1. Toshiba Corporation, 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki, Kanagawa 212–8582, Japan
2. National Institute of Animal Health, National Agriculture and Food Research Organization, Sapporo, Hokkaido 062–0045, Japan
3. National Institute of Animal Health, National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305–0856, Japan
4. School of Veterinary Medicine, Azabu University, 1–17–71, Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252–5201, Japan