Host-parasite relationships in experimental airborne tuberculosis. V. Lack of hematogenous dissemination of Mycobacterium tuberculosis to the lungs in animals vaccinated with Bacille Calmette-Guérin.

The influence of bacille Calmette-Guérin (BCG) on the pathogenesis of experimental airborne tuberculosis was studied. In a model that approximates the conditions under which man is vaccinated and infected, BCG-vaccinated and unvaccinated guinea pigs were infected by the respiratory route with an inoculum that resulted in the inhalation and retention (by each animal) of approximately three virulent tubercle bacilli (Mycobacterium tuberculosis strain H37Rv). Hematogenous seeding of the lungs