J1.36.w4, B294.10.w10)

Immunohistochemistry (IHC):

  • Immunohistochemistry is the main method which has been used for validated diagnosis of CWD. It is accepted as the "gold standard" for diagnosis of CWD and will be the gold standard in the long term. (D110.w3, J1.36.w4, P40.1.w17)
    • This test, as performed by APHIS’ National Veterinary Services Laboratories (NVSL) and State/university laboratories with which NVSL has contracted, has been designated by USDA as an official test for CWD surveillance. (W30.12Apr03.CWD2)
    • This test may be used for the detection of PrPres (PrPCWD) in the brains of clinically affected and (presumably) preclinically infected cervids. (B294.10.w10)
    • This test may be used also for the detection of PrPCWD in biopsies of tonsillar tissue from mule deer (Odocoileus hemionus - Mule deer) and white-tailed deer (Odocoileus virginianus - White-tailed deer). See: How to tell if an animal is carrying CWD (below). (P40.1.w18)
    • Recent studies have shown that abnormal prions are usually detectable by immunohistochemistry in the retropharyngeal lymph nodes of both deer (Odocoileus spp.) and elk (Cervus elaphus nelsoni - Rocky Mountain Elk (Cervus elaphus - Red deer)) , and in the tonsils of deer, earlier in infection than they are detectable in the obex. However on rare occasions a deer may be positive by IHC in the obex but not in lymphoid tissues. (P10.67.w1, J3.151.w3, J212.15.w2)
    • Immunohistochemistry (IHC) may be used for diagnosis even in samples with some degree of autolysis. (J64.11.w3)
    • Prior to IHC staining, treatment of tissue with formic acid and proteases is required to denature normal PrPC which would otherwise be detected by the antibodies used in testing, which cannot distinguish between the normal and abnormal forms of PrP. (J64.21.w17)
    • The main disadvantage of this method is that it is time consuming, expensive, and interpretation requires the skill of a pathologist. (P40.1.w17)
    • Concerns have been expressed regarding the capacity of laboratories to carry out large volume testing of samples for CWD using immunohistochemistry.
      • The Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids (D110)  noted that "approximately 100 assays/day/machine can be run using IHC. The use of 15 laboratories with two machines each would allow 750,000 samples per year to be tested, or the estimated 177,000 surveillance samples to be tested in three months for rapid management action." (D110.w3)

ELISA:

  • ELISA tests have been developed for the detection of BSE in cattle. They may allow more rapid testing of samples, since they use fresh tissue, removing the delay (three days) while tissue is fixed. Additionally they do not require a trained pathologist to interpret results, which could increase the efficiency of large scale surveillance. However it is important to ensure that such tests are fully validated for similarity to immunohistochemistry in terms of both sensitivity and specificity before use, ensure that data collected using different tests are comparable. (J40.66.w1, P40.1.w17, D118)
  • To date [January 2004] ELISA-based test kits from three different companies have been approved by USDA Center for Veterinary Biologicals for use as screening tests for the detection of CWD in free-ranging cervid populations, but none has been approved for use for testing of farmed cervids in regulatory programmes. (W253.Jan04.CWD1, W475.Jan04.CWD1, W399.08Jan04.CWD1, W399.08Jan04.CWD2)

Western blotting:

  • Western blot tests have been developed for the detection of BSE in cattle. They may allow more rapid testing of samples, since they use fresh tissue, removing the delay (three days) while tissue is fixed prior to IHC. Additionally they do not require a trained pathologist to interpret results, which could increase the efficiency of large scale surveillance. However it is important to ensure that such tests are fully validated for similarity to immunohistochemistry in terms of both sensitivity and specificity before use, ensure that data collected using different tests are comparable. (J40.66.w1, P40.1.w17, D118) Western blotting may be used for detection of PrPres in the brains of clinically affected and (presumably) preclinically infected cervids. (B294.10.w10)

For more information and data sources, see also:

The Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids (D110) includes the following overview in relation to diagnosis of CWD (text copied directly):

CWD assays currently in use and development are, and will be, validated only for epidemiological or disease control purposes.  Immunohistochemistry (IHC) will be the source of validated results in the short-term and will be the gold-standard test long-term.  High-throughput assays may be available for use in laboratories (not animal-side) in the fall 2002 hunting/control season on an experimental basis, but will not be validated prior to the season. The assays may be validated for use by early 2003, allowing retrospective use of their results. Laboratory capacity should be sufficient using approved State/university laboratories as part of a network. However, as the volume and rate of sample submission is uncertain, reporting results may be delayed. Laboratories should be approved first to use the standardized IHC, which will allow them to assist in validating, and then use high-throughput assays. Samples collected may exceed diagnostic needs until research will assist in the selection of tissues for optimum testing; in turn, the samples may be applied to further research and management strategies. Funding for testing populations, new assay validation, and laboratory space/equipment is needed.

