Viral-bacterial pneumonia in calves: effect of bovine herpesvirus-1 on immunologic functions

The temporal relation between a lung infection with bovine herpesvirus-1, suppression of some immune functions, and susceptibility to secondary bacterial infection resulting in fibrinous pneumonia prompted a study to determine the mechanism(s) involved in the apparent immunosuppression. In six independent experiments employing from five to 40 calves, we studied the immunologic parameters of migration of and superoxide anion production by polymorphonuclear neutrophils, lectin-induced lymphocyte proliferation, interleukin-2 production, natural cytotoxicity, interferon and antibody formation, as well as complement activation and hematologic parameters. Chemotaxis of polymorphonuclear neutrophils, natural cytotoxicity, and mitogen response of peripheral blood leukocytes were depressed, whereas superoxide anion generation by polymorphonuclear neutrophils was transiently increased and interleukin-2 production was only marginally affected. The assumption that virus-induced interferon might be a common cause for the various changes could not be substantiated. However, the results did suggest that mechanisms other than lack of T helper cell activity, accessory cell activity of macrophages, or development of suppressor T cells were the cause of suppressed mitogen responses. None of the immunologic parameters appeared to have consistent prognostic value with respect to outcome of the infection.