Systemic and secretory antibody responses to sequential bovine respiratory syncytial virus infections in vaccinated and nonvaccinated calves.

To investigate the influence of humoral immunity on the severity of disease caused by infection with bovine respiratory syncytial virus (BRSV), an experimentally induced infection study was performed on vaccinated and nonvaccinated calves. Fifteen weanling calves were allotted to 3 groups: 1 group of 6 calves was exposed to 2 live virus aerosols, 35 days apart; another group of 6 calves was vaccinated prior to the same aerosol exposures; and the remaining 3 calves served as controls. Clinical signs of infection were converted to a numerical score for evaluating disease severity. For 14 days after each virus exposure, BRSV-specific IgG and IgM concentrations in serum and BRSV-specific IgA concentration in nasopharyngeal exudate and lung lavage fluid were measured by ELISA. Serum BRSV-specific IgG and IgM and secretory BRSV-specific IgA concentrations did not correlate with disease sign expression. There was a strong correlation between viral isolation and disease scores. Vaccination prior to virus exposure appeared to have little or no effect on severity of the disease, but it did appear to affect disease persistence. Findings indicate that the immunoglobulins evaluated may be primarily protective in nature and do not contribute to disease severity.