1. Kenhoku Livestock Hygiene Service Center, Ibaraki Prefecture, 966-1 Nakagachi, Mito, Ibaraki 310-0002, Japan
2. Viral Disease and Epidemiology Research Division, National Institute of Animal Health, National Agriculture and Food Research Organization, 3-1-5 Kannondai, Tsukuba, Ibaraki 305-0856, Japan