Introduction
In recent years, researchers have become increasingly interested in the mechanisms of transmissible spongiform encephalopathies (TSE’s). The interest grew after discovering that one of the TSE’s, bovine spongiform encephalopathy (BSE), is linked to the human variant Creutzfeldt-Jacob disease (Williams, 2005). TSE’s are infectious diseases entailing the refolding of prion proteins resulting in death (Spraker et al., 2002). The following animal diseases are categorized as TSE’s: scrapie in small ruminants, transmissible mink encephalopathy (TME), BSE, feline spongiform encephalopathy (FSE), Exotic ungulate encephalopathy (EUE) in zoological greater kudu, nyala and oryx, and chronic wasting disease (CWD) in cervids. CWD is the only known TSE occurring primarily in free-ranging animals.
Chronic wasting disease was first recognized in 1967 among captive mule deer in Colorado, though at that time it was known as clinical wasting syndrome (Bourne, 2015). In 1978 it was renamed, after researchers diagnosed the disease as a spongiform encephalopathy through histopathological examination of affected brains (Williams and Miller, 2002). To this day, five species within the family Cervidae are identified as species naturally susceptible to CWD: mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), elk (Cervus canadensis), moose (Alces alces) and reindeer (Rangifer tarandus tarandus) (Miller and Fischer, 2016). Several studies have revealed that domestic livestock and laboratory mice are not naturally susceptible to CWD, these species are only experimentally susceptible through intracerebral inoculation (Bruce et al., 2000; Hamir et al., 2001).
Hitherto, no cases of human disease owing to contact with CWD infected meat have been reported. Though infection of human prion protein with CWD has been achieved at the molecular level (Barria et al., 2011; Davenport et al., 2015), transgenic mice with human prion protein did not develop any signs of chronic wasting disease when infected (Kong et al., 2005; Wilson et al., 2012). Thus, no consensus regarding the zoonotic potential of chronic wasting disease could be obtained. Besides, CWD is still spreading within North America and globally, making the transmission of CWD a public health concern.
For several decades only three species were thought to be naturally susceptible to CWD, yet a decade ago CWD was detected in free-ranging moose in North America (Baeten et al., 2007). Recent outbreaks in Norway have shown that free-ranging reindeer should also be included in the list of susceptible species (Benestad et al., 2016). This range expansion in susceptible host species points out that multiple cervid species may be susceptible, which adds to the difficulty of disease containment. To date, the CWD is detected in various states in North America, Canada, South Korea and Norway. Retrospective studies deduced that the origin of the outbreak in South Korea was likely to be subclinical infected animals imported from Canada (Sohn et al., 2002). The origin of the CWD outbreak in Norway has not yet been identified (Benestad et al., 2016). Now more than ever, it is prudent to limit the occurrence and distribution of CWD before the disease becomes too widespread and uncontrollable. Therefore, the aim of this thesis is to assess useful containment strategies of chronic wasting disease, by providing an overview of successes and failures in implemented control strategies.
Materials and methods
Literature search method
In this thesis, a dual methodology was applied to search for relevant peer-reviewed literature. First, a search strategy was established and imported in search engine Scopus. Several limitations were placed on the search strategy to retrieve the most relevant articles; articles needed to be written in English, published between the years 2000-2017, conducted in North America. The following search strategy was imported in Scopus and resulted in 64 hits: (“chronic wasting disease” OR TSE OR prion* OR cwd AND cervid* OR deer OR moose OR elk OR reindeer AND control OR preven* OR containment PRE/2 strateg* OR management) AND (“chronic wasting disease”) AND (LIMIT-TO(LANGUAGE,”English “) )
The titles and abstracts of the articles were screened and further selection was made, when a title or abstract did not contain sufficient information the introduction and discussion were screened as well. In addition, articles were excluded when only the abstract was available. This resulted in seven relevant articles based on the abovementioned inclusion criteria.
Second, snowballing was applied to identify articles that were not retrieved based on the search strategy that was imported in Scopus. Snowballing comprises of the use of an article’s reference list or citations to the article to retrieve additional articles (Wohlin, 2014). The seven articles selected from Scopus were used as the start set for snowballing. In total 44 articles were retained
An additional search for information was carried out in grey literature. National and state government documents, such as the EFSA report, were viewed for the latest information regarding CWD containment strategies that was not included in peer-reviewed literature. In total three reports were retained.
