1. Department of Veterinary Medicine, College of Bioresource Sciences, Nihon University, Fujisawa, Kanagawa 252–0880, Japan
2. Northernfarm, Koka, Shiga 529–1812, Japan
3. Hidaka Training and Research Center, Japan Racing Association, 535–13 Aza-Nishicha, Urakawa-cho, Urakawa-gun, Hokkaido 057–0171, Japan
4. Present address: Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido 080–8555, Japan
5. Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, 080–8555, Japan
6. College of Veterinary Medicine, Seoul National University, Seoul 151–742, Korea
7. RIKEN, Wako, Saitama 351–0198, Japan
8. Hidaka Agriculture Mutual Aid Association, Hidaka, Hokkaido 059–3105, Japan