ORAL IMMUNIZATION OF PIGS WITH VIABLE OR INACTIVATED ACTINOBACILLUS-PLEUROPNEUMONIAE SEROTYPE-9 INDUCES PULMONARY AND SYSTEMIC ANTIBODIES AND PROTECTS AGAINST HOMOLOGOUS AEROSOL CHALLENGE

A dose-defined aerosol infection of pigs was used to study the immunog enic and protective potentials of oral immunization with dead or live Actinobacillus pleuropneumoniae serotype 9 reference strain CVI 13261 against an aerogenic challenge. Pigs were vaccinated with a single dos e of 10(11) CFU of viable (n = 8) or inactivated (n = 8) A. pleuropneu moniae given orally in a gelatin capsule. After 3 weeks, vaccinated pi gs and nonvaccinated controls were challenged aerogenically with a dos e of 10(8) CFU of A. pleuropneumoniae CM 13261. The protective efficac y of oral immunization was evaluated by clinical and postmortem examin ations. Bronchoalveolar lavage in pigs was performed during the experi ment to obtain lavage samples for assessment of local antibodies. Isot ype-specific antibody responses in sera and in bronchoalveolar lavage fluids were determined by enzyme-linked immunosorbent assays based on whole-cell antigen. Oral immunization did not induce clinical side eff ects. After aerosol challenge, two animals of both vaccinated groups ( 25% in each case) showed a moderate fever for 2 days, whereas all four pigs (100%) of the nonvaccinated control group developed severe fever . In contrast to the controls, which developed severe pleuropneumonia, the vaccinated pigs had only mild pulmonary lesions. Three weeks afte r challenge, 13 of 16 vaccinated pigs (81%) were found to be free of p athomorphological changes of the lungs. From two of these pigs immuniz ed with live bacteria we were able to reisolate A. pleuropneumoniae. A significant systemic and pulmonary increase in the concentrations of immunoglobulin A (IgA), IgM, and IgG antibodies reactive with A. pleur opneumoniae was detectable after aerosol challenge in both vaccinated groups. Immunization with viable bacteria was found to induce signific antly higher concentrations of each Ig isotype in bronchoalveolar lava ge fluids and sera than immunization with inactivated A. pleuropneumon iae. These serological findings were not reflected in the reduction in clinical disease after challenge in comparison to the case for the pi gs vaccinated with inactivated bacteria. We concluded that a single or al administration of A. pleuropneumoniae provides partial clinical pro tection against aerosol challenge infection in the respiratory tract.