(D110 - Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids - Text copied Directly)

 

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Live animal testing for CWD

It IS NOT possible to diagnose CWD based on clinical signs, since several other diseases may result in similar physical or behavioural signs in cervids. (N8.18.w6, D119)
  • Loss of body condition is the most obvious sign but this is highly non-specific and is more commonly due to other causes such as malnutrition. (J64.11.w3, B294.10.w10)
  • Behavioural changes are frequently subtle and detection requires knowledge of the individual animals affected. (J64.11.w3)
  • Clinical pathology does not generally reveal diagnostic information. (J64.11.w3)

It IS possible to diagnose CWD in live deer (Odocoileus spp.) by testing for the presence of PrPCWD in tissue taken from the tonsillar crypt. 

  • Biopsies of tonsils and retropharyngeal lymph nodes followed by examination using IHC may be used to detect infection in live deer and in subclinically infected deer (individuals with pre-clinical CWD). Unfortunately differences in the progression of CWD in elk (Cervus elaphus nelsoni - Rocky Mountain Elk (Cervus elaphus - Red deer)) makes examination of these tissues less useful for such testing in this species. (J40.66.w1, J223.83.w1) 
    • A negative sample cannot be used as proof that the animal is free of infection as the tonsils will be IHC-negative early in the disease process, before accumulation of PrPCWD occurs in lymphoid tissues. (J223.83.w1)
  • Tonsillar biopsy requires that the deer be restrained and anaesthetised. It is most applicable to the testing of captive animals and is likely to be useful only in very limited circumstances in wild deer. (J40.66.w1, J40.66.w2)
    • Live trapping and radiotelemetry may be of use in strictly limited situations for research purposes, particularly if serial samples could be obtained from individual free-ranging individuals. (D118)
  • Data from tonsillar biopsies (with IHC testing) of live deer may be taken to be equivalent to data from tonsillar IHC in culled or harvested deer; this method may be used for estimating prevalence. (J40.66.w2)
    • The results will give a slight underestimate of true prevalence as PrPCWD is not present in tonsillar tissue early in the course of infection with CWD. (J40.66.w2).
  • See CWD Individual Techniques: Tonsillar Biopsy in Deer (Cervidae) for Diagnosis of CWD (Techniques) for detailed description.
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Aims of surveillance

The main aims of surveillance are:

  • Detection: 
    • For populations of cervids (free-living or captive) in which CWD infection has never been detected, to detect new foci of CWD as soon as possible. (D110.w3, D114.III.w3, D118, D119, D123, D127)
      • If surveillance fails to detect CWD in an area, the likelihood that CWD is absent should be estimated. (D127)
        • It is important to recognise that without testing all animals in an area it is not possible to be 100% certain that the disease is not present in that population. Well-designed surveillance programmes can allow a high degree of confidence that the disease is not present above a selected prevalence level. (D127)
      • Surveillance programmes which detect CWD while the disease is present in only a small number of animals provide the best opportunity for disease elimination in an area. (D127)
  • Assessment: 
    • For populations of cervids (free-living or captive) in which CWD infection has been detected, to determine the spatial distribution and prevalence of CWD. (D110.w3, D114.III.w3, D118, D119, D124, D127)
      • This information may be used to determine appropriate management actions. (D127)
  • Monitoring: 
    • To determine changes in the prevalence and distribution of infection in an area or population over time. (D118, D127)
    • To determine the effectiveness of management actions. (D110.w3, P10.67.w1, D127)
    • To confirm that a population is CWD-free.
      • For captive herds, a minimum of five years of complete surveillance of all juvenile and adult mortalities is emerging as a standard for demonstrating CWD-free status. (D109.w5)
      • The criteria which could be used to remove an area, in which CWD has been detected in free-living cervids and in which depopulation has been used to eradicate the disease, from CWD-positive status have not yet been established. (D109.w5)
        • It has been suggested that, based on current scientific knowledge, statistically-valid monitoring of cervids in an area for five years, with no CWD-positive animals being detected during this time, would be required before the area could be considered to be CWD-free. (W430.27Mar03.CWD11)
    • Research: 
      • Adding to our understanding of CWD epidemiology and supporting modelling of disease spread. (D109.w4, D127)
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Known risk factors for CWD in free-living populations

