1. National Institute of Animal Health, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-0856, Japan
2. Shizuoka Prefectural Swine and Poultry Experiment Station, 2780 Nishikata, Kikukawa, Shizuoka 439-0037, Japan
3. Animal Health Laboratory, SDS Biotech K.K., 2-1 Midorigahara, Tsukuba, Ibaraki 300-2646, Japan
4. Nippon Veterinary and Animal Science University, 2-1 Kyonancho, Musashino, Tokyo 180-0023, Japan