CROSS-PROTECTION EXPERIMENTS IN PIGS VACCINATED WITH ACTINOBACILLUS-PLEUROPNEUMONIAE SUBTYPES 1A AND 1B

Cross-protection experiments were conducted to determine whether antig enic differences located within the lipopolysaccharides (LPS) of Actin obacillus pleuropneumoniae subtypes 1A and 1B were important with resp ect to the efficacy of whole cell, formalin-inactivated bacterins. Bas ed on clinical signs, lung lesion scores and mortality rates, pigs imm unized with A. pleuropneumoniae subtype 1A were partially protected ag ainst severe challenge with both subtypes 1A and 1B. In contrast, 1B v accinated pigs were not protected against severe challenge with subtyp e 1A but were partially protected against 1B challenge. Cross-reactive serum antibody levels were measured with an ELISA using outer membran es of subtype 1A or 1B as the coating antigen. Serum antibodies were d etected against both subtypes within 2 weeks after the first immunizat ion. Antibody levels increased with time and were generally higher aga inst the homologous subtype coating antigen. We conclude that antigeni c variation within a capsular serotype, due to antigenic variation wit hin LPS, can result in the failure of whole cell bacterins to provide protection against challenge with the same capsular serotype. This lac k of cross-protection within a capsular serotype provides partial expl anation for vaccination failures observed under field conditions.