Risk factors for introduction of CWD into an area:
  • Area is adjacent to an area in which CWD-positive free-living animals are known to be present;
  • Area is adjacent to land on which TSE-positive farmed or free-living animals have lived;
  • Area has large numbers of farmed/captive elk or deer;
  • Area has received translocated deer/elk from regions in which CWD is known to occur;
  • Area permits transport of hunter-killed deer/elk carcasses from CWD-infected areas.

Risk factors which may amplify CWD presence in an area:

  • High deer or elk population density;
  • Area has a history of the presence of CWD-positive animals / CWD-contaminated environments;
  • Area has a low abundance of large predators (this may allow sick animals to survive longer and act as a source of agent for a longer period of time, thus increasing the likelihood of transmission of the agent to other cervids);
  • Areas with artificial concentrations of free-ranging deer/elk due to e.g. baiting, feeding, water development, other human-related modification of habitat.

(D127)

  • N.B. full evaluation of risk factors for CWD in free-ranging populations requires an understanding of distribution, movement social behaviour, population characteristics and dynamics of affected deer and elk populations. (D127)
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Choice of areas and populations for surveillance in free-living populations

Geographical targeting:

Surveillance for initial detection of CWD in free-living populations may be geographically targeted based on assessment of risk, concentrating on areas which known risk factors for introduction of CWD. 

Risk factors for introduction of CWD into an area:
  • Area is adjacent to an area in which CWD-positive free-living animals are known to be present;
  • Area is adjacent to land on which TSE-positive farmed/captive or free-living animals have lived;
  • Area has large numbers of farmed/captive elk or deer;
  • Area has received translocated deer/elk from regions in which CWD is known to occur;
  • Area permits transport of hunter-killed deer/elk carcasses from CWD-infected areas.

(J40.66.w1, D119, D123, D124, D126, D127)

Risk factors which may amplify CWD presence in an area:

  • High deer or elk population density;
  • Area has a history of the presence of CWD-positive animals / CWD-contaminated environments;
  • Area has a low abundance of large predators (this may allow sick animals to survive longer and act as a source of agent for a longer period of time, thus increasing the likelihood of transmission of the agent to other cervids);
  • Areas with artificial concentrations of free-ranging deer/elk due to e.g. baiting, feeding, water development, other human-related modification of habitat.

(D127)

  • N.B. full evaluation of risk factors for CWD in free-ranging populations requires an understanding of distribution, movement social behaviour, population characteristics and dynamics of affected deer and elk populations. (D127)

Within a state/jurisdictional area, surveillance may be targeted near the borders with neighbouring states/jurisdictional areas (i.e. areas where the disease may enter from an adjacent state/jurisdictional area).

Sample area size:

  • Smaller sample areas containing fewer individuals may be required if infection is to be detected early, before it becomes widespread in the population and while management actions are most likely to be able to eliminate the disease in the area. (D127)
    • If a surveillance plan is designed to have a high probability of detecting the disease at a prevalence in the population exceeding 1%, if the total target population is 200,000 this would mean 2,000 cervids would be infected at the time of detection, while if the target population for surveillance is 20,000 surveillance would expect to detect the disease when only 200 infected animals were present. (D127)
      • Setting smaller sample areas in this way will require greater sampling effort. (D127)

Appropriate sampling units:

  • Depending on the size of the area in which surveillance is to be carried out, the area may need to be divided into smaller regions for efficient surveillance, e.g. within a state, counties or game management units. (D127)

Species:

  • It may be necessary to consider as separate surveillance data from elk, mule deer and white-tailed deer, since rates of infection may vary among populations of different cervid species within a given geographical area. (D127)

Populations:

  • Within one geographical area there may be distinct populations, separated by either behavioural or physical (environmental) barriers, there may be biological subgroupings within a population which need to be considered separately. (D127)
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Forms of surveillance in free-living populations

A variety of forms of surveillance may be used for the detection and monitoring of CWD. Which form or forms are appropriate in a given location will vary with the aims of surveillance for that area.

