1. Department of Veterinary Pathology, Faculty of Veterinary Science, University of Liverpool, Leahurst Campus, Chester High Rd., Neston, Cheshire CH64 7TE, United Kingdom
2. Department of Production Animal Clinical Sciences, Norwegian School of Veterinary Science, Kyrkjevegen 332/334, N-4325 Sandnes, Norway
3. Department of Bacteriology, National Veterinary Institute, SE-751 89, Uppsala, Sweden