Brucella suis S2, Brucella melitensis Rev. 1 and Brucella abortus S19 living vaccines: residual virulence and immunity induced against three Brucella species challenge strains in mice

Live attenuated Brucella suis S2 vaccine was compared to living vaccines B. abortus S19 and B. melitensis Rev.1 in mice. Residual virulence was estimated by ability to multiply and persist in spleen and lymph nodes. Immunogenicity was estimated by spleen counts of control and vaccinated mice challenged either with the reference B. abortus 544 strain or with virulent B. melitensis H38 and B. suis 1330 strains. S2 vaccine had lower residual virulence; expressed as 50% recovery time, persistence was 4.3 weeks, compared to 7.1 and 9.0 weeks for S19 and Rev.1 vaccines. Immunity induced by the three vaccines was similar 45 days after vaccination. At 150 days, immunity by S19 and Rev.1 was still similar against the three challenge strains. In contrast, immunity induced by S2 had declined against the B. melitensis strain. Thus, a recall vaccination may be required for vaccination of sheep to confer a long-lasting immunity.