Cross-protection experiments in pigs vaccinated with Actinobacillus pleuropneumoniae subtypes 1A and 1B

Cross-protection experiments were conducted to determine whether antigenic differences located within the lipopolysaccharides (LPS) of Actinobacillus pleuropneumoniae subtypes 1A and 1B were important with respect to the efficacy of whole cell, formalin-inactivated bacterins. Based on clinical signs, lung lesions scores and mortality rates, pigs immunized with A. pleuropneumoniae subtype 1A were partially protected against severe challenge with both subtypes 1A and 1B. In contrast, 1B vaccinated pigs were not protected against severe challenge with subtype 1A but were partially protected against 1B challenge. Cross-reactive serum antibody levels were measured with an ELISA using outer membranes of subtype 1A or 1B as the coating antigen. Serum antibodies were detected against both subtypes within 2 weeks after the first immunization. Antibody levels increased with time and were generally higher against the homologous subtype coating antigen. We conclude that antigenic variation within a capsular serotype, due to antigenic variation within LPS, can result in the failure of whole cell bacterins to provide protection against challenge with the same capsular serotype. This lack of cross-protection within a capsular serotype provides partial explanation for vaccination failures observed under field conditions.