Scrapie epidemic in a fully PrP-genotyped sheep flock.
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 (> or =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.
Baylis
M
M
Institute for Animal Health, Compton, Newbury, Berkshire RG20 7NN, UK1.
Goldmann
W
W
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
Houston
F
F
Institute for Animal Health, Compton, Newbury, Berkshire RG20 7NN, UK1.
Cairns
D
D
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
Chong
A
A
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
Ross
A
A
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
Smith
A
A
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
Hunter
N
N
Neuropathogenesis Unit, Institute for Animal Health, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK2.
McLean
A R
AR
Department of Zoology, South Parks Road, Oxford OX1 3PS, UK3.
D000367:Age Factors; D000818:Animals; D004196:Disease Outbreaks; D005260:Female; D005838:Genotype; D008297:Male; D011328:Prions / Q000235:genetics*; D012608:Scrapie / Q000453:epidemiology* / Q000209:etiology / Q000401:mortality; D012756:Sheep; D015996:Survival Rate
D011328:Prions