Electron microscope study of experimental enteric infection in neonatal dogs with a canine coronavirus.

: Neonatal dogs, inoculated orally with coronavirus 1-71, grown in canine kidney cell cultures, developed diarrhea and a severe enteritis characterized by atrophy of the villi, changes in the enterocytes, and accelerated epithelial cell loss. Electron microscopy of the mucosal epithelium, 4 days after challenge, showed that the virus penetrated into the enterocytes between microvilli, possibly by pinocytotic mechanism. In the enterocytes, virions were most often enclosed, singly or in groups, in cytoplasmic vesicles. They were less frequently found in the cisternae of the Golgi apparatus, the endoplasmic reticulum, or in the dilated perinuclear space and only rarely, free in the cytoplasm. Virions replicated by budding only on the smooth.membranes of the cytoplasmic vesicles. The infected cells showed a variety of cytopathic effects, some nonspecific, such as disruption of the microvilli, loss of density of the cytoplasm, presence of lipid inclusions, alteration of mitochondria, and dilation of the endoplasmic reticulum and Golgi cisternae and of the perinuclear space. Other cytopathic effects, characteristic of the coronavirus infection, consisted of formation of dense filamentous structures and of membrane-bound bodies. Progeny virions appeared to discharge into the gut lumen through the disrupted cell membranes of infected enterocytes still in situ or following their premature shedding.