Simulation of the options for a national control programme to eradicate scrapie from Great Britain
Introduction
Following exposure to contaminated feed during the 1980s sheep in Great Britain (GB) might have been infected with bovine spongiform encephalopathy (BSE; Ferguson et al., 2002, Kao et al., 2002) and hence, could pose a risk to public health (Butler, 1998, Ferguson et al., 2002). However, scrapie and BSE are difficult to distinguish clinically (Foster et al., 1993, Foster et al., 2001) so it is only possible to target control measures at sheep transmissible spongiform encephalopathies (TSEs) in general. Historically, such control measures have involved culling affected animals or flocks to remove infection from the population (Sigurdarson, 1991, Detwiler, 1992, Hopp et al., 2001, Detwiler and Baylis, 2003). The risk of scrapie is associated with polymorphism in the prion protein (PrP) gene (see, for example, Hunter et al., 1996, Elsen et al., 1999; see Table 1). Consequently, a selective breeding programme to increase the frequency of the allele associated with lowest risk of scrapie (ARR) while reducing the level of the allele associated with the highest risk (VRQ) is another potential strategy to control disease (Dawson et al., 1998, Detwiler and Baylis, 2003).
In Great Britain, the National Scrapie Plan (NSP) aims to eradicate scrapie from the national flock (Defra, 2003). As part of the NSP a ram-genotyping scheme to increase the prevalence of TSE resistance in British sheep was launched in 2001 for registered pedigree flocks and was extended in 2002 to include non-registered purebred flocks. European Commission (EC) legislation requires European Union (EU) member states to take action in scrapie-affected flocks (EC, 2003). Consequently, there is an urgent need to investigate the potential impact of different control strategies for scrapie at a national level.
Earlier modelling studies have focused on identifying general methods that could be effective at controlling infection within an affected flock (for example, reducing horizontal transmission or identifying and culling infected animals prior to the onset of clinical signs; Woolhouse et al., 1998, Matthews et al., 1999). At a regional level, models for flock-to-flock transmission have been used to investigate general questions about scrapie control (for example, whether a breeding programme should target rams in all flocks or rams and ewes in ‘highly-affected’ flocks; Kao et al., 2001). None of these studies, however, considered specific strategies that could be implemented in the field. A further study investigated the impact of different breeding programmes on the PrP genotype profile of the GB sheep flock (Arnold et al., 2002), although the effect of the programmes on the prevalence of infection were not considered. Consequently, models have yet to be used to assess the impact of specific strategies for the control of scrapie on the prevalence of scrapie in Great Britain.
We used a model developed to describe the spread of scrapie between flocks (Gubbins, 2005) to investigate the impact of different control strategies. The efficacy of each strategy was assessed by estimating the probability of eradication and, where infection is eradicated, the time taken to do so. The primary focus of our analyses was to investigate the relative efficacy of the different strategies and to identify the key factors influencing the efficacy of a scheme. In particular, we consider the impact of scrapie reporting and, hence, the ability to identify affected flocks, on control efficacy. For each strategy we also estimated the number of flocks recruited, the number of animals culled and the number of animals genotyped.
Section snippets
Control of scrapie
Designing a control strategy for scrapie is complicated by several factors. First, the role of different transmission routes—especially of environmental contamination—remains unclear (Hoinville, 1996, Detwiler and Baylis, 2003) Second, animals can be infected for years prior to the development of clinical signs. Finally, there is currently no ante mortem diagnostic test available, which is suitable for large-scale screening. Consequently, control must be targeted at removing animals, which are
Efficacy of control
The model predictions indicate that it is possible to eradicate scrapie from the national flock, but whether a particular strategy does so depends critically on the underlying long-term dynamics of scrapie and the proportion of flocks that report scrapie (Fig. 2). Moreover, it is likely to be decades before the disease is eliminated (Fig. 2). Formal tests indicated that the eradication times shown in Fig. 2 differ significantly (P = 0.05) amongst the strategies.
For the two scenarios in which
Discussion
Recently it has been suggested that scrapie in GB is a single 200-year-long epidemic and, consequently, the infection should eventually disappear from the national flock even without any intervention (Bradley, 2001, Woolhouse et al., 2001). Although a recent modelling study has provided some support for this view, there were scenarios consistent with the available data in which scrapie remained endemic (Gubbins, 2005). Consequently, some form of control programme, in addition to the removal of
Conclusions
Our analyses suggest that it is feasible to eliminate scrapie from the British sheep flock, but that to do so would require an expensive, long-term control programme. Moreover, farmer co-operation is essential for the success of any scheme; steps must be taken to ensure notification of scrapie cases by farmers and their subsequent recruitment to any strategy implemented to control disease.
Acknowledgements
This work was funded by the Department for Environment, Food and Rural Affairs (Defra). We thank Mark Arnold (VLA) for discussions on modelling aspects of the work and Danny Matthews (VLA), Kumar Sivam (VLA) and John Wilesmith (Defra) for comments on earlier versions of the manuscript.
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