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

The specificity and sensitivity of agglutination, complement fixation, and enzyme-linked immunosorbent assay (ELISA) procedures in the detection of antibodies to Haemophilus somnus was investigated. H. somnus rabbit immune sera were found to agglutinate Pasteurella multocida, Staphylococcus aureus, and Haemophilus agni and, in some instances, also Pasteurella 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 tests with sonicated or heat-extracted antigens, cross-reactions were seen with the two Pasteurella spp. and with H. agni. When whole cells and saline extracts were used as antigens in ELISAs, 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 tests with saline extract of H. somnus as antigen. It is concluded that such saline extract, which appears to consist largely of H. somnus common antigen, has the potential of being a useful diagnostic reagent in the study by ELISA of antibody response to H. somnus.