5.2. Subunit vaccines
The major constraints related to existing Brucella vaccines are related to virulence, the inability to provide sterile immunity, and for S19, the inability to readily distinguish infected from vaccinated animals. The availability of the genomic sequences for pathogenic Brucella spp. and their hosts has allowed for better understand- ing of the host-pathogen interactions and allows the discovery of subunit antigens several of which have been targeted for vaccine development (Gomez et al., 2013). Subunit vaccines commonly use a purified protein or DNA approach to stimulate immune responses in animals (Yang et al., 2013). As these subunit vaccines are non-viable and not infectious, they cannot cause disease and are therefore safer than live vaccines. They also have the advan- tage of protecting against multiple Brucella species because of their extensive gene homologies (Yang et al., 2013). Several subunit Bru- cella antigens have been tested as potential vaccines (Gomez et al., 2013).