1. Centre forTropicalVeterinary Medicine, University of Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, UK
2. WellcomeTrust Centre for the Epidemiology of Infectious Disease, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
3. Institute for Animal Health BBSRC/MRC Neuropathogenesis Unit, Ogston Building,West Mains Road, Edinburgh EH9 3JF, UK