Effect of immunization with attenuated Mycobacterium bovis on experimental toxoplasmic retinochoroiditis

Administration of attenuated Mycobacterium bovis (Bacillus Calmette-Guérin or BCG) provides nonspecific resistance to a variety of microbial infections and tumors. This is associated with a state of augmented immunologic responsiveness. Mustering defenses against intracellular parasites, such as Toxoplasma gondii, presents a special problem that can be met only by measures that alter the intracellular environment. Our study was designed to evaluate the effect of prior immunization of rabbits with BCG on experimental toxoplasmic retinochoroiditis. One group of rabbits was immunized by the intravenous administration of BCG, another group by the retrobulbar injection of BCG, and a third group, unvaccinated, served as a control. Intravenous immunization provided significant protection against Toxoplasma organisms injected into the suprachoroidal space. In the immunized rabbits, the onset of Toxoplasma retinochoroiditis was delayed and the severity of the disease reduced. Although Toxoplasma was isolated from the chorioretinal tissues of both BCG-immunized and control rabbits, Toxoplasma antibody was not (with one exception) detected in the sera of BCG-immunized rabbits. While vaccination by the retrobulbar route produced little or no effect, intravenous administration of BCG provided nonspecific resistance to Toxoplasma retinochoroiditis in rabbits.