1. Department of Veterinary Microbiology; Obihiro University of Agriculture and Veterinary Medicine; Obihiro Hokkaido 080-8555 Japan
2. Osaka Prefectural Institute of Public Health; Osaka Osaka 537-0025 Japan
3. Institute of Basic Medical Sciences; University of Tsukuba; Tsukuba Ibaraki 305-8557 Japan
4. Hokkaido Institute of Public Health; Sapporo Hokkaido 060-0819 Japan