1. Graduate School of Frontier Sciences, The University of Tokyo, 5F Environmental Building, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan
2. Division of Virology and Medical Zoology, Chiba Prefectural Institute of Public Health, 666-2 Nitona-cho, Chuo-ku, Chiba 260-8715, Japan
3. Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan