1. Division of Pathology and Pathophysiology, National Institute of Animal Health, 4 Hitsujigaoka, Toyohira, Sapporo, Hokkaido 062-0045, Japan
2. Current address: Division of Viral Disease and Epidemiology, National Institute of Animal Health, 3-1-5 Kannondai, Tsukuba, Ibaraki 305-0856, Japan
3. Current address: Division of Bacterial and Parasitic Disease, National Institute of Animal Health, 3-1-5 Kannondai, Tsukuba, Ibaraki 305-0856, Japan