Performance of skin tests with allergens from B. melitensis B115 and rough B. abortus mutants for diagnosing swine brucellosis

Swine brucellosis by Brucella suis biovar 2 is an emerging disease whose control is based on serological testing and culling. However, current serological tests detect antibodies to the O-polysaccharide (O/PS) moiety of Brucella smooth lipopolysaccharide (S-LPS), and thus lack specificity when infections by Yersinia enterocolitica O:9 and other gram-negative bacteria carrying cross-reacting O/PS occur. The skin test with the protein-rich brucellin extract obtained from rough B. melitensis B115 is assumed to be specific for discriminating these false positive serological reactions (FPSR). However, B115 strain, although unable to synthesize S-LPS, accumulates O/PS internally, which could cause diagnostic problems. Since the brucellin skin test has been seldom used in pigs and FPSR are common in these animals, we assessed its performance using cytosoluble protein extracts obtained from B. abortus rough mutants in manBcore or per genes (critical for O/PS biosynthesis) and B. melitensis B115. The diagnostic sensitivity and specificity were determined in B. suis biovar 2 culture positive and brucellosis free sows, and apparent prevalence in sows of unknown individual bacteriological and serological status belonging to B. suis biovar 2 naturally infected herds. Moreover, the specificity in discriminating brucellosis from FPSR was assessed in brucellosis free boars showing FPSR. The skin test with B. abortus ΔmanBcore and B. melitensis B115 allergens performed similarly, and the former one resulted in 100% specificity when testing animals showing FPSR in indirect ELISA, Rose Bengal and complement fixation serological tests. We conclude that O/PS-free genetically defined mutants represent an appropriate alternative to obtain Brucella protein extracts for diagnosing swine brucellosis.