  • "Surveillance data should be gathered using appropriate biological and epidemiological standards to assure the reliability of results and inferences drawn from the data." (D126)
  • A combination of forms of surveillance may be appropriate to gather the maximum epidemiological information. Targeted surveillance, harvest based surveillance and live animal testing (tonsillar biopsy) have all been used to determine the extent and severity of endemic CWD in northeastern Colorado. (P46.1.w3)
  • Long-term active surveillance is required if changes in distribution and prevalence over time are to be detected and to evaluate the effects of management actions. (J64.21.w17, J40.66.w1)

Three main forms of surveillance which may be used for detection and/or monitoring of CWD in wild cervid populations are targeted surveillance, active, generally harvest-based surveillance and live-animal surveillance. 

Targeted Surveillance:

  • Targeted surveillance involves examination of "suspects": deer showing clinical signs consistent with CWD, or deer which have died recently under questionable or unknown circumstances. (J1.36.w4, P40.1.w4, P42.12.w1, D119, W414.10Apr03.CWD1)
    • The brainstem (medulla oblongata at the obex) of such animals is examined by histopathology and IHC. (J1.36.w4, P40.1.w4)
    • At a minimum, this form of surveillance should be undertaken. (D126)

Appropriate use:

  • As a supplement to active surveillance methods. (D127)

  • For initial detection of CWD in areas where the risk of CWD is considered to be low, resources are limited and adequate samples from hunter-harvested deer are not available for active surveillance. (D127)

  • May be more appropriate in limited areas with high human density where there is a high probability that animals with clinical signs will be detected. (D127)

Advantages:

  • Reduced surveillance costs compared with other form of surveillance, while allowing a large geographical area to be surveyed for the disease. (P40.1.11)
    • "The objective of targeted surveillance is to locate and test the animals most likely to be infected, thereby reducing surveillance costs, while at the same time enabling coverage of a large geographic area." This approach has been successful in detecting CWD in areas in which it had not been recorded previously. (P40.1.11)
  • In terms of resources, this may be an efficient method of initial detection: 

    • This method of surveillance appears to be effective for the detection of new foci of CWD infection in free-ranging cervids. (P40.1.w4, J1.36.w4)

    • Targeted surveillance "continues to be our most efficient means of detecting new foci of infection throughout the state [Colorado]." (P40.1.w5)
    • This approach has been successful in detecting CWD in areas in which it had not been recorded previously. (P40.1.w11)
    • Targeted surveillance of clinical suspects appears to be the most sensitive approach for initially detecting CWD in deer and elk populations". (N9.1997.w2, N9.1998.w1)
  • May provide information on other diseases present in the population. (D127)

Limitations:

  • Highly dependent on animals with clinical signs being detected and sampled. (D127)
  • Only animals in the late stages of CWD show clinical signs and are therefore likely to be detected. (D127)
  • May not detect the presence of CWD in an area while it is present at only a very low level:
    • It has been shown that in most areas clinical cases have not detected until the prevalence in the local cervid population was greater than 1%. (J1.36.w4)
  • Does not appear to be useful for either assessment or monitoring of disease in an area.
    • Data from targeted surveillance are obviously biased and are therefore of little use in estimating prevalence. (P40.1.w4)
    • It has been shown that the number of clinical cases submitted for examination from a particular area is a poor predictor of the local prevalence of CWD. (J1.36.w4)
      • Human population distribution appears to have a strong influence on detection of suspect cases. (J1.36.w4)
  • Costs per individual deer for collection of samples may be greater than with e.g. harvest-based surveillance. (D127)

Active/Harvest-based Surveillance:

