1. Obihiro University of Agriculture and Veterinary Medicine, Inada, Obihiro, Hokkaido 080-8555, Japan
2. Veterinary Medical Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
3. Ibaraki Prefectural Kenpoku Livestock Hygiene Service Center, 966-1 Nakagachicho, Mito, Ibaraki 310-0002, Japan