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Summary Seven conventional newborn calves were inoculated orally with the coronavirus strain LY‐138, originally recovered from naturally‐infected diarrheic calves. All calves developed diarrhea during the first 24 h after inoculation. The presence and excretion of virus was monitored by electron microscopy. Grossly, the changes were subtle and not striking. The intestinal tract was distended and contained a yellow clear fluid. The small intestinal wall was thin and transparent. Histologic changes were characterized by atrophy and fusion of the villi, decreased height and vacuolation of the intestinal epithelial cells, by a reduced number of goblet cells, crypt hypertrophy and dilatation, and by edema and increased cellularity of the lamina propria mucosae. Ultrastructurally the viral agent infected the intestinal absorptive epithelial cells, undifferentiated epithelial cells and goblet cells of the villi, and occasionally fibroblasts and endothelial cells of the lamina propria mucosae. Morphologic evidence indicated that virus multiplication occurred only in the cytoplasm of infected cells. The rough and smooth endoplasmic reticulum became dilated, and electron‐dense, granulo‐fibrillar material filled their cysternae to form smooth membrane‐bounded viral factories in the supranuclear cytoplasm of infected intestinal epithelial cells. Ultrastructural cytologic changes in this viral infection consisted of dilation of the endoplasmic reticulum network, cellular ribosome depletion, extensive endoplasmic vacuolation and vesiculation of Golgi elements of intestinal epithelial cells, swelling and degeneration of mitochondria, loss of normal nuclear polarity with pyknosis and karyolysis of the nuclei, loss of cytoplasmic matrix, and a broad spectrum of pathologic changes involving the microvilli and terminal web. Zusammenfassung Lichtmikroskopische und Ultrastrukturelle pathologische Veränderungen bei intestinaler Coronavirusinfektion neugeborener Kälber Sieben konventionelle neugeborene Kälber wurden oral mit dem Coronavirus‐Stamm LY‐138, der ursprünglich von natürlich infizierten Kälbern mit Diarrhoe stammt, infiziert. Alle Kälber entwickelten Durchfall während der ersten 24 Stunden nach Inokulation. Vorhandensein und die Ausscheidung des Virus wurden elektronenoptisch verfolgt. Sichtbare Veränderungen waren gering und wenig auffallend. Der Darmtrakt war erweitert und enthielt eine gelbe, klare Flüssigkeit. Die Dünndarmwand war dünn und durchscheinend. Histologische Veränderungen waren charakterisiert durch Atrophie und Verschmelzung der Zotten, verminderte Höhe und Vakuolisierung der intestinalen Epithelzellen, durch eine reduzierte Anzahl von Gobletzellen, Kryptenhypertrophie und ‐erweiterung, sowie Ödematisierung und erhöhte Zellularität der lamina propria mucusae. Ultrastrukturell infizierte das Virus die intestinalen absorptiven Epithelzellen, nichtdifferenzierte Epithelzellen und Gobletzellen der Zotten, und gelegentlich Fibroblasten sowie Endothelzellen der Lamina propria mucosae. Die morphologischen Veränderung...

Abstmct. Ultrastructural changes induced by chlamydia1 infection of niucosal cells of the ileum of ncwborn calves after oral inoculation were investigated. Depending on the stage of chlamydia1 development, the organelles of all infected cells became damaged. The damage was degenerative and included vesiculation of microvilli and swelling of the terminal web of absorptive epithelial cells. The mitochondria were swollen and had fragmented cristae. The endoplasmic reticulum was dilated and vesiculated, and infected cells gradually lost their ribosomes. The lateral junctional complex between infected cells became occasionally dislocated and fragmented. The basal lamina was thrown into folds, became discontinuous and separated from the basal border of the epithelial cells. The nuclei of infected cells were affected last, lost their chromatin pattern, and ultimately became pyknotic.Enteric disease represents a serious problem for the newborn of many different animal species and man. The newborn encounters numerous infectious agents during the postnatal period when the intestinal microbial flora is being established. Intestinal infections with viruses and other obligate intracellular parasites such as chlamydiae are also known to occur during this period.Chlamydia1 agents have been isolated from feces of clinically normal young calves [ 151 and from specimens of newborn calves with diarrhea and polyarthritis [ 121. After oral inoculation with specific chlamydia1 strains, young calves developed diarrhea and died rapidly because of water and electrolyte losses, but only minor gross and histopathologic lesions were detected in the intestine [5]. Since chlamydiae replicate by going through a welldefined developmental cycle, infected cells can be identified ultrastructurally. The purpose of this investigation was to analyze ultrastructural changes induced by chlamydiae in the ileal mucosa of calves that developed diarrhea after oral inoculation.