A stochastic model to estimate the prevalence of scrapie in Great Britain using the results of an abattoir-based survey

In 1997/1998, an abattoir survey was conducted to determine the likely expo sure of the human population to transmissible spongiform encephalopathy (TS E) infection in sheep submitted for slaughter in Great Britain. The survey examined brain material from 2809 sheep processed through British abattoirs . Sampling was targeted by age: 45% of animals tested were greater than or equal to 15 months old. All samples of adequate quality (98%) were tested f or signs of scrapie infection using histopathology and scrapie-associated f ibril (SAF) detection and 500 were tested using immunohistochemistry (IHC). No conclusive positive animals were found using either histology or IHC. T en animals were positive by SAF. Standard statistical analyses suggest (wit h 95% confidence) that the prevalence of detectable (by histopathology) inf ection in the slaughter population was less than or equal to0.11%. However, the incubation period of scrapie is long (usually around 2-3 years) and no ne of the tests used in the survey is capable of detecting scrapie infectio n in the early stages of infection. We present an age-structured stochastic model incorporating parameters for the incubation period of scrapie, preva lence of infection by age and test sensitivity. Using the model, we demonst rate that the negative results, obtained for all samples using IHC and hist opathology are consistent with a true prevalence of infection in the slaugh ter population of up to 11%. This suggests that up to 300 of the animals te sted might have been infected but the infection was not sufficiently advanc ed in these animals to be delectable by IHC or histopathology. The survey w as designed to detect a prevalence of 1% with a precision of +/-0.5% and a confidence level of 95% in each age group assuming that diagnostic tests we re 100% specific and sensitive from a known stage in the incubation period. The results of the model demonstrate that to estimate a true prevalence of scrapie infection of 1% with an accuracy of +/-0.5% would have required a far larger sample size. An accurate estimate of the required sample size is complicated by uncertainty about test sensitivity and the underlying infec tion dynamics of scrapie. A pre-requisite for any future abattoir survey is validation of the diagnostic tests used in relation to both stage of incub ation and genotype. Sampling in the < 15-month age group was of no value in this survey because the diagnostic test, used were thought to be ineffecti ve in most of the animals in this age group. (C) 2001 Published by Elsevier Science B.V.