Avirulent rotavirus infections protect calves from disease with and without inducing high levels of neutralizing antibody.

Two bovine rotaviruses, C3-160 and 17/4, which multiplied in calves without inducing disease, were studied for their ability to protect against disease caused by a virulent rotavirus, CP-1. Rotavirus 17/4 and the virulent virus CP-1 cross-neutralized poorly and, on the basis of 20-fold differences in neutralization titres, belonged to different serotypes. Rotaviruses C3-160 and CP-1 were more closely related: neutralization of the CP-1 virus by C3-160 antisera was within 20-fold of the homologous titre although neutralization of the C3-160 virus by CP-1 antisera was not. Nine gnotobiotic calves infected with either C3-160 or 17/4, had rotavirus antibody in their faeces and/or serum 21 days after oral inoculation as detected by indirect immunofluorescence and IgG, IgM and/or IgA antibodies by ELISA. As expected from the antigenic relationships between the viruses, the sera and faeces from the four calves infected with C3-160 contained moderate levels of neutralizing antibody to the virulent virus CP-1 and the sera and most of the faeces from the five calves infected with 17/4 contained undetectable or low levels. When challenged with CP-1 on day 21, four age-matched controls developed disease whereas all of four calves previously infected with C3-160 and four of five calves previously infected with 17/4 were protected from disease. It was concluded that avirulent rotavirus infection provided protection against disease caused by a virulent rotavirus even when one of the avirulent viruses was poorly related to the virulent virus by neutralization. Mechanisms other than neutralizing antibody appeared to be important in protection.