  • Harvest based surveys involve geographically-targeted random surveillance of harvested cervids. Sections of brainstem (specifically the medulla oblongata at the obex) are collected and examined using IHC; more recently collection of tonsil or retropharyngeal lymph node has been used also. Stage of infection may be assessed by the distribution of IHC staining and by histopathology. Data from such surveys represent relatively unbiased point estimates of CWD prevalence. (P40.1.w4)
  • The number of cervids to be tested per area (e.g. per Deer management Unit or per county) must be calculated for sufficient probability of detecting the disease if it is present at a given level. for example in Michigan a number of 50 deer per county was calculated to give a 95% probability of detecting the disease if it was present at a prevalence of at least 5%. For detection of CWD in free-living deer in the event of the disease being detected in a privately owned captive cervid facility, it was calculated that about 300 deer of at least 18 months old would need to be harvested within a five mile radius (approximately 79 square miles) around the facility to give 95% confidence of detecting CWD at a prevalence of at least 1%. (D119)
  • Spatially explicit data are required if the distribution of the disease is to be well-defined, particularly if the distribution is clumped. (D118)
  • "Surveillance must be continued over time and sample sizes must be sufficient to detect changes in prevalence and distribution of the disease."(D118)

Appropriate Use:

  • Surveillance for CWD may be carried out by harvest surveys in both endemic areas and areas not known to be endemic. Hunter surveys have been used to estimate prevalence in endemic areas, and by expanding the surveillance areas outside the known endemic area, to better define the distribution of CWD. Initial surveillance in other western states during 1996, by examination of the brains of hunter-harvested deer and elk, found all animals to be CWD-negative. (B294.10.w10)
  • Harvest based surveys in combination with road-kill based surveys are recommended for estimation of prevalence, monitoring of prevalence trends and comparison of prevalence among data analysis units (DAUs) once the disease has been detected in a given area. (N9.1997.w2) [June 1997]
    • It has been noted that the number of samples from harvested deer and elk is increased if hunters are required to participate in the survey, with submission rates being three to five times higher in units/seasons where regulations required submission of heads. (N9.1997.w2)
    • Active surveillance programs have been effective in detecting CWD at locations in the USA (South Dakota, Nebraska and Wisconsin) and in Canada (Saskatchewan). (P40.1.11)
  • Examples of the use of harvest based surveys:
    • Annual harvest-based surveys are important for estimating prevalence and monitoring epidemic trends in Colorado. (P40.1.w5)
    • In Nebraska hunter-harvest surveillance was conducted statewide during the 2002 firearm deer hunt, with emphasis placed on the known CWD endemic area.(P40.1.w7, V.w48)
    • In Minnesota a geographically-focused CWD Monitoring Program was initiated during the Fall 2002 Deer Hunting Firearm Season. The objective of this program was to collect and test brain stems from 5,00-6,000 hunter-harvested deer in 16 of the state's Permit Areas. The areas where harvest-based surveillance were to be carried out were selected on criteria such as proximity to captive cervid facilities, density of deer, the number of check stations in the area and availability of samples and proximity to areas (e.g. Wisconsin) in which the disease had been detected recently. (D121)

Advantages:

  • Large numbers of deer are harvested annually by hunters in many areas. Harvest-based surveillance utilises these harvested deer.
  • This method appears to provide relatively unbiased data on prevalence. (J1.36.w3, J1.36.w4)
  • An additional advantage of harvest-based surveillance is that, if a hunter has provided adequate and legible contact information, the individual may be informed regarding the CWD status of their individual deer. This can assist the hunter in deciding whether or not to consume venison from their deer. (D126)
    • The WHO recommends that CWD-positive deer should not be consumed. (W244.09Apr2002.CWD1)

Limitations:

