1. Division of Infection Control and Disease Prevention, Department of Veterinary Medical Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
2. Laboratory of Veterinary Epizootiology, Department of Veterinary Medicine, Nihon University, 1866 Kameino, Fujisawa, Kanagawa 252-0880, Japan
3. Iwate University Faculty of Agriculture Veterinary Medicine, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan
4. National Institute of Animal Health, National Agriculture and Food Research Organization, 3-1-5 Kannondai, Tsukuba, Ibaraki 305-0856, Japan
5. Animal Resource Science Center, Graduate School of Agricultural and Life Science, The University of Tokyo, 3145 Yorii, Kasama, Ibaraki 319-0206, Japan
6. Private investigator, Izumimachi, Shikaoi, Katogun, Hokkaido 081-0212, Japan