1. Department of Veterinary Medicine, University of Cambridge , Cambridge CB3 0ES , UK
2. Medical Research Council Metabolic Diseases Unit, Wellcome–MRC Institute of Metabolic Science–Metabolic Research Laboratories, University of Cambridge , Cambridge CB2 0QQ , UK
3. Department of Microbiology, Immunology and Pathology, Colorado State University , Fort Collins, CO , USA
4. Wallenberg Center for Molecular Medicine, Department of Clinical and Experimental Medicine, Linköping University , 581 83 Linköping , Sweden
5. UMR INRA ENVT 1225 -Hôtes-Agents Pathogènes, Ecole Nationale Vétérinaire de Toulouse , 31076 Toulouse , France
6. Calgary Prion Research Unit, Faculty of Veterinary Medicine, University of Calgary, TRW 2D10 , Calgary, AB , Canada