1. Department of Veterinary Microbiology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan,1 and
2. Taiwan Provincial Research Institute for Animal Health, Tam-Sui, Taipei,2
3. Department of Veterinary Medicine, National Taiwan University, Taipei,3and
4. Department of Veterinary Medicine, National Chung Hsing University, Taichung 40227,4 Taiwan, Republic of China