Table III.
Summary of the models used to inform scrapie surveillance.
| Author(s) |
Aims |
Modelling approach |
Conclusions |
| Gubbins et al. [30] |
Estimate prevalence of scrapie infection in GB based on an abattoir survey in 1997/1998 |
-
Simple age-structured prevalence model
-
Probability of detection dependent on stage of incubation
-
Diagnostic tests less than 100% specific
|
-
Prevalence of scrapie 0.22% (95% CI: 0.01–0.97%)
-
All tests used very specific (> 99%), with only one less than 100%
|
| Gubbins [35] |
Estimate prevalence of classical scrapie in GB by integrating data on reported cases and the results of abattoir and fallen stock surveys for 2002 |
|
-
Prevalence ranges from 0.33% to 2.06% depending on stage of incubation at which diagnostic test able to detect infected animals
-
Risk of infection much higher than the risk of clinical disease
-
Analysis of surveillance data needs to account for PrP genotype
|
| Gubbins and McIntyre [36] |
Estimate prevalence of classical scrapie in GB for 1993–2007 by integrating data on reported cases (1993–2007) and the results of abattoir and fallen stock surveys (2002–2007) |
-
Back calculation approach
-
Probability of detection dependent on stage of incubation and PrP genotype
-
Baseline risk of infection changes over time
-
Frequency of PrP genotypes in a birth cohort changes over time
-
Proportion of cases reported changes over time
|
-
Prevalence was approximately constant for 1993–2003 and was estimated to be 0.3% to 0.7% depending on stage of incubation at which diagnostic test able to detect infected animals
-
Prevalence declined by around 40% between 2003 and 2007
|
| Hopp et al. [44] |
Assess the efficacy of different strategies for identifying scrapie-affected flocks in Norway |
-
Stochastic simulation of strategy based on the probability of detecting an infected animal through each surveillance stream
-
Includes effect of PrP genotype on risk of scrapie, incubation period and probability of detection
|
-
Less than 9% of affected flocks are identified by either abattoir or fallen-stock surveillance
-
Samples sizes much lower for fallen stock than abattoir surveys
-
Abattoir surveillance most affected by an increase in test sensitivity
|
| Webb et al. [78] |
|
-
Simple age-structured prevalence model
-
Probability of detection dependent on stage of incubation
-
Stochastic simulation of survey
|
-
Survey results consistent with a prevalence in the slaughter population of up to 11%
-
Sample sizes need to be larger
-
Diagnostic tests need to be assessed in relation to genotype and stage of infection
|
