1. Department of Medical Zoology1 and
2. Kobe University School of Medicine, Kobe 650-0017, School of Veterinary Medicine, Rakuno-gakuen University, Ebetsu 069-8501,2
3. Third Division of the Department of Medicine,3
4. Hyogo Red Cross Blood Center, Kobe 651-0062,4
5. National Institute of Infectious Diseases, Shinjuku, Tokyo 162-8640,5 and
6. Central Institute of Experimental Animals, Nogawa, Kawasaki 216-0001,6 Japan