1. Department of Virology and Parasitology, Fujita Health University School of Medicine, Toyoake, Aichi 470-1192,1
2. Department of Hygiene, School of Medicine, Sapporo Medical University, Sapporo 060-0061,2
3. Department of Virology, Aichi Prefectural Institute of Public Health, Nagoya, Aichi 462-8576,3 and
4. Department of Virology II, National Institute of Infectious Diseases, Tokyo 162-8640,4 Japan