Comparative Evaluation of Five Serological Methods for the Diagnosis of Ovine Brucellosis: Agreement and Molecular Confirmation of Seroreactive Samples

This cross-sectional study compared five serological methods for ovine brucellosis—the Rose Bengal test (RBT), agglutination test (AT), complement fixation test (CFT), Saiduldin test (ST), and enzyme-linked immunosorbent assay (ELISA)—in 575 sheep from four rural districts of the Ili District, Almaty Region, Kazakhstan. All samples were tested by the five methods, whereas PCR was performed only on samples positive by at least one serological test. At least one positive result was obtained in 40/575 animals (6.96%). Positive-result frequencies were 6.61% for ST, 6.43% for ELISA, 3.30% for RBT and CFT, and 1.57% for AT. The highest pairwise agreement was observed between RBT and CFT (κ = 0.946) and between ST and ELISA (κ = 0.929). Brucella spp. DNA was detected in 8/40 seroreactive samples; all eight were positive by all five serological methods. ST titers were higher in PCR-positive than in PCR-negative ST-positive samples (median 1:80 vs. 1:10; p = 0.002). Because PCR was restricted to seroreactive samples and no independent reference standard was available, diagnostic sensitivity, specificity, and predictive values could not be estimated.