1. Royal Veterinary College, Departments of Veterinary Clinical Sciences (S.H., D.P.F., E.J.W., S.T.L., I.M.S.), University of London, North Mymms, Hatfield, Hertfordshire AL9 7TA, United Kingdom
2. Pathology and Infectious Diseases (D.W.), University of London, North Mymms, Hatfield, Hertfordshire AL9 7TA, United Kingdom
3. Department of Veterinary Clinical Science and Animal Husbandry (H.D.), Faculty of Veterinary Science, University of Liverpool, Leahurst, Neston CH64 7TE, United Kingdom
4. Centre for Veterinary Science (C.E.B.), Department of Clinical Veterinary Medicine, University of Cambridge, Cambridge CB3 0ES, United Kingdom