1. Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, Japan.
2. National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.
3. Vietnam Research Station, Nagasaki University, c/o National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.
4. Graduate School of Medicine, Hokkiado University, Hokkaido, Japan.
5. Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan.