1. Department of Internal Medicine, Veterinary Medical Teaching Hospital, College of Veterinary Medicine, Seoul National University, San 56-1, Shillim-dong Kwanak-gu, Seoul 151-742, Korea
2. Department of Microbiology and Immunology, Faculty of Agriculture, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657, Japan