1. Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Iwate University, Morioka, Iwate 020-8550, Japan
2. Department of Biological Sciences, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0808, Japan
3. National Institute of Livestock and Grassland Science, Tsukuba, Ibaraki 305-0901, Japan