1. Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
2. National Institute of Hygiene and Epidemiology, Hanoi 100000, Vietnam
3. Department of Bacteriology I, National Institute of Infectious Diseases, 1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8640, Japan
4. Hanoi Center for Disease Control, No 70 Nguyen Chi Thanh, Dong Da district, Hanoi 100000, Vietnam
5. Vietnam Research Station, Center for Infectious Disease Research in Asia and Africa, Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki-shi, Nagasaki 852-8523, Japan