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%

Back calculation approach

Probability of detection dependent on stage of incubation and PrP genotype

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

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

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

Estimate prevalence of scrapie infection in GB based on abattoir survey data

Assess design of an abattoir survey

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