Immunity to pasteurellosis in compromised rabbits.

Pasteurellosis in the rabbit inoculated with a malignant variant of Shope fibroma virus (SFV-MV) is presented as a model for the study of immunosuppression and immunoprophylaxis in pasteurellosis. The rabbits, before the inoculation, were healthy carriers of Pasteurella multocida. They were intradermally inoculated with SFV-MV, and 3 to 6 days later, a primary tumor appeared at the site of inoculation. By postinoculation day (PID) 7 or 8, the rabbits had snuffles, conjunctivitis, and tumor metastases; death occurred on PID 10 to 14. Rabbits given the nonmalignant Patuxent strain of SFV developed local primary tumors, but not pasteurellosis nor metastases. In SFV-MV-inoculated rabbits, there was decreased responsiveness of spleen lymphocytes to B and T cell mitogens by day 6, and of spleen and peripheral blood lymphocytes by day 10. In addition, SFV-MV antigen was detected (by immunofluorescence) in mononuclear phagocytes in all major organs and in epithelial cells of the conjunctiva and nasal mucosa. Both nasal and conjunctival epithelia showed squamous metaplasia as well. These changes did not appear in SFV-infected rabbits. With SFV-MV-inoculated rabbits, we obtained partial protection against pasteurellosis by immunization with heat-killed P multocida or a cross-protective core lipopolysaccharide mutant of Escherichia coli (J5). Rabbits were immunized before the inoculation with SFV-MV which precipitated "spontaneous" pasteurellosis due to impaired defenses. Rabbits immunized with J5 or P multocida had less severe conjunctivitis and snuffles than nonimmunized controls, indicating that immunization with the J5 mutant may be useful as prophylaxis against pasteurellosis in compromised hosts.