1. Laboratory of Laboratory Animal Science and Medicine, School of Veterinary Medicine, Kitasato University, Towada, Aomori 034-8628, Japan
2. Department of Laboratory Animal Medicine, Research Institute, National Center for Global Health and Medicine, Tokyo 162–8655, Japan
3. Laboratory of Anatomy, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Hokkaido 060–0818, Japan
4. Department of Infectious Diseases, Section of Animal Models, Research Institute, National Center for Global Health and Medicine, Tokyo 162–8655, Japan