1. Department of Veterinary Microbiology, Gifu University, Yanagido 1-1, Gifu City 501-11, Japan
2. Department of Veterinary Public Health, Gifu University, Yanagido 1-1, Gifu City 501-11, Japan
3. Department of Preventative Dentistry, Higashi Nippon Gakuen University, Ishikari 061-02, Hokkaido, Japan. Reprints or correspondence: Dr. Katsuya Hirai, Department of Veterinary Microbiology, Faculty of Agriculture, Gifu University, Yanagido 1-1, Gifu City 501-11, Japan