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Prevalence of scrapie in sheep in Great Britain estimated from abattoir surveys during 2002 and 2003

SCRAPIE is a transmissible spongiform encephalopathy of sheep and goats, in which abnormal prion protein (PrPSc) accumulates in the brain. An affected animal succumbs to progressive neurological degeneration and death. While no evidence of the zoonotic potential of scrapie has been reported, the possible link between bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease in human beings (Bruce and others 1997, Hill and others 1997, Weissmann and Aguzzi 1997) has raised concerns about the safety of small ruminant products for human consumption. In order to evaluate any potential risk to public health, a robust estimate of the national prevalence of scrapie infection is required. In Great Britain, two abattoir surveys were conducted as part of a programme of statutory European Union (EU) surveillance for sheep scrapie, for which the sample size for each member state was defined (Anon 2001). These surveys were completed in 2002 and 2003. The aim of each survey was to collect and test samples from 60,000 sheep over 18 months of age per 12-month period. Sheep were selected from the 42 abattoirs with a throughput of more than 10,000 adult sheep, which slaughtered 93 per cent of adult sheep in Great Britain. A systematic sampling scheme was used and the number of samples required from each of the abattoirs was proportional to its throughput; there were three or four sampling sessions per week. From each sheep, a brainstem sample, which comprised the medulla oblongata from the proximal spinal cord to the cerebellar peduncles, was removed through the foramen magnum using a specially designed ‘spoon’. Only samples with intact diagnostic sites in the obex were tested. Between January 2002 and March 2003 (survey 1), the EU-approved biochemical tests of either the BioRad Platelia (ELISA) (Grassi and others 2001), or the Prionics Western blot (Schaller and others 1999) were used. The use of two different tests and hence a difference in testing protocols arose as a result of logistic constraints imposed on the survey after its commencement, to accommodate the increase in sample size requested by the EU. Samples that were positive by ELISA and all those tested with the Western blot were subjected to immunohistochemistry (IHC) at the diagnostic sites in the obex. An anti-PrP monoclonal antibody R145 diluted 1:16,000 was used in an avidin-biotin peroxidase technique (Terry and other 2003). A positive animal was defined as one that exhibited diseasespecific immunostaining for prion protein (PrP) in the neurones and neuropil of specific neuroanatomical areas of the brainstem at the level of the obex as defined in Terry and others (2003). This technique is recognised by the World Organisation for Animal Health (OIE) as an official confirmatory test for scrapie in sheep (Anon 2004). A requirement of the statutory testing was to identify the presence of any abnormal forms of scrapie in PrPSc-positive animals through genotyping (Anon 2001). However, genotyping was extended to all animals tested in survey 1, in order to obtain a distribution of genotypes in the national flock. The PrP genotype was determined at codons 136, 154 and 171 of the PrP allele in DNA extracted from brain tissue at either the Veterinary Laboratories Agency (VLA) – Weybridge using the Qiagen Tissue Extraction Kit and an Applied Biosystems 377 automated sequencer, or at Cellmark Orchid. Results of the genotyping showed that all confirmed positive PrPSc samples were confined to genotypes of types 3, 4 or 5 of the National Scrapie Plan, as was expected. Between April 2003 and December 2003 (survey 2), all suitable samples were tested with the BioRad Platelia ELISA. Only those samples that were positive were subject to confirmatory testing with IHC and their genotyping of the PrP protein. Estimates of the prevalence of scrapie infection in the national flock were extrapolated from the survey results using an existing epidemiological model, which allowed for changes in prevalence with age, and the sensitivity and specificity of the diagnostic test (Gubbins and others 2003). Between January 2002 and March 2003 (survey 1) a total of 54,145 sheep were sampled, of which 50,630 were suitable for testing. Of these, 67 were classified as being positive by IHC (crude prevalence 0·13 per cent, 95 per cent confidence intervals (CI) 0·1 to 0·17 per cent). The extrapolated prevalence estimate was 0·33 per cent (95 per cent CI 0·24 to 0·44 per cent). Between April 2003 and December 2003 (survey 2), a total of 54,554 sheep were sampled, of which 50,735 were suitable for testing. Of these, 28 were classified as being positive by IHC (crude prevalence 0·06 per cent, 95 per cent CI 0·04 to 0·08 per cent). The extrapolated prevalence estimate was 0·27 per cent (95 per cent CI 0·18 to 0·38 per cent). The differences between the extrapolated prevalence estimates and the crude prevalence estimates arise for two reasons. First, the probability of detecting an infected animal is less than 100 per cent, therefore, not all infected animals will be detected. Secondly, the prevalence of infection is likely to be lower in older animals, such as those sampled in these surveys in which the majority of sheep were more than 36 months of age. Thus, there is an apparent increase in prevalence when extrapolating to the population as a whole. In several EU member states, the introduction of largescale active surveillance programmes for scrapie in small ruminants has led to estimates of prevalence greater than those that were previously available (Anon 2004). However, the prevalence estimates for Great Britain derived here do not differ significantly from those obtained from a previous smaller abattoir survey in 1997/98 (Simmons and others 2000, Gubbins and others 2003). When comparing estimates of prevalence both between years and between countries, it is important that differences in survey methodologies, the background populations and the structure of the sheep industry are considered.