The Brucellin skin test as a tool to discriminate false positive serological reactions in bovine brucellosis.

Three experiments were performed in order to assess the diagnostic value of the Brucellin allergic skin test (AST) in a brucellosis false positive serological reactions (FPSR) context. First, 1259 cattle from 20 Brucella-free herds in a FPSR area were tested twice with AST to estimate its specificity. Secondly, AST and serological tests (complement fixation test [CFT], tube agglutination test, dithiothreitol-microagglutination test and ELISA) sensitivities were evaluated on 111 cattle positive to the Rose Bengal test (RBT) belonging to 15 Brucella-infected herds. Thirdly, AST was used in a field trial to discriminate FPSR from true brucellosis reactions. AST specificity in non-vaccinated cattle was very high (99.83%; confidence interval 95% [CI95%]: 99.67-99.96%). Skin thickening 72 h post-injection was significantly higher on vaccinated cattle (1.42 vs 0.15 mm). In this sub-population, AST specificity decreased significantly to 78% (CI95%: 68-87%). Individual sensitivity of AST relative to Rose Bengal test was 64% (CI95%: 54-72%), while all infected herds were AST positive (n = 15). When associated with CFT, it detected 95% (CI95%: 90-98%) of the infected cattle. These results were consistent with the field trial. In a FPSR context, AST was more specific than RBT or CFT. Therefore, this test could be used at herd level as a confirmation test, on cattle non vaccinated against brucellosis.