1. Laboratories of Infectious, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan
2. Department of Veterinary Pathology, College of Veterinary Medicine, Chonnam National University, Kwangju 300, Korea
3. Department of Veterinary Research, Kwangju City Institute of Health and Environment, Kwangju 379-2, Korea
4. Department of Veterinary Public Health, College of Veterinary Medicine, Cheju National University, Cheju 690-756, Korea
5. Laboratories of Radiation Biology, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan