1. Department of Veterinary Pathology, School of Veterinary Medicine, Kitasato University, 23-35-1, Higashi, Towada, Aomori 034-8628, Japan
2. Research Promotion Project, Oita University, 1-1 Idaigaoka, Hasama-machi, Yufu, Oita 879-5593, Japan
3. Department of Veterinary Science, National Institute of Infectious Diseases, Toyama 1-23-1, Shinjuku-ku, Tokyo 162-8640 Japan
4. Department of Microbiology, Faculty of Medicine, Oita University, 1-1 Idaigaoka, Hasama-machi, Yufu, Oita 879-5593, Japan