Abstract
Neonatal calf diarrhea induced by a reovirus-like agent was studied by electron microscopy. Gnotobiotie calves were inoculated with virulent and cell-cultureadapted isolates of viruses, and similar results were obtained with both isolates. The virus infected mature villous epithelial cells of the small intestine and replicated predominantly within cisternac of the endoplasmic reticulum. Macrophages within small intestinal villi also contained viral particles. Shedding of virus into the feces apparently resulted from desquamation of infected epithelial cells. Replacement epithelium did not contain the rcovirus-like agent, was of cuboidal or squamous type, and ultrastructurally resembled immature epithelium normally found in the crypts. Escherichia coli was nonpathogenic when inoculated alone and did not adhere to the epithelium when inoculated with the virus. The reovirus-like agent was a primary pathogen since it caused diarrhea and lesions when inoculated alone or with E. coli.