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