1. Department of Veterinary Pathology; Rakuno Gakuen University; 582 Bunkyodai-Midorimachi Ebetsu Hokkaido 069-8501 Japan
2. Department of Veterinary Virology, School of Veterinary Medicine; Rakuno Gakuen University; 582 Bunkyodai-Midorimachi Ebetsu Hokkaido 069-8501 Japan
3. Department of Veterinary Pathology, School of Veterinary Medicine; Obihiro University of Agriculture and Veterinary Medicine; Obihiro Hokkaido 080-8555 Japan
4. Department of Virology; Research Institute for Microbial Disease, Osaka University; Suita Osaka 565-0871 Japan