Comparison of serological tests based on outer membrane or internal antigens for detecting antibodies to Brucella ovis in infected flocks.

The aim of this work was to compare the performance of 6 serological tests using outer or internal antigens from Brucella for the diagnosis of Brucella ovis infection in sheep in an endemic area. Outer membrane antigens included a hot saline extract (HS) and the rough lipopolysaccharide (R-LPS) from B. ovis. Internal antigens were LPS-free total cytosolic proteins (CP) and an 18-kDa cytosolic protein (p18) from Brucella spp. Sera from 200 sheep from naturally infected flocks were assayed by agar gel immunodiffusion test (AGID) and by complement fixation test (CFT), both using HS, and by 4 ELISA using HS, R-LPS, CP, and p18, respectively. The percentage of positive results was 45.5% for ELISA with HS, 42.0% for ELISA with p18, 39.5% for CFT, 33.5% for ELISA with R-LPS, 29.0% for ELISA with CP, and 18.0% for AGID. Taking CFT as the reference test for calculating relative test parameters, the ELISA with HS had the best sensitivity (96.2%), while AGID and the ELISA with R-LPS had the best specificity (96.6%). The ELISA with CP was not more sensitive than the ELISA with p18 (67.1% vs. 79.7%) in spite of the higher number of antigens in CP. The lower relative sensitivity of tests using internal antigens might reflect a lack of antibodies to cytosolic proteins in some infected animals or a shorter persistence of these antibodies relative to antibodies to outer membrane components after recovery from infection.