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Institute for Animal Health, Compton, Newbury, Berkshire RG20 7NN, UK1
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2
Department of Zoology, South Parks Road, Oxford OX1 3PS, UK3
Author for correspondence: Matthew Baylis. Fax +44 1635 577237. e-mail matthew.baylis{at}bbsrc.ac.uk
In scrapie-affected sheep flocks, host PrP genotype plays a vital role in determining which sheep will succumb to scrapie and the incubation period. Consequently, within-flock scrapie dynamics is best understood within the context of the genotype profile of the flock. Here we describe a 17 month epidemic of scrapie in a commercially farmed flock of 230 genotyped Texel sheep. At the start of the study, 70% of the sheep were of three genotypes only: ARR/ARQ, ARH/ARQ and ARQ/ARQ. Only 15% of sheep encoded the disease-associated VRQ allele and only a single sheep (0·4%) was of the most susceptible VRQ/VRQ genotype. For susceptible genotypes there was a marked deficit (P<0·025) of older animals (
3 years), implying that some cases of scrapie had occurred previously. In the ensuing 17 months, 18 sheep of known genotype were confirmed positive for the disease: seven VRQ/ARQ, six VRQ/ARH, two VRQ/ARR, three ARQ/ARQ. Median ages at death were 2·7, 2·8, 4·2 and 3·8 years respectively. Mortality rates were 55, 86, 13 and 3% respectively. Survival analysis revealed a highly significant effect of genotype on survivorship, but no difference between VRQ/ARQ and VRQ/ARH, or between VRQ/ARR and ARQ/ARQ. There was no difference in the survivorship of middle- and older-age cohorts of susceptible sheep. Scrapie risk group (as defined by PrP genotype) was not associated with submission as a scrapie suspect but later found to be negative, or with dying of unknown causes on the farm.
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