1. Department of Veterinary Microbiology; The University of Tokyo; Bunkyo-ku Tokyo 113 Japan
2. Department of Biomedical Science; The University of Tokyo; Bunkyo-ku Tokyo 113 Japan
3. Department of AIDS Research, Institute of Tropical Medicine; Nagasaki University; Nagasaki Nagasaki 852 Japan
4. Department of Veterinary Microbiology; Kagoshima University; Kagoshima Kagoshima 890 Japan
5. Department of Veterinary Microbiology; Yamaguchi University; Ube Yamaguchi 735 Japan
6. Central Laboratory; Kyoritsu Shouji Co., Ltd.; Ibaraki 300-12 Japan