Mastitis was produced in three first calf heifers by inoculation of the udder with the virus of infectious bovine rhinotracheitis. These animals were killed at different times during the course of the disease and tissues were harvested and examined histologically. The basic lesion resulted from the necrotizing effect of the virus upon the epithelial cells of the alveoli and ducts. Eosinophilic intranuclear inclusion bodies were demonstrated in tissues from animals killed at three and six days postinoculation, but not at nine days. Of interest was the presence of a large Reed-Sternberglike cell in tissues of the last two animals killed.
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