Cross-reactions between Actinobacillus (Haemophilus) pleuropneumoniae strains of serotypes 1 and 9.

Actinobacillus pleuropneumoniae strains of serotypes 1 and 9 were studied for their serological properties by means of agglutination, coagglutination (CoA), indirect hemagglutination (IHA), Co-IHA, ring precipitation (RP), and immunodiffusion (ID) tests. Particulate and soluble antigens of unheated and heat-treated bacterial cells were used in various serological tests. Agglutination, CoA, and RP tests demonstrated common antigens between strains of serotypes 1 and 9. Quantitative estimation of serotype-specific antigenic activity by CoA, RP, and ID tests proved useful in differentiating strains of serotypes 1 and 9. IHA and Co-IHA tests using sheep erythrocytes sensitized with unheated or heat-treated whole-cell saline extract and the ID test using boiled whole-cell saline extract as antigen distinguished the strains of serotypes 1 and 9. In studies of absorption of rabbit antisera with heterologous whole-cell antigens there was no absorption of antibodies in tube agglutination and IHA tests, suggesting that serotype 1 and 9 strains belong to two distinct serogroups. It appears that the cross-reactivity between serotype 1 and 9 strains could be due to common epitopes associated with cell wall antigens.