  • Tests used cannot detect CWD infection in the very early stages therefore although an individual that tests positive for CWD can be said definitely to be infected, on an individual-animal basis a deer/elk that tests negative for CWD cannot be guaranteed not to have the disease.
  • In some areas the number of harvested deer may be low. (D126)
  • If the proportion of infected animals in the population is very low then large number of animals must be tested in order to give a high probability that an infected animal will be detected and therefore to be able to say that, if no infected animals were detected, there is a high probability that the disease is not present. (B318.13.w13, B127.7.w7).
  • CWD is not randomly distributed across the landscape; there are "hot-spots" of increased prevalence and other areas of lower prevalence. (D118, D127) This may affect the results of harvest-based surveys.
  • Hunters do not sample (harvest) deer at random nor at an even spacing within an area. This will tend to reduce the probability that sampling will detect a CWD-positive animal. (B127.7.w7, D127)
  • The behaviour of CWD-infected deer might affect the probability that they will be harvested. This could be an increase in probability (e.g. if less sensitive to external stimuli, therefore easier to approach and shoot), which would result in the estimate being higher than the real prevalence, or a decrease (e.g. if lethargic and remaining hidden, or avoided by hunters as not looking healthy) which would result in the estimate being lower than the real prevalence. (J1.36.w3)
    • A within-year trend, therefore a possible bias was detected in harvest-based CWD-prevalences using data from four game management units in Larimer County, Colorado, 1996-1998. "Field data strongly suggest that the proportion of CWD-positive deer harvested increased across the three main sampling periods (rifle seasons) of each yr but not across yr." It was concluded that if data was pooled across all the sampling periods for a given year a relatively unbiased prevalence could be obtained. (J1.36.w3)
    • Analysis of data over a period of several harvest seasons may be required for estimation of prevalence and in order to identify temporal trends and design a sampling strategy which minimises bias or allows for it in estimating prevalence. (J1.36.w3)
  • If insufficient samples are available from hunters additional collection of samples may be required. (D118)

Active live-animal based Surveillance:

  • This involves capture of animals, collection of samples, marking, release of sampled animals and follow-up elimination of animals identified as CWD-positive. (D127)
  • See above section on "Live animal testing for CWD"

Appropriate use:

  • For limited areas where culling for sampling purposes is problematic (e.g. protected areas, refuges) and population densities are low, so only relatively few animals needing to be tested. (D127)

Advantages:

  • Killing of CWD-infected animals is not required.(D127)
    • This may be important in areas where there would be strong public opposition to such killing. (D127)

Disadvantages:

  • A high level of skill is required to live-capture cervids and take samples from live animals. (D127)
  • Live animal testing requires more personnel time and is much more expensive than collecting samples from hunter-killed animals. (D127)

Additional forms of surveillance which may be utilised include:

Mortality-based Surveillance:

  • This involves testing of all natural mortalities, or all mortalities regardless of proximate cause. 
    • The brainstem of such animals is examined by histopathology with IHC as an adjunct. 
  • This surveillance method is more aggressive than targeted surveillance. 
  • These data contain inherent bias and are therefore of limited use in estimating prevalence. 

    (P40.1.w4)

Road-kill based Surveillance:

  • This involves the testing of animals killed on the roads.
  • This may be useful where the number of samples provided by harvest-based surveys are low, to augment surveillance designed to detect new foci or to assist in better definition of distribution in known foci. 

    (D126)

Outbreak Surveillance:

  • Outbreak surveillance involves the collection of specified numbers of animals from an area in which CWD is known to exist. (D110.w3)
  • May be used to determine the rate of infection and the extent of the infected area (which may have been detected by targeted or hunter-based surveillance). (D110.w3)
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Surveillance in farmed cervids

Because of the difficulties in managing infected herds/premises, aggressive surveillance is important to prevent commercial movements of infected individuals. (J64.21.w17, P10.67.w1)

For effective surveillance of farmed elk and deer, individual cervids must be uniquely and permanently identifiable and adequate records must be kept. 

  • If records are adequate and individual animals are identifiable, epidemiological investigations can be used to identify populations which potentially have been exposed to the disease and are at risk of infection. This has been used in combination with mortality-based surveillance.
    • It is worth noting that "routine surveillance, tracing and population efforts" identified 23 CWD-infected farmed elk herds in six states (Colorado, Kansas, Montana, Nebraska, Oklahoma and South Dakota) following identification of the disease in an elk herd in South Dakota in 1997. [2002 data]. (P40.1.w14)
Individual animal identification and registration:
  • Two forms of identification, one of which is permanent, should be required for each individual cervid. (W414.10Apr03.CWD1)
  • Registration data should include "total number of animals, species, sex, age, deaths, causes of deaths, movement of animals into and out of the operation, and records of disease-testing."  (W414.10Apr03.CWD1)

Standard mortality-based surveillance:

