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Genotype-level variation in lifetime breeding success, litter size and survival of sheep in scrapie-affected flocks

¹ Centre for Tropical Veterinary Medicine (CTVM), Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, UK
² Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK
³ Institut National de la Recherche Agronomique, Station d'Amélioration Génétique des Animaux, BP 27, 31326 Castanet-Tolosan Cedex, France
⁴ Institut National de la Recherche Agronomique, Unité de Mathématiques et Informatique Appliquées, 78352 Jouy-en-Josas Cedex, France
⁵ Sustainable Livestock Systems Group, Scottish Agricultural College, Bush Estate, Penicuik, Midlothan EH26 0PH, UK
⁶ Institute for Animal Health, Neuropathogenesis Unit, Ogston Building, West Mains Road, Edinburgh EH9 3JF, UK

Correspondence
Margo E. Chase-Topping
margo.chase{at}ed.ac.uk

Five different sheep flocks with natural outbreaks of scrapie were examined to determine associations between individual performance (lifetime breeding success, litter size and survival) and scrapie infection or PrP genotype. Despite different breed composition and forces of infection, consistent patterns were found among the flocks. Regardless of the flock, scrapie-infected sheep produced on average 34 % fewer offspring than non-scrapie-infected sheep. The effect of scrapie on lifetime breeding success appears to be a function of lifespan as opposed to fecundity. Analysis of litter size revealed no overall or genotype differences among the five sheep flocks. Survival, however, depends on the individual's scrapie status (infected or not) and its PrP genotype. Susceptible genotypes appear to perform less well in lifetime breeding success and life expectancy even if they are never affected with clinical scrapie. One possible explanation for these results is the effect of pre-clinical scrapie. Additional evidence supporting this hypothesis is discussed.

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