VARIATION IN ROTAVIRUS VIRULENCE - A COMPARISON OF PATHOGENESIS IN CALVES BETWEEN 2 ROTAVIRUSES OF DIFFERENT VIRULENCE

Variation in virulence between two bovine rotaviruses was investigated using ten female and ten male 10-day-old gnotobiotic calves of five b reeds or cross breeds that were inoculated with a virulent strain or a strain of low virulence. Similar numbers of infectious viral particle s were detected in feces of calves inoculated with either virus, but d iarrhea, xylose malabsorption, and reduction of villus height occurred only after inoculation with virulent virus. The mean percentage of th e area of the villus epithelium per villus immunostained for rotavirus antigen was eight times greater in calves inoculated with virulent vi rus, and the mean percentage of villi on which immunostained enterocyt es were detected was twice as large in calves inoculated with virulent virus than in calves inoculated with the virus of low virulence. Mean crypt death and mean crypt cell production rates were increased after inoculation with either virus. Virulence was associated with extensiv e spread of infection through the small intestine, preferential coloni zation of the proximal small intestine, and marked damage to enterocyt es and villi. The virus of low virulence infected the proximal small i ntestine poorly, and although it infected more enterocytes in the mid and distal small intestine and replicated in them, causing cytopathic effects, it did not damage intestinal structure and affect function.