1. Laboratory of Public Health, Department of Environmental Veterinary Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818,1 Japan, and
2. Institute for Animal Experimentation, Hokkaido University School of Medicine, Hokkaido University, Sapporo 060-8638,2 and
3. Swedish Institute for Infectious Disease Control and Microbiology and Tumorbiology Center, Karolinska Institute, Stockholm, Sweden3