1. Epizootic Research Station, Equine Research Institute, Japan Racing Association, 1400-4 Shiba, Kokubunji-machi, Shimotsuga-gun, Tochigi 329-04, Japan
2. Department of Animal Hygiene, Faculty of Agriculture, Kobe University, Kobe-shi, Hyogo 657, Japan
3. Mitsuishi Animal Clinic Center, Agriculture Mutual Aid Association of Hidaka District, 200 Higashi-hohrai, Mitsuishi-cho, Mitsuishi-gun, Hokkaido 059-323, Japan