title: Factors influencing temporal variation of scrapie incidence within a closed Suffolk sheep flock.
abstract: Several studies have shown that transmission of natural scrapie can occur vertically and horizontally, and that variations in scrapie incidence between and within infected flocks are mostly due to differences in the proportion of sheep with susceptible and resistant PRNP genotypes. This report presents the results of a 12-year period of scrapie monitoring in a closed flock of Suffolk sheep, in which only animals of the ARQ/ARQ genotype developed disease. Among a total of 120 of these, scrapie attack rates varied between birth cohorts from 62.5 % (5/8) to 100 % (9/9), and the incidence of clinical disease among infected sheep from 88.9 % (8/9) to 100 % (in five birth cohorts). Susceptible sheep born to scrapie-infected ewes showed a slightly higher risk of becoming infected (97.2 %), produced earlier biopsy-positive results (mean 354 days) and developed disease at a younger age (median 736 days) than those born to non-infected dams (80.3 %, 451 and 782 days, respectively). Taken together, this was interpreted as evidence of maternal transmission. However, it was also observed that, for the birth cohorts with the highest incidence of scrapie (90-100 %), sheep born to infected and non-infected dams had a similar risk of developing scrapie (97.1 and 95.3 %, respectively). Compared with moderate-attack-rate cohorts (62.5-66.7 %), high-incidence cohorts had greater numbers of susceptible lambs born to infected ewes, suggesting that increased rates of horizontal transmission in these cohorts could have been due to high levels of environmental contamination caused by infected placentas.
authors.affiliation: Animal Health and Veterinary Laboratories Agency (AHVLA), Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: González
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Mark P
authors.initials: MP
authors.lastname: Dagleish
authors.affiliation: Animal Health and Veterinary Laboratories Agency (AHVLA), Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Stuart
authors.initials: S
authors.lastname: Martin
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Jeanie
authors.initials: J
authors.lastname: Finlayson
authors.affiliation: Animal Health and Veterinary Laboratories Agency (AHVLA), Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Sílvia
authors.initials: S
authors.lastname: Sisó
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Samantha L
authors.initials: SL
authors.lastname: Eaton
authors.affiliation: The Roslin Institute and R(D)SVS University of Edinburgh, Easter Bush, Midlothian EH25 9RG, UK.
authors.forename: Wilfred
authors.initials: W
authors.lastname: Goldmann
authors.affiliation: Animal Health and Veterinary Laboratories Agency (AHVLA), Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Janey
authors.initials: J
authors.lastname: Witz
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Scott
authors.initials: S
authors.lastname: Hamilton
authors.affiliation: The Roslin Institute and R(D)SVS University of Edinburgh, Easter Bush, Midlothian EH25 9RG, UK.
authors.forename: Paula
authors.initials: P
authors.lastname: Stewart
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Pang
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Philip
authors.initials: P
authors.lastname: Steele
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Hugh W
authors.initials: HW
authors.lastname: Reid
authors.affiliation: Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Francesca
authors.initials: F
authors.lastname: Chianini
authors.affiliation: Animal Health and Veterinary Laboratories Agency (AHVLA), Pentlands Science Park, Bush Loan, Midlothian EH26 0PZ, UK.
authors.forename: Martin
authors.initials: M
authors.lastname: Jeffrey
mesh_terms: D000818:Animals; D005260:Female; D020022:Genetic Predisposition to Disease; D015994:Incidence; D018445:Infectious Disease Transmission, Vertical / Q000662:veterinary*; D008297:Male; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology / Q000235:genetics / Q000378:metabolism / Q000662:veterinary*; D011328:Prions / Q000235:genetics / Q000378:metabolism*; D012608:Scrapie / Q000453:epidemiology / Q000235:genetics* / Q000378:metabolism / Q000635:transmission*; D034561:Sheep, Domestic
chemical_list: D011328:Prions