Abstract
The Yamaguchi strain exhibited a rapid loss of pathogenicity for calves upon serial passage in suckling hamsters, suckling mice and hamster kidney BHK21-WI2 cells. Calves developed clinical symptoms after intravenous inoculation with the virus up to the fourth passage in suckling hamsters, the third passage in suckling mice, and the seventh or eighth passage in cell cultures, while the virus further passaged did not induce any clinical illness. Almost at the same time the virus lost the ability to produce antibody and resistance to challenge with virulent virus. The attenuated virus only stimulated production of antibody and immunity to virulent virus after repeated inoculations. These findings provide additional evidence for the similarity of bovine epizootic fever to bovine ephemeral fever.
