1. Animal Center for Biomedical Research; Faculty of Medicine, The University of Tokyo; Bunkyo-ku Tokyo 113-0033 Japan
2. Institute of Special Pathology and Veterinary Medical Clinic; Faculty of Veterinary Medicine, The University of Milan, Via Celoria; I-20133 Milan Italy
3. Department of Virology; Veterinary Laboratories Agency (Weybridge); New Haw, Addlestone Surrey KT15 3NB U.K.