1. Neuropathogenesis Unit; Institute for Animal Health; Ogston Building, West Mains Road Edinburgh EH9 3JF
2. Department of Food Safety and Veterinary Public Health; Istituto Superiore di Sanita; Viale Regina Elena 299 00161 Rome Italy
3. Department of Pathology; National Veterinary Institute; PO Box 8156 Dep N-0033 Oslo Norway