Abstract. Procedurally similar competitive enzyme-linked immunoassay (cELISA) methods were developed for the serodiagnosis of Babesia equi and Babesia caballi (piroplasmosis), Trypanosoma equiperdum (dourine), and Burkholderia mallei (glanders) infections in horses. Apparent test specificities for the B. equi, B. caballi, T. equiperdum, and B. mallei cELISAs were 99.2%, 99.5%, 98.9%, and 98.9%, respectively. Concordances and kappa values between the complement fixation (CF) and the cELISA procedures for the serodiagnosis of B. equi, B. caballi, T. equiperdum, and B. mallei infections in experimentally exposed horses were 76% and 0.55, 89% and 0.78, 97% and 0.95, and 70% and 0.44, respectively. The cELISA method may be a technically more reproducible, objective, and convenient approach for piroplasmosis, dourine, and glanders serodiagnosis in qualifying animals for international movement and disease eradication programs than the CF systems currently in use. Use of the cELISA method also obviated the problems associated with testing hemolyzed or anticomplementary sera.Piroplasmosis, dourine, and glanders are internationally reportable, serious infectious diseases of Equidae. 1,3 The etiologic agents of these diseases are unrelated. Piroplasmosis results from Babesia equi or Babesia caballi infection, whereas dourine and glanders result from Trypanosoma equiperdum and Burkholderia mallei infections respectively. 1,3 Many nations require that Equidae presented for importation be serologically negative to B. equi, B. caballi, T. equiperdum, and B. mallei. 1,3,10 Traditionally, a panel of 4 separate complement fixation (CF) procedures has been the primary serodiagnostic tool prescribed by the Office International des Epizooties (OIE) for this purpose. 2,3 The OIE has also recognized alternative serodiagnostic methods for each etiologic agent, including separate indirect fluorescent antibody tests (IFATs) for B. equi and B. caballi, enzyme-linked immunosorbent assay (ELISA) for B. mallei, and ELISA, IFAT, or agar gel immunodiffusion procedures for T. equiperdum serodiagnosis. 4,6,7,12,13,17 In practice, the battery of 4 CF procedures has been the most convenient, rapid, single test system for accomplishing the required piroplasmosis, dourine, and glanders serodiagnostic testing. Unfortunately, the CF tests require careful continuous titration of numerous labile reagents and, for piroplasmosis, the laborious expensive preparation of antigen from infected splenectomized horses. 2,3 These CF procedures do not From the Diagnostic Bacteriology Laboratory, National Veterinary Services Laboratories, US Department of Agriculture, Animal and Health Inspection Services, Ames, IA 50010.Received for publication January 19, 1999. function with sera having anticomplementary activity, test interpretation is often subjective, test sensitivity is low relative to more modern immunoassay methods, and CF test sensitivity declines as the serologic responses of exposed animals shift from initial IgMbased reactions to those of other immu...