Literature analysis method
Analysis of prevention and control strategies was performed using the epidemiological triangle as point of departure. The epidemiological triangle is a model developed for studying infectious diseases. The model helps to understand the role of agent, host and environment in disease transmission and control (Figure 1) (Scholthof, 2007). An intervention can break one of the ‘legs’ of the triangle to control or stop an epidemic. An epidemic may even be completely stopped, when one of the factors or element of the triangle is interfered or removed, resulting to stop continuation of the disease along its mode of transmission (Timmreck, 2002).
Results
The results are divided into two sections. The first section describes the epidemiological triangle of chronic wasting disease. The second section summarizes control strategies in relation to the epidemiological triangle.
The Epidemiological triangle
Agent
The agent in the epidemiological triangle (figure 1) is the cause of the disease (Timmreck, 2002). The agent in this triangle is the prion disease: chronic wasting disease. The origin of CWD has never been determined, as the ‘primary case’ was not identified. There are three theories currently circulating regarding the origin of the disease. Firstly, CWD could be a derivative from scrapie, considering CWD has corresponding features with scrapie (Williams, 2005). Secondly, CWD possibly emerged de novo in mule deer, as a spontaneous mutation of the prion protein and metamorphosed into a transmissible disease within the Cervidae family (Williams et al., 2002). Thirdly, Williams (2005) suggests an acquired infection for a yet unknown source. The first theory is favoured above the second and third theory in literature.
Pathogenesis
Strains
At present, two strains of chronic wasting disease have been described (Benestad et al., 2016). These two strains were classified on the location of neuropathology and incubation time, hence short-incubation time CWD1 strain and long-incubation time CWD2 strain were identified (Angers et al., 2010). The two strains also differ in distribution of prion proteins, CWD1 strain has symmetrical bilateral lesions, whereas CWD2 strain displayed an asymmetrical distribution of prion proteins, which is also a feature in certain scrapie strains (Angers et al., 2010). Apart from the incubation time and distribution of the prion protein in the central nervous system, the two strains are indistinguishable without the use of bioassays (Benestad et al., 2016).
Host
The host in the epidemiological triangle (figure 2) is an organism that holds the disease (Timmreck, 2002). In this thesis, the host is one of the five species of the Cervidae family mentioned in the introduction.
Host susceptibility
MM 132 LM132 uitleggen! Genotype verschillen
infected elk with the genotype MM132 were correctly diagnosed almost twice as often as elk with the ML132 genotype. Prior research indicates MM132 elk exhibit shorter incubation times for CWD than those with ML132 (Hamir et al., 2006), which likely results in faster accu- mulation of PrPCWD in the rectal mucosa and a greater likelihood of accurate prion diagnosis using rectal biopsies (Monello et al., 2013)
no immunity in none of the susceptible species
Species barrier
Various studies have researched the possibility of cross-species transmission of CWD, which generated conflicting results (Kong et al., 2005; Barria et al., 2011; Wilson et al., 2012; Davenport et al., 2015). These studies all have in common that they did not investigate what promoted or impaired the conversion of the prion protein (PrP), only whether CWD was transferred from donor to recipient. Kurt et al. (2015) researched the species barriers, especially, which part of the tertiary structure of the PrP is responsible for the impairment of effective prion conversion. This study suggests that a human-specific amino acid sequence within the β2-α2 loop of the cellular PrP prevents conversion of PrP into misfolded and aggregated PrP. The human β2-α2 loop might therefore be one of the mechanisms in the CWD species barrier (Kurt et al., 2015).
Clinical signs and diagnosis
The clinical signs of CWD are not very distinct, noteworthy aspects of the disease in cervids are emaciation by means of reduced feed uptake, and changes in behaviour (Williams and Miller, 2002). The changes in behaviour might be separation from the herd, decreased or increased interaction with caretakers in the case of captive infected cervids and stereotypic behaviour such as walking in patterns. During the course of the disease many animals develop additional health problems, being; polyuria and polydipsia, wide-based stance, incoordination, ataxia, tremors, increased saliva production resulting in drooling and oesophageal dilatation (Williams et al., 2002). Due to their deteriorating health, animals regularly acquire aspiration pneumonia (Bourne, 2015).
