1. Animal Research Center, Tokyo Medical College, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo 160, Japan
2. Department of Veterinary Microbiology, Nippon Veterinary and Animal Science University, 1-7-1 Kyonan-cho, Musashino-shi, Tokyo 180, Japan
3. Department of Veterinary Public Health, Institute of Public Health, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108, Japan