Open Access
Review

Table II.

Summary of models used to describe the transmission of scrapie between sheep flocks.

Author(s) Aims Modelling approach Conclusions
Durand et al. [15] Develop a regional model for spread between flocks
  • One-dimensional arrangement of flocks

  • Winter transmission only between neighbouring flocks

  • Summer transmission also between flocks which share grazing

  • Gene-flow between flocks

  • Selective breeding programmes

Model developed, which can in future be used to assess control strategies
Gravenor et al. [27] Estimate the flock-to-flock force of infection for scrapie in Great Britain Simple SI model for flocks
  • Force of infection: 0.0045 per farm per year

  • Mean outbreak duration: 5 years

  • No evidence for an increase in the force of infection before, during or after the BSE epidemic in British cattle

Gravenor et al. [28]
  • Estimate transmission parameters for scrapie in Cyprus

  • Investigate the impact of control measures

Simple SEI model for flocks, including effect of culling and quarantine
  • R 0 = 1.4–1.8

  • Early identification and quarantine of affected flocks most effective for control of disease

Gubbins [31] Develop modelling approach to describe the spread of scrapie between sheep flocks in Great Britain
  • Stochastic, spatial flock-level model

  • Acquisition of infection depends on trade

  • Probability and duration of a within-flock outbreak depends on flocks size and PrP genotype profile

  • Model is able to capture the spatial dynamics of scrapie

  • There is considerable uncertainty when predicting long-term trends for disease

Gubbins and Webb [32] Assess the efficacy of control strategies to eradicate scrapie from Great Britain
  • Feasible to eradicate scrapie, but it will take decades to do so

  • The most-effective strategy is whole-flock culling, though whole-flock genotyping and selective culling is also effective

Gubbins and Roden [33] Assess the impact of selective breeding programs on prevalence and incidence of scrapie
  • Simple age- and genotype-structured SI model

  • Flock structure ignored (same force of infection for all sheep)

  • Selective breeding strategies will reduce the prevalence and incidence

  • Targeting only the VRQ allele is sufficient to have a large impact on disease occurrence

Gubbins et al. [34] Estimate basic reproduction number (R 0) and mean outbreak duration (D) for spread between flocks in the Shetland Isles Simple SIR model for flocks
  • R 0 = 1.47 (95% CI: 1.45–1.50)

  • D = 2.11 years (95% CI: 2.01–2.23)

Hagenaars et al. [40] Use surveillance data to estimate key epidemiological parameters Simple SI model for flocks
  • Large proportion of cases (80%) go undetected

  • Occurrence of scrapie may provoke changes in flock management which reduces outbreak duration

  • Within-flock R 0 = 1.5–6.0

Kao et al. [50]
  • Formulate a flock-to-flock model of scrapie spread

  • Assess potential control programmes

  • SEI epidemic model with affected flocks of “low” and “high” risk (depending on genetic structure) which determines whether they experience outbreaks following exposure

  • Acquisition of infection depends on trade

  • R 0 = 1.1–1.2

  • High risk flocks predicted to comprise 3–20% of the national population

  • Targeted programmes predicted to eradicate scrapie more quickly than those aimed at the general population

Truscott and Ferguson [75]
  • Develop a model for spread of scrapie in UK sheep population

  • Use the model to estimate infection prevalence (overall and by breed), and to evaluate possible long-term persistence of scrapie

  • Metapopulation model based on the coupling of fairly detailed within-flock SI epidemic models (genotype, age, and infection stage structure)

  • Flock-level acquisition of infection occurs by breeding, trading, or through homogeneous low-level contamination generated by all flocks

  • Flock-differ in breed, size, and PrP allelic composition

  • Detection/reporting probability of 16% (12–17)

  • Prevalence of infected animals in the population estimated to be 0.15%

  • 9% of flocks estimated to be infected overall, rising to 60% in Shetland and 75% in Swaledale flocks

Truscott and Ferguson [76]
  • Assess impact of different strategies for control of scrapie in UK sheep

  • UK National Scrapie Plan (NSP) is the most effective scheme

  • NSP and UK Compulsory Scrapie Flock Scheme (CSFS) both reduce the case incidence, but CSFS is less effective in decreasing the susceptible allele frequency

  • Trading restrictions have a limited impact compared to selective breeding and culling