The additional health problems are often reason for misdiagnosis, as presently the leading diagnosis of CWD is through examination of the brain for spongiform lesions or immunohistochemistry (IHC) on a sample of the brain (Williams and Miller, 2002). The most important section for sampling is the parasympathetic vagal nucleus in the dorsal side of the medulla oblongata at the obex (figure 2). Besides these methods of diagnosis, antemortem testing for CWD is becoming increasingly more reliable. In mule deer, palatine tonsils and rectal mucosa were biopsied in live animals for PrPCWD testing with IHC assays (Wild et al., 2002; Keane et al., 2009). Similar antemortem tests were performed on elk, in both animal species the rectal mucosal biopsy was found to be less reliable than tonsillar biopsy (Monello et al., 2013). Misdiagnosis occurs primarily in the early stages of the disease, when prions have not yet accumulated in rectal mucosal region (Keane et al., 2009; Monello et al., 2013).
Environment
CWD can be dispersed through direct contact or via contaminated environments. Environmental contamination occurs in live animals via shedding of prions in urine, feces, saliva and velvet (Uehlinger et al., 2016). Prions are also an environmental contaminant in deceased animals, as prions are released during decomposition and saturate the soil (Uehlinger et al., 2016). Various papers state that CWD is foremost spread via lateral transmission within species, but also between the species (Miller et al., 2000; Gross and Miller, 2001; Bourne, 2015).
Up to now, models have predicted that CWD is a self-sustaining disease in captive and free-ranging cervids (Williams and Miller, 2002; Miller, Hobbs and Tavener, 2006). It is proposed that horizontal transmission of the disease and contaminated environments are the cause of the self-sustainability.
Geographic distribution
– Spatial dispersion
– geographic extent of endemic CWD in free-ranging wildlife is relatively limited and natural rate of expansion has been slow, more widely through market-driven movements of infected farmed elk.
Time
Time entails the incubation period and duration of the disease, specifically the life expectancy of host and agent (Timmreck, 2002). The average incubation period of CWD is estimated at 2 to 4 years, because it concerns free-ranging animals it is difficult to establish an exact timeline of the incubation period (Williams, 2005). Most animals die within 4 months, once the disease has manifested itself (Williams and Miller, 2002). When carcasses of infected cervids decompose, they form a reservoir for prion infectivity, which remains in the soil long after the carcass is decomposed. According to Uehlinger et al. (2016) the prions remain infectious for at least 2.5 years.
Control strategies
At present, management of CWD is foremost aimed at controlling the disease in free-ranging populations and preventing further spread to unaffected populations or across state lines (Conner et al., 2008). Table 1 provides an overview successes and failures of implemented control strategies. The nine studies are classified into two study designs to evaluate control strategies: predictive modelling and observational study. Eight of the studies were conducted in North America and one study in Canada. No host species selection was made in the search strategy, but all nine studies have deer as its target population.
Interventions
The evaluated studies can be categorized in three groups: non-selective culling, selective culling of infected animals and spatial dispersion. Non-selective culling was described with the use of predictive modelling as well as observational studies. The results ranged from no effect on CWD prevalence to decreased CWD prevalence and possible eradication. Selective culling was only outlined in predictive modelling, in both studies there was a decrease in CWD prevalence. One observational study researched spatial dispersion after group removal, which generated favourable to mixed results.
Non-selective culling
Here, non-selective culling (e.g. population reduction, recreational killing, non-selective animal culling at specific locations and hunter harvest) is described by two studies that used predictive modelling and three observational studies. Both predictive modelling studies deemed intensive non-selective culling as a valid strategy for disease management (Wasserberg et al., 2009; Potapov et al., 2016). Two out of three observational studies suggested non-selective culling as an effective way of disease control (Mateus-Pinilla et al., 2013; Manjerovic et al., 2014). Conner et al. (2007) concluded that the control strategy had no effect on the CWD prevalence.
