There is not a clear correlate with mechanisms of protection. In contrast to the predominant role of cell-mediated immunity in protection against smooth Brucella species, several studies showed that antibodies play a predominant role in protective immunity to B.ovis in mice and rams. Immunization with detergentextracted recombinant Omp31 induced bactericidal antibodies against outer membrane proteins and rough lipopolysaccharide which are able to kill B.ovis in the presence of complement.