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Estimation of the relative risk of developing clinical scrapie: the role of prion protein (PrP) genotype and selection bias
  1. S. C. Tongue, BVSc, MSc, MRCVS1,
  2. D. U. Pfeiffer, DrMedVet, PhD, MACVSc, DiplECVPH3,
  3. R. Warner, MA, DPhil4,
  4. H. Elliott, BVSc, BVBiol, PhD, MACVSc, MRCVS2 and
  5. V. Del Rio Vilas, BVSc, MBA, MSc, MRCVS1
  1. 1Scrapie Epidemiology Group, Centre for Epidemiology and Risk Analysis
  2. 2Neuropathology Unit, Department of Pathology, Veterinary Laboratories Agency Weybridge, Woodham Lane, New Haw, Addlestone, Surrey KT15 3NB
  3. 3Epidemiology Division, Department of Veterinary Clinical Sciences, Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA
  4. 4National Scrapie Plan Administration Centre, DEFRA, Whittington Road, Worcester WR5 2SU Dr Elliott’s present address is Institute for Animal Health, Pirbright Laboratory, Woking, Surrey GU24 0NF


Prion protein (PrP) genotype data from statutory confirmed cases and from three non-case datasets have been used to calculate the odds ratio (OR) for the development of clinical scrapie for an individual sheep of a given PrP genotype, compared with one possessing the ‘wild-type’ ARQ/ARQ genotype. Logistic regression has been used to estimate the ORs, and a multiple-test procedure has been used to evaluate the statistical significance of each comparison. The results are similar to those observed in other studies: the VRQ/VRQ genotype has OR point estimates greater than 20; the ARQ/VRQ and ARH/VRQ genotypes have OR point estimates between 5 and 20; AHQ/VRQ between 0·03 and 0·1; ARR/VRQ 0·4 and 0·5; all the other PrP genotypes, excluding ARR/ARR, ARR/ARH and AHQ/ARH for which no clinical cases have been recorded have OR point estimates of less than 0·3. The estimates derived from each dataset are comparable, but not identical. This can be explained by plausible biases inherent in the sampling of the non-case populations

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