Variation in virulence of bovine rotaviruses.

Forty-six gnotobiotic calves aged less than 16 days or 42-116 days were infected with three strains of bovine rotavirus designated C3-160, CP-1 and PP-1. Each virus was passaged and cloned in cell culture (cloned viruses) but CP-1 and PP-1 were also used before culture (faecal viruses). Infection of calves aged less than 16 days with faecal or cloned CP-1 caused disease whereas cloned C3-160 and faecal or cloned PP-1 caused subclinical infections. The clinical signs of disease were change in faecal colour to pale yellow or cream, increase of 2- to 7-fold in the volume of faecal output and, usually, anorexia. With the virulent CP-1 virus and the avirulent C3-160, similar amounts of virus were excreted in the faeces for 4-6 days. Infection of calves aged 56-116 days with faecal CP-1 produced disease of similar severity to that seen in calves aged 7-10 days infected with the same virus. No differences in clinical signs, virus excretion or levels of convalescent antibody were seen between the two groups. With cloned CP-1, 5 of 8 older calves developed disease but 3 showed only mild signs of infection. It was concluded that two strains of rotavirus caused sub-clinical infections in young calves while a third was virulent in calves up to at least 116 days of age.