Cross-reactivity of Haemophilus somnus antibody in agglutination and complement fixation tests and in the enzyme-linked immunosorbent assay

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Abstract
The specificity and sensitivity of agglutination, complement fixation and enzyme-linked immunosorbent assay (ELISA) procedures in the detection of antibodies to H. somnus was investigated. H. somnus rabbit immune sera agglutinated Pasteurella multocida, Staphylococcus aureus and Haemophilus agni and, in some instances, P. haemolytica, Salmonella dublin, Streptococcus agalactiae and Corynebacterium pyogenes. In complement fixation tests with saline extracts as antigens, only H. agni reacted with H. somnus antisera to any significant degree. In ELISA with sonicated or heat-extracted antigens, cross-reactions were seen with the 2 Pasteurella spp. and with H. agni. When whole cells and saline extracts were used as antigens in ELISA, only H. agni showed any cross-reactivity. The greatest specificity in distinguishing homologous from heterologous reactions was achieved by ELISA with saline extracts as antigens. Escherichia coli and Brucella abortus antigens failed to react with H. somnus antibody in any of the tests. A rabbit serum containing antibody to bovine type isolates of P. multocida, P. haemolytica, S. aureus, S. agalactiae, S. dublin, C. pyogenes and E. coli gave no positive reaction in ELISA with saline extract of H. somnus as antigen. Thus, saline extract, which appears to consist largely of H. somnus common antigen, is a potentially useful diagnostic reagent in the ELISA of antibody response to H. somnus.