1. Department of Health Sciences, Kobe University School of Medicine, Kobe 654-0142,1
2. Department of Medical Zoology, Kobe University School of Medicine, Kobe 650-0017,2 and
3. Department of Microbiology, Kinki University School of Medicine, Osaka-sayama 589-8511,3 Japan, and
4. Plum Island Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Greenport, New York 119444