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SUMMARY A coronavirus-like agent isolated in fetal bovine kidney ( fbk ) cell culture from feces of a calf having diarrhea was adapted to secondary fbk cells. The virus was attenuated by serial passage in consecutively higher passages of fbk cells originating from the fetus used for adaptation. The mononucleated cells and syncytia in infected fbk cells and the epithelium of intestine from diarrheal calves inoculated with virulent and cell culture-propagated virus (16th passage) fluoresced with the same conjugate. The electron microscopic structural characteristics of cell culture-propagated virions were identical to those of the virus in diarrheal calf feces. In ultrathin sections of detached cells from infected fbk culture, virions were present within membrane-bound vesicles of cytoplasm of cells and had an average overall diameter of 78 nm. Calves inoculated orally with attenuated cell culture virus were resistant to oral challenge inoculation with virulent virus of fecal origin.
Mebus et al. (1973) studied this question.