Quantitation of serotype-specific and cross-reacting group-specific antigens by coagglutination and immunodiffusion tests for differentiating Actinobacillus (Haemophilus) pleuropneumoniae strains belonging to cross-reacting serotypes 3, 6, and 8.

There are strong cross-reactions among strains of Actinobacillus pleuropneumoniae belonging to serotypes 3, 6, and 8. Various serological tests were used to differentiate these serotypes from each other. Tube agglutination, coagglutination, and indirect hemagglutination tests were not sufficiently sensitive to differentiate strains of serotypes 3, 6, and 8. However, higher antibody titers were obtained with a 2-mercaptoethanol agglutination test in homologous rabbit antisera. Absorption of immune sera with homologous and heterologous serotypes as well as quantitative estimation of antigenic activity in the unheated and heat-treated bacterial cell suspensions of reference strains with rabbit homologous and heterologous antisera revealed serotype-specific and cross-reacting group-specific antigens. Usually, serotype-specific antigens were major and dominant over group-specific antigens. The coagglutination test could be used quantitatively to measure the ratio of serotype-specific and group-specific antigens with rabbit hyperimmune sera against serotypes 3, 6, and 8. The highest antigen content for a particular serotype reflected serotype-specific antigen. For strains showing equal amounts of antigen for two or more serotypes in the coagglutination test, the immunodiffusion test with boiled cell-saline extract as the antigen and rabbit antisera against whole-cell suspensions of serotypes 3, 6, and 8 clearly revealed the serotype-specific antigen. It is suggested that coagglutination and immunodiffusion tests could be used successfully to determine the exact serotype of strains belonging to serotypes 3, 6, and 8.