1. Veterinary Epidemiology Unit, School of Veterinary Medicine, Rakuno Gakuen University, 582 Bunkyodai-Midorimachi, Ebetsu, Hokkaido 069-8501, Japan
2. Veterinary Epidemiology Unit, Graduate School of Veterinary Medicine, Rakuno Gakuen University, 582 Bunkyodai-Midorimachi, Ebetsu, Hokkaido 069-8501, Japan
3. Department of Veterinary Sciences, Faculty of Agriculture, University of Miyazaki, 1-1 Gakuenkibanadai Nishi, Miyazaki 889-2192, Japan
4. Center for Animal Disease Control, University of Miyazaki, 1-1 Gakuenkibanadai Nishi, Miyazaki 889-2155, Japan