1. Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d'Aosta, 10154 Turin, Italy.
2. Neuropathogenesis Division, The Roslin Institute and Royal (Dick) School of Veterinary Studies, University of Edinburgh, Roslin, Midlothian, EH25 9PS, UK.
3. Department of Animal Pathology, Faculty of Veterinary Medicine, University of Zaragoza, 50013 Zaragoza, Spain.
4. Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, Midlothian EH26 0PZ, UK.
5. Department of Animal Productions, Epidemiology and Ecology, Faculty of Veterinary Medicine, University of Turin, 10095 Grugliasco, Italy.