Author's Organization:Department of Epizootiology,Faculty of Veterinary Medicine,Hokkaido University Department of Epizootiology,Faculty of Veterinary Medicine,Hokkaido University Takikawa Animal Husbandry Experimental Station of Hokkaido Department of Comparative Pathology,Faculty of Veterinary Medicine,Hokkaido University Department of Comparative Pathology,Faculty of Veterinary Medicine,Hokkaido University