1. Laboratory of Veterinary Epizootiology, Department of Veterinary Medicine, Nihon University, Kameino 1866, Fujisawa, Kanagawa 252-0880, Japan
2. Ibaraki Kenhoku Animal Hygiene Service Station, Nakakawauchi 966-1, Mito, Ibaraki 310-0002, Japan
3. Laboratory of Large Animal Clinical Sciences, Department of Veterinary Medicine, Nihon University, Kameino 1866, Fujisawa, Kanagawa 252-0880, Japan