In predictive modelling, it is of importance to know whether a disease has a density-dependent (DD) transmission or a frequency-dependent (FD) transmission. In DD transmission, the infection rates are related to the population densities, when the population increases so do the contact rates with infectious animals (Uehlinger et al., 2016). In FD transmission, the infection rates are not dependent on population densities. The mode of transmission for CWD is still not fully understood. This makes it hard to predict the effectiveness of certain models, as the mode of transmission is critical in designing disease management strategies. Any type of culling or population reduction has more effect on a DD transmission mode than FD transmission.
Wisconsin data fitted both FD and DD models, thus Wasserberg et al. (2009) developed both models in relation to the subsequent effect on disease management. Important is that CWD outbreaks need to be contained otherwise prevalence levels of 30% to 60% could be reached. When CWD has a DD mode of transmission, CWD could be eliminated from a population and still have a viable population left (Wasserberg et al., 2009). An initial increase in prevalence should be considered when intensive culling of infected animals is implemented, but after some years the success of the control strategy will be visible. When the transmission is FD, the results of the study showed that eradication of the host population is necessary to eliminate CWD from the population. With FD transmission, the prevalence never decreases when population reduction strategies are employed (Wasserberg et al., 2009)Table 1 – overview of CWD interventions
Author and publication date
Study location
Study design
Intervention
Mode of impact in epidemiological triangle
Outcome
Gross & Miller, 2001
Colorado
Predictive modelling
Selective culling
Host – Environment
Decrease in CWD prevalence; success
Wasserberg, Osnas, Rolley, & Samuel, 2009
Wisconsin
Deterministic matrix model,
Predictive modelling
Recreational killing and population reduction
Host – Environment
Slight decrease in CWD prevalence;
Success
Wild, Hobbs, Graham, & Miller, 2011
Colorado
Predictive modelling
Selective and non-selective predation
Host – Environment
Decrease in CWD prevalence;
Success
O’Hara Ruiz, Kelly, Brown, Novakofski, & Mateus-Pinilla, 2013
Illinois and Wisconsin,
2003-2009
Predictive modelling
Illinois: sharpshooting
Wisconsin: eradication and management zones
Host – Environment
Reallocation of clusters with CWD in and near Wisconsin;
Success in Illinois, failure in Wisconsin
Potapov, Merrill, Pybus, & Lewis, 2016
Alberta, hunter harvest 2006-2012
Predictive modelling
Non-selective culling
Host – Environment
Decrease in CWD prevalence, possible eradiation;
Success
Conner, Miller, Ebinger, & Burnham, 2007
Colorado,
1996-2005
Observational study, before-after-control-impact (BACI)
Non-selective animal culling at specific locations
Host – Environment
No effect on CWD prevalence; Failure
Mateus-Pinilla, Weng, Ruiz, Shelton, & Novakofski, 2013
Illinois,
2003-2008
Observational study
Sharpshooting: permits for deer population control permits and nuisance deer removal
Host – Environment
Decrease in CWD prevalence; Success
Manjerovic, Green, Mateus-Pinilla, & Novakofski, 2014
Illinois and Wisconsin,
2003-2012
Observational study
Illinois: government culling program and hunter harvest
Wisconsin: foremost hunter harvest
Host – Environment
Maintaining low CWD prevalence in Illinois; Success
Increase CWD prevalence in Wisconsin after reduction culling program; Failure
Tosa, Schauber, & Nielsen, 2016
Illinois, 2011-2014
Observational study, before-after-control impact (BACI)
Experimental group removal
Host – Environment
Adult female: limiting spread of disease due to shorter contacts
Juveniles: results inconclusive
Success or failure inconclusive
The predictive model developed by Potapov et al. (2016) uses only FD mode of transmission, but assumes that recruitment and survival are density-dependent. Potapov et al. (2016) advocate a control strategy focused on targeting male deer, as they have the highest CWD prevalence. In all simulations, the male: female ratio decreases due to removal of male deer. In addition, there is an increase in juvenile: female ratio. This is related to disease dilution and the higher survival rate of juveniles at low population densities. There is relatively more feed for the juvenile in low population densities compared to high population densities. Only focusing on the animals with the highest CWD prevalence is not enough, to fully eradicate CWD in a population almost 80% of the males need to be removed annually (Potapov et al., 2016).
