Aerosol exposure of pigs to viable or inactivated Actinobacillus pleuropneumoniae serotype 9 induces antibodies in bronchoalveolar lining fluids and serum, and protects against homologous challenge

A dose-defined nose-only inhalation system for pigs was used to study the immunogenic and protective potentials of a single aerosol application of viable or killed Actinobacillus pleuropneumoniae serotype 9. Respiratory volumes were measured for each pig to calculate inhaled individual doses. Eight pigs inhaled 107 CFU A. pleuropneumoniae CVI 13261 reference strain for serotype 9. Another eight pigs received an identical dose of killed actinobacilli. After three weeks the pigs and nonexposed controls were challenged with 108 CFU of the homologous strain by aerosol. Bronchoalveolar lavage (BALF) in pigs was performed during the experiment to obtain lavage samples for assessment of local antibodies. Isotype-specific antibody responses in serum and BAL fluids were measured by ELISAs based on whole-cell antigens. The protective efficacy of aerosol immunization was evaluated by clinical and post-mortem examinations. The controls developed fever and severe pleuropneumonia, whereas previously exposed pigs had less fever and less extensive gross pulmonary lesions. After the first aerosol exposure pulmonary IgM, and IgG antibodies reactive with A. pleuropneumoniae increased significantly in both aerosol exposed groups. IgA in BALF and serum concentrations of each Ig class were significantly increased in the group exposed to viable bacteria when compared to the non-exposed controls. After aerosol challenge a pronounced increase of systemic and pulmonary IgA, IgM, and IgG antibodies was detected in both exposure groups. Aerosol application of whole-cell A. pleuropneumoniae bacterins induced similar protective effects against aerosol challenge infection as administration of an identical dose of viable bacteria. Inhalation of A. pleuropneumoniae may lead to asymptomatic carriers in some pigs that could spread the disease under field conditions.