title: The time-course of a scrapie outbreak.
abstract: BACKGROUND
  Because the incubation period of scrapie has a strong host genetic component and a dose-response relationship, it is possible that changes will occur during an outbreak, especially in the genotypes of cases, age-at-onset of disease and, perhaps, the clinical signs displayed. We investigated these factors for a large outbreak of natural scrapie, which yielded sufficient data to detect temporal trends.
  
  
  RESULTS
  Cases occurred mostly in two genotypes, VRQ/VRQ and VRQ/ARQ, with those early in the outbreak more likely to be of the VRQ/VRQ genotype. As the epidemic progressed, the age-at-onset of disease increased, which reflected changes in the genotypes of cases rather than changes in the age-at-onset within genotypes. Clinical signs of cases changed over the course of the outbreak. As the epidemic progressed VRQ/VRQ and VRQ/ARQ sheep were more likely to be reported with behavioural changes, while VRQ/VRQ sheep only were less likely to be reported with loss of condition.
  
  
  CONCLUSIONS
  This study of one of the largest scrapie outbreaks in the UK allowed investigation of the effect of PrP genotype on other epidemiological parameters. Our analysis indicated that, although age-at-onset and clinical signs changed over time, the observed changes were largely, but not exclusively, driven by the time course of the PrP genotypes of cases.
authors.affiliation: Institute for Animal Health, Pirbright Laboratory, Ash Road, Pirbright, Surrey, UK. marie.mcintyre@bbsrc.ac.uk
authors.forename: K Marie
authors.initials: KM
authors.lastname: McIntyre
authors.forename: Simon
authors.initials: S
authors.lastname: Gubbins
authors.forename: Wilfred
authors.initials: W
authors.lastname: Goldmann
authors.forename: Emily
authors.initials: E
authors.lastname: Stevenson
authors.forename: Matthew
authors.initials: M
authors.lastname: Baylis
mesh_terms: D000375:Aging; D000818:Animals; D004196:Disease Outbreaks / Q000662:veterinary*; D020022:Genetic Predisposition to Disease; D012608:Scrapie / Q000453:epidemiology*; D012756:Sheep / Q000235:genetics*; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology