1. 1)Laboratory of Animal Health and Hygiene, Department of Food and Life Sciences, College of Agriculture, Ibaraki University, 3-21-1, Chuuo, Ami, Inashiki, Ibaraki 300-0393, Japan
2. 2)Nippon Institute for Biological Science, 9-2221-1 Shin-machi, Ome, Tokyo 198-0024, Japan
3. 3)Laboratory of Veterinary Pathology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan