1. Viral Disease and Epidemiological Research Division, National Institute of Animal Health, National Agriculture and Food Research Organization, 3–1–5 Kannondai, Tsukuba, Ibaraki 305–0856, Japan
2. Pathology and Pathophysiology Research Division, National Institute of Animal Health, National Agriculture and Food Research Organization, 3–1–5 Kannondai, Tsukuba, Ibaraki 305–0856, Japan
3. Dairy Hygiene Research Division, National Institute of Animal Health, National Agriculture and Food Research Organization, Hitsujigaoka, Toyohira-ku, Sapporo, Hokkaido 062–0045, Japan