Monoclonal antibody-mediated, immunodiagnostic competitive enzyme-linked immunosorbent assay for equine monocytic ehrlichiosis.

Competitive enzyme-linked immunosorbent assay (CELISA), mediated by a monoclonal antibody designated HybI, was developed for the diagnosis of equine monocytic ehrlichiosis. Inhibition of binding of HybI by the horse antibodies to Ehrlichia risticii was optimum at dilutions of 1:20 for serum and 1:10,000 for HybI. Mean optical densities (ODs) of positive and negative sera were 0.158 and 0.855, respectively. A comparison of ODs obtained by CELISA and indirect enzyme-linked immunosorbent assay (ELISA) indicated a marked tendency of positive and negative samples to cluster separately with respect to CELISA ODs, whereas the ELISA results displayed a continuum of ODs from negative to positive. Analysis of diagnosis by indirect fluorescent-antibody test (IFA), ELISA, and CELISA for 66 field-collected serum samples indicated that CELISA was superior to IFA and ELISA. Among 11 acute-phase serum samples negative by IFA which were obtained from horses that subsequently seroconverted, CELISA clearly demonstrated antibodies in 8 of these acute-phase sera, whereas 5 were borderline positive by ELISA. The presence of agent-specific humoral antibodies could be demonstrated conclusively by 14 days after infection. The results suggest that CELISA is more sensitive than IFA and ELISA and, owing to the marked differences between positive and negative samples, can be easily adapted for use in the field for detection of horse antibodies to E. risticii.