1. Laboratory of Veterinary Infectious Diseases, Faculty of Veterinary Medicine, Nippon Veterinary and Life Science University, Tokyo, Japan
2. Research Center for Animal Life Science, Nippon Veterinary and Life Science University, Tokyo, Japan
3. Influenza Virus Research Center, National Institute of Infectious Diseases, Tokyo, Japan
4. Department of Infectious Diseases and Host Defense, Gunma University Graduate School of Medicine, Gunma, Japan