1. National Institute of Animal Health, 3–1–5 Kannondai, Tsukuba, Ibaraki 305–0856, Japan
2. National Institute of Animal Health (Kyushu Research Station), 2702 Chuzan, Kagoshima, Kagoshima 891–0105, Japan
3. Iwate University, 3–18–8 Ueda, Morioka, Iwate 020–8550, Japan
4. Emeritus Professor, The University of Tokyo, 1–1–1 Yayoi, Bunkyo, Tokyo 113–8657, Japan