1. Department of Bacteriology, Okayama University Graduate School of Medicine and Dentistry, Okayama 700-8558
2. Nanbu Livestock Hygiene Service Center, Kanazawa, Ishikawa 920-3101
3. Department of Veterinary Epidemiology, University of Osaka Prefecture, Sakai, Osaka 599-8531, Japan
4. National Water Research Institute, Burlington, Ontario L7R 4A6, Canada