1. Hokkaido Research Station, National Institute of Animal Health, Toyohira, Sapporo 062-0045
2. Obihiro University of Agriculture and Veterinary Medicine, Inada, Obihiro 080-8555
3. National Institute of Animal Health, Tsukuba Science City 305-0856
4. Kyushu Research Station, National Institute of Animal Health, Kagoshima 891-0105, Japan