The observational study by Conner et al. (2007) used before-after-control-impact estimates to evaluate localized animal culling at management evaluation sites in Colorado. No effect was measured between the treated sites and the control sites. The study opted various reasons why no effect was detected between the sites. First, seven of 16 sites did not contain any CWD positive culling, thus for these sites no effect could be related to localized culling. Second, female deer were left out of the model, solely male deer older than 1.3 years were included. Third, a flawed study design could be the reason for no effect, especially the chosen management evaluation sites pose as a possible improvement of the study with a potential different outcome. Another reason might be that the treatment period was to short the see any changes, as Wasserberg et al. (2009) explained that it takes some years before the prevalence of a chronic disease declines.
The two observational studies that noted intensive culling as a valid strategy to control CWD were both performed in Illinois. Mateus-Pinilla et al. (2013) evaluated the non-selective culling (i.e. governmental culling and hunter harvest) based on three parameters: frequency (duration), intensity (average number of deer per year through governmental culling) and effort (total number of deer cleared during control strategy period). The parameter effort had the highest reduction in CWD prevalence with the removal of 13-50 deer in the five-year period of the intervention. Intensity had a stronger association with a decreased CWD prevalence in fawns and yearlings than in adults. The parameter frequency had a negative association with CWD prevalence change in yearlings and fawn, when repeated yearly with the intervention period.
Manjerovic et al. (2014) compared the disease management strategies of the adjoining states Illinois and Wisconsin. The researchers concluded that intensive culling by the government, described as sharpshooting, is responsible for maintaining the low CWD prevalence in Illinois. Further below, the comparison of disease management between Illinois and Wisconsin is thoroughly described.
Selective culling
Here, selective culling is described by two predictive modelling studies and both models predicted a decrease in CWD prevalence. Gross and Miller (2001) simulated various scenarios, in which the affected populations were managed via selective culling. They suggest that only with low CWD prevalence and intensive culling, a population could return CWD-free. One condition of these scenarios is that more than 10-20% of the infected animals is culled, otherwise CWD will not be eliminated from the population. In addition, when the prevalence is higher than 5% the chance of eliminating CWD from a population reduces significantly. Furthermore, the social structure and spatial dispersion of a deer population could in a long-term persistence of CWD.
The long-term persistence of CWD in the population was also forecasted by Wild et al. (2011), though in their models it was under non-selective culling conditions. Hunting would decrease the prevalence, but not eliminate CWD. To eliminate CWD in a closed system, they propose the use of predators as a means of selective culling. The assumption is made that due to the deteriorating health of the infected cervids, it becomes easier for predators such as wolves to catch the affected animals. This allows for halving the CWD prevalence within a decade (Wild et al., 2011).
Both studies performed their predictive models in optimal conditions, which did not include the dispersion of infected animals to neighbouring groups. It is known that cervids have a fidelity to a certain area. Cervids do mingle with neighbouring groups at feeding and drinking areas at which they could transfer the disease to other groups. Furthermore, both studies did not account for carcasses infecting the soil, although Wild et al. (2011) did mentioned that carnivores can reduces the environmental contamination of carcasses through scavenging and selective predation of dying animals.
Spatial dispersion after group removal
Little research has been performed regarding spatial dispersion after dismantling the stable social structure that female and juvenile cervids live in. In Illinois, Tosa, Schauber and Nielsen (2016) investigated the spatial dispersion of adult female and juvenile deer after group removal in relation to disease management. The study revealed that group removal has different responses on remnant adult females and remnant juveniles. Adult females had shorter contacts with neighbour groups and almost no change in spatial dispersion and contact rates. However, remnant juveniles increased their spatial overlap with neighbouring groups (Tosa, Schauber and Nielsen, 2016). The difference in response was ascribed to the difference in age. Where adult females have a social status, the juveniles probably need to establish their status, which they establish through contact with neighbouring groups. Though the results regarding the remnant juveniles was inconclusive, the article suggests to remove entire social groups to decrease the spread of CWD (Tosa, Schauber and Nielsen, 2016). This is consistent with previous studies with respect to disease management. Oyer and Porter (2004) proposed that stable social deer groups have fidelity to a particular area, which means that one entire social group is culled, the other groups in the area will not change position.
Comparison disease management Illinois and Wisconsin
Two studies, one based on predictive modelling and the other observational, compared the CWD prevalence of the adjoining states Illinois and Wisconsin. Both studies concluded that the CWD prevalence in Illinois is low. This is supported by facts and figures from Illinois Department of Natural Resources (IDNR, 2016). By contrast, the CWD prevalence in Wisconsin has risen since the detection in 2002. No facts or figures regarding CWD prevalence were posted in the response plan developed by Wisconsin Department of Natural Resources (WDNR, 2017), but Manjerovic et al. (2014) estimated the CWD prevalence in Wisconsin at 5%. In literature, the disease management approach implemented by Illinois has been referred to as successful, whereas Wisconsin has been exemplified as a failure after two failed management approaches.
Since the first case of CWD in Illinois, the aim has been to keep the CWD prevalence low and to prevent the disease from spreading within the state via population reduction in known CWD infected areas. This was achieved through increased hunter harvest activities and government culling in relative small areas of 64 km2 (Manjerovic et al., 2014). Government culling consists of sharpshooting, which is performed by trained personnel in areas that are poorly accessible or where hunters are not permitted to hunt. Illinois has kept the same management approach during the last 15 years, which resulted in a low CWD prevalence. However, this approach has also shown that CWD will not be eliminated via this strategy.
The state Wisconsin implemented a completely different approach compared to Illinois after the first detection of CWD in 2002. In that year, Wisconsin established an eradication zone around all CWD positive harvested animals with the goal of killing all deer within this zone. The eradication zone was buffered by a herd management zone with a radius of 64 km2 from the centre of the eradication zone, in which the goal was to reduce the number of deer to 26 deer/km2 (Vercauteren and Hygnstrom, 2011). In 2003, a CWD positive deer was detected outside the eradication zone, resulting in a second eradication zone. At that time, Wisconsin discontinued the eradication goal and adopted a similar strategy in disease management as Illinois. The goal was no longer deer eradication, but foremost herd reduction and controlling the spread. Until 2007, when public resistance and reduced legislative support diminished government culling as a disease management approach (Manjerovic et al., 2014). Currently, the management strategy is herd control via public hunting (WDNR, 2017).
In the period 2003-2007, Wisconsin and Illinois had similar disease management strategies and had similar CWD prevalence. Figure 3 shows the course of CWD prevalence of both states during the different management approaches, illustrated with the dashed line. The increase in CWD prevalence in the last ten years in Wisconsin suggests that the current management strategy, non-selective hunting based on hunter harvest alone, is not working. Hunters in Wisconsin do not seem to be aware of the necessity of reducing or eliminating CWD (Vercauteren and Hygnstrom, 2011). In addition, hunters do not necessarily hunt in areas with high CWD risk or high CWD prevalence (Manjerovic et al., 2014).
Despite the fact that Illinois and Wisconsin have different disease management strategies, both states have difficulty with managing the spatial dispersion of the disease. O’Hara Ruiz et al. (2013) investigated the influence of management and landscape factors on the spatial patterns in Illinois and Wisconsin. This study noted that CWD risk areas are changing over time, in particular the risks areas are shifting towards Wisconsin (O’Hara Ruiz et al., 2013). Furthermore, the article stated that CWD risk is higher in Wisconsin than in Illinois. This is not entirely based on the management strategies, as landscape conditions, such as soil type and forest coverage, also pose a threat to disease management. Nevertheless, O’Hara Ruiz et al. (2013) and Manjerovic et al. (2014) both conclude that intensive management in high risk CWD areas, as implemented in Illinois, is still a valid control strategy.
Discussion
Key findings
– none of the interventions can be single out as the most effective control strategy.
– Tailor-made strategies needed for every state based on geography, population density,
Interventions in relation to the epidemiological triangle
As can be seen in table 1, all interventions are based on the leg ‘host – environment’ of the epidemiological triangle. To this day, this is the only feasible manner to disrupt the mode of transmission of this disease. Whereby, non-selective or selective culling by the government was preferred above harvest hunting. Harvest hunting by recreational hunters alone does not seem to be enough to keep CWD prevalence in check. The problem is that recreational hunters do not necessarily kill cervids that are infected. To account for this, Dugal et al. (2013) proposes target hunting distribution based on kill sites and host selection. This manner of selective hunting by recreational hunters could more effective in tackling the CWD prevalence.
One predictive modelling study included the potential use of a vaccine within a management strategy, to disrupt the leg ‘agent – host’. Although, a vaccine is not available, the study anticipates the development of a vaccine in the future. Potapov, Merrill and Lewis (2012) developed a predictive model, in which they assumed that CWD is a frequency-dependent transmission disease and they adopted transmission coefficients from Wasserberg et al. (2009). With these assumption, the introduction of vaccination in a harvest management strategy seems necessary for the elimination of CWD in a population. Without vaccination, CWD would maintain in a population similar to the current situation in respect to CWD management.
Interventions disrupting the leg ‘agent – environment’ have yet to be conceived for this situation. There is still not much known about the infectivity of prion in soil. It has been research that certain types of soil result in higher infectivity and better preservation of prions (O’Hara Ruiz et al., 2013), but no intervention has been proposed to reduce the infectivity or even better degeneration of prions in the environment. The difficult part is to affect the misfolded prions, but leave alone the healthy prion proteins.
Public awareness
Disease management strategies as described in the results section involves more than just culling animals. An important part of disease management is public awareness. Illinois and Wisconsin are two example of states that have different disease management, but both include educating the public and especially hunters. Hunters are educated in recognizing affected cervids and taking the appropriate precautions. Currently, Wisconsin is dependent on the motivation of hunters to kill the infected animals, but according to Vercauteren and Hygnstrom (2011) the hunters are not convinced of the risks related to CWD. Hence, hunters in Wisconsin kill less deer than they are allowed to shoot, because they do not recognize the high priority of disease management.
Raising public awareness became even more difficult after the United States Department of Agriculture (USDA) cut the primary source of funding for CWD management. Since 2012, states must come up with their own funding for CWD monitoring and surveillance (Evans, Schuler and Walter, 2014). Some states started to ask a fee for sampling harvested deer. This resulted in a 90% drop of submitted samples in Colorado (Evans, Schuler and Walter, 2014). Other states adapted their strategy and implemented weighted surveillance. Weighted surveillance means using demographic surveillance streams, e.g. sample sources such as road-kill, hunter harvest and (non)-selective culling, to observe differences in CWD prevalence (Walsh and Miller, 2010). The combining of sample sources makes weighted surveillance cost-effective. In addition, fewer animals need to be culled for weighted surveillance compared to traditional surveillance, so public resistance towards culling animals for surveillance might also decrease.
Model parameter estimates
predictive models lots of uncertainties (Uehlinger et al., 2016) because the magnitude and severity of this epidemic were recognized only recently, long-term trends in CWD dynamics have not been observed. Our model projections are generally consistent with the few estimated parameters from field data that are available for comparison. Modeled stage and age-specific prevalence, as well as rates of change in prevalence over time, are all consistent with estimates derived from studies of affected deer populations. To the extent that modelled mechanisms of CWD transmission appear to offer at least a reasonable approximation of disease process occurring in nature, it follows that this model provides plausible forcasts of future epidemic trends (gross and miller, 2001)
•
• mode of transmission still not clear maar erg belangrijk
o “in all models, the assumed mode of CWD transmission had a strong impact on the outcome” Uehlinger et al 2016
Choice of research area
• Focus America – uitbraak in europa nog te kort geleden om interventie te evalueren. Lessons can be learned from the aanpak uit de amerikaanse staten.
• South Korea buiten beschouwing gelaten in resultaten omdat het daar ging om captive animals
• NY: niet te achterhalen waar de infected cervids vandaan kwamen.. free-ranging of ontsnapt? The CWD positive deer seemed isolating incidents, instead of a start of an new area with CWD outbreaks. Therefore, not included in this thesis.
Implication for Europe?
• Buffer zones?
Conclusion
– Research in other areas of management should be encouraged. Targeting hunter and selective culling via predation are a possibility, though predators should not be introduced into the wild without careful consideration.