1. Yale Medical School, New Haven, CT 06510, USA.
2. Center for Emerging Infectious Diseases, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan.
3. Department of Molecular Microbiology and Immunology, Nagasaki University Graduate School of Medicine, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan.