[1] Juntendo Univ, Dept Infect Control Sci, Fac Med, Tokyo, Japan
[2] Rakuno Gakuen Univ, Sch Vet Med, Dept Vet Sci, Ebetsu, Hokkaido 069, Japan
[3] Nippon Vet & Life Sci Univ, Dept Vet Hyg, Tokyo, Japan
[4] Gifu Univ, United Grad Sch Vet Sci, Gifu, Japan
[5] Nihon Univ, Coll Bioresource Sci, Dept Vet Med, Lab Vet Publ Hlth, Fujisawa, Kanagawa, Japan