1. Department of Veterinary Science, National Institute of Infectious Diseases, 1-23-1, Toyama, Shinjuku, Tokyo 162-8640, Japan.
2. CREST, JST, Saitama 332-0012, Japan.
3. Gene Research Center, Tokyo Institute of Technology, Kanagawa 226-8501, Japan.
4. Tokai University, School of Health Sciences, Kanagawa, Japan.