Gross, immunofluorescent, and light microscopic findings in seven gnotobiotic calves inoculated orally with a Reo-like neonatal calf diarrhea virus were compared to findings in three control gnotobiotic calves. Neonatal calf diarrhea virus infected primarily the villous epithelium of the small intestine. Calves examined within 1.5 h after onset of diarrhea had tall columnar immunofluorescent villous epithelial cells in the middle and lower small intestine. Calves examined 2–4.5 h after onset of diarrhea had cuboidal to squamous villous epithelial cells and an increase in reticulum-like cells in the villous lamina propria of the middle and lower small intestine. Viral tilers were 106 and 108 in colonic contents from two calves inoculated with cell-culture-adapted virus and necropsied, respectively, 2 and 6 h after onset of diarrhea.

Abstract. Neonatal calf diarrhea induced by a reovirus-like agent was studied by electron microscopy. Gnotobiotic 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 cisternae of the endoplasmic reticulum. Macrophages within small intestinal villi also contained viral particles. Shedding of virus into the feces apparently resulted from desquaniation of infected epithelial cells. Replacement epithelium did not contain the reovirus-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.Diarrhea of neonatal calves has been a serious and persistent problem of the dairy industry for many years. This disease has become a major problem on ranches with cow-calf operations. Apparently an increased frequency of neonatal calf diarrhea in beef-type calves has been caused by calving during a relatively short period of time in the late winter and early spring when inclement weather causes stress. Cows and their offspring have been concentrated into small areas during the calving season, and transmission of infectious diseases to susceptible calves has been favored.A virus designated initially as neonatal calf diarrhea virus was isolated in 1968 and reported as an etiologic agent in field cases of neonatal calf diarrhea [9]. The disease was reproduced experimentally in specific pathogen-free and gnotobiotic calves inoculated with bacteria-free filtrates obtained from infected calves. Fecal material purified by ultracentrifugation and examined by

One- to four-day-old gnotobiotic piglets were inoculated orally with a reovirus-like agent obtained from human infants with acute gastroenteritis. Diarrhea developed in the piglets two to seven days after inoculation and was reproduced for five serial passages in one sequence and for three passages in another. Nineteen of 21 inoculated piglets developed diarrhea; reovirus-like particles were observed in intestinal contents and/or fecal samples from 17 animals with illness and from two inoculated piglets that did not develop diarrhea. One piglet, for which daily fecal samples were examined by electron microscopy, shed the largest number of virus particles at the onset of diarrhea. Immunofluorescent antibody responses to the reovirus-like agent were detected in sera from the seven inoculated animals that were tested.