1. Yamagata Prefectural Central Livestock Health and Sanitation Office; Yamagata 990-2151
2. Laboratory of Veterinary Public Health; School of Veterinary Science; Nippon Veterinary and Life Science University; Tokyo 180-8602
3. Dairy Hygiene Research Division
4. Viral Disease and Epidemiology Research Division; National Institute of Animal Health, National Agriculture and Food Research Organization; Ibaraki 305-0856 Japan