Mitsuaki Sakashita
Hirofumi Sakai
Hiromi Kohno
Hong-Kean Ooi
Yuzaburo Oku
Masao Kamiya
Department of animal disease control, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060, Japan.

Kinpei Yagi
Hokkaido Institute of Public Health, Sapporo 060, Japan.

Mamoru Ito
Central Laboratory for Experimental Animals, Kawasaki, Japan.