EVIDENCE FOR A HIGH-RATE OF FALSE-POSITIVE RESULTS WITH THE INDIRECT FLUORESCENT-ANTIBODY TEST FOR EHRLICHIA-RISTICII ANTIBODY IN HORSES

Objective-The original objective was to determine seroprevalence of Eh rlichia risticii antibody among horses in California. On the basis of the unexpected results of the survey, an investigation into the accura cy and reproducibility of results of the indirect fluorescent antibody (IFA) test for E risticii was carried out. Design-Prospective, seropr evalence study. Animals-Healthy horses (n = 655) and horses with clini cal signs of equine monocytic ehrlichiosis (EME; n = 514) from various regions of California. Procedure-The IFA test was performed. Results were compared with results of an ELISA and with results of western imm unoblot analysis. Results-Overall, 104 of 655 (15.9%) healthy horses h ad evidence of an antibody response. However, 84 of 514 (16.3%) horses with clinical signs of EME also had positive test results, and of the 8 seropositive diseased horses for which paired (acute and convalesce nt) samples had been submitted, only 1 had a rise in antibody titers b etween the acute and convalescent samples. Comparison of results for t he IFA test, ELISA, and western immunoblot analysis revealed a high ra te of false-positive results for the IFA test. Subsequent studies sugg ested that routine vaccination of horses with non-E risticii vaccines may have contributed to the false-positive reactions. Clinical Implica tions-The data failed to provide conclusive evidence of E risticii inf ection among California horses. Owing to the high percentage of false- positive test results, caution is advised when using the IFA test to d iagnose EME in horses or to determine the necessity for E risticii vac cination.