[1] Hokkaido Univ, Sch Med, Inst Anim Experimentat, Kita Ku, Sapporo, Hokkaido 0608638, Japan
[2] Hokkaido Univ, Grad Sch Vet Med, Dept Environm Vet Sci, Publ Hlth Lab, Sapporo, Hokkaido 0600818, Japan
[3] Natl Inst Infect Dis, Div Expt Anim Res, Tokyo 2080011, Japan
[4] Chinese Acad Prevent Med, Inst Virol, Beijing 100052, Peoples R China