1. Department of Disease Control, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Hokkaido 060–0818, Japan
2. Department of Environmental Veterinary Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Hokkaido 060–0818, Japan
3. Hokkaido Institute of Public Health, Sapporo, Hokkaido 060–0819, Japan
4. National Institute of Infectious Diseases, 1–23–1 Toyama, Shinjuku-ku, Tokyo 162–8640, Japan