Use of the complement fixation and brucellin skin tests to identify cattle vaccinated with Brucella abortus strain RB51

Summary In the European Union, RB51 vaccine can be used only under strictly controlled conditions for the immunisation of cattle at risk of infection with Brucella abortus. A diagnostic system is therefore necessary to distinguish vaccinated from unvaccinated animals. To avail of a rapid and accurate diagnostic tool, the authors produced, controlled and evaluated an experimental brucellin prepared from strain RB51 (RB51 brucellin). The potency of this brucellin was evaluated in guinea-pigs sensitised with B. abortus RB51 and compared with a commercially available brucellin. Both allergens produced similar biological activity in guineapigs. The RB51 brucellin skin test was performed in 10 cattle 414 days after calfhood vaccination with RB51 when they were negative to the complement fixation test with RB51 antigen (RB51-CFT). The skin test revealed 60% sensitivity (with a 95% confidence interval [CI] of 30.8%-83.3%) and 100% specificity (CI 60.7%-100%). These findings limit the use of the skin test only for screening to detect RB51 vaccinated herds, not individual animals. Nevertheless, following intradermal inoculation of RB51 brucellin, a transient antibody increase to the RB51-CFT was observed from day 9 to day 20 post inoculation with RB51 brucellin. This transient antibody increase, when evaluated in parallel with the RB51 brucellin skin test results, enables detection of individual vaccinated animals (sensitivity 100%; CI 76.2%-100%).