Evidence of immunosuppression by bovine respiratory syncytial virus.

Respiratory syncytial virus (RSV) is a major respiratory pathogen in human infants and calves. Calves and lambs infected with bovine RSV show mild clinical signs but they are more susceptible to secondary infection with Pasteurella haemolytica. Lambs infected with P. haemolytica 6 days after experimental infection with bovine RSV had significantly greater magnitudes of fever, higher disease and lesion scores and higher mortality rates than those infected with P. haemolytica or bovine RSV alone (P less than 0.05). Experimental infection with bovine RSV is characterized by alterations in lymphocyte subpopulations and down-regulation of some of their functions. For example, the number of T helper cells is significantly reduced during the first week of infection and peripheral blood mononuclear cells obtained from bovine RSV-infected lambs were less responsive to the mitogen phytohaemagglutinin but more susceptible to P. haemolytica cytotoxin than those obtained from control lambs. Infection with bovine RSV does not significantly affect the humoral immune responses of lambs against P. haemolytica cytotoxin. Bovine RSV does not appear to affect the capacity of alveolar macrophages to present antigens in vitro.