  • Standard surveillance includes mandatory reporting of all deaths and testing of all animals over a stated age (e.g. 16 months).
    • This would include all animals that die naturally, all animals slaughtered, and all animals harvested on shooting preserves.
  • Efforts are underway to develop a single CWD herd certification program. This would include farmed elk (Cervus elaphus nelsoni - Rocky Mountain Elk (Cervus elaphus - Red deer)), white-tailed deer (Odocoileus virginianus - White-tailed deer), mule deer (Odocoileus hemionus - Mule deer) and red deer (Cervus elaphus - Red deer). (P40.1.w14)
    • "The national surveillance plan for farmed cervid herds includes mandatory death reporting and CWD testing of all animals, except calves, that are slaughtered or die on the premises." (D110.w3)

  • The APHIS Revised Draft CWD Program Plan (June 2001) Herd Certification Program Standards includes "surveillance based on testing of all deaths over 16 months of age is required. This includes surveillance at slaughter and surveillance of animals killed in shooter bull operations." It notes that "good quality sampling is essential for surveillance." (W30.23May02.CWD1)

  • This method has detected several infected elk farms. (P40.1.w4)

(D115, W414.10Apr03.CWD1)

Trace-forward exposed herds:

  • The APHIS Revised Draft CWD Program Plan (June 2001) Herd Certification Program Standards defines a "trace-forward" herd as "a herd that has received an animal from a CWD positive herd within 60 months prior to the diagnosis of CWD in the positive herd." (W30.23May02.CWD1)

    • A herd plan would be drawn up for any trace-forward exposed herd, including herd surveillance consisting of "mandatory death reporting and CWD testing of all age animals" for a period of five years. (W30.23May02.CWD1)

Trace-back exposed herds:

  • A "trace-back" herd is defined as "a herd in which a CWD positive animal resided in any of the 60 months prior to diagnosis of CWD in the positive herd." (W30.23May02.CWD1)
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Surveillance in zoos

AZA member institutions are required already, by accreditation standards, to perform necropsies on all collection animals to determine the cause of death of the animal; however since this does not always involve removal and examination of the brain additional requirements are needed. (J2.34.w1, W253.Jun03.CWD2)

The AAZV Infectious Disease Committee has drafted guidelines for AZA-accredited institutions regarding CWD. (W253.Jun03.CWD2)

  • In order to align with planned federal certification status for cervid producers, AZA institutions should maintain cervid herds for a minimum of 5 years with no evidence of CWD. Additionally it is strongly recommended that all cervids from "at risk" species that die or are euthanased over the age of 12 months should have the head (specifically, obex of the brain) submitted to a certified laboratory for CWD testing. This should be a requirement for any individuals from "high risk" species."
  • It is recommended that all cervids in zoos dying with clinical signs of a wasting and/or neurological disease should be tested for CWD at a certified laboratory.
  • From "high risk" and "at risk" species, lymphoid tissue, particularly from the tonsils, should be considered for testing in order to contribute data on the detection of CWD in these tissues.
  • It is not know at this time [2003] whether the proposed national surveillance plan for farmed cervids, requiring mandatory death reporting and testing for CWD, by a certified laboratory, of all cervids other than calves, slaughtered or dying on the premises, will be applied to zoos.

Any free-ranging cervid that dies or is euthanased on zoo grounds or nearby should be tested for CWD.

(J2.34.w1, W253.Jun03.CWD2)

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Further research

For more effective diagnosis and surveillance of CWD it will be necessary to:
  • Evaluate existing diagnostic tests for accuracy and utility; 
  • Improve the accuracy, speed, and capacity of diagnostic tests;
  • Establish a standardized yet flexible national sampling protocol for testing; 
  • Develop tests that provide early detection of disease; 
  • Develop a cost effective live animal test that can be applied in the field; 
  • Develop tests for detection of environmental contamination;
  • Developing a GIS that can elucidate patterns of disease–host population characteristics.

(D110.w3, D121)

There is a fundamental need for research to improve understanding of the epidemiology of CWD. For effective design of surveillance programmes and utilisation of data in modelling programs, improved knowledge associated with CWD required regarding:

  • Cervid behaviour, population characteristics and population dynamics associated with CWD occurrence and spread;
  • Risk factors associated with occurrence and spread of CWD;
  • Factors influencing seasonal movements of cervids;
  • Factors associated with the patchy distribution of CWD;
  • Cervid interactions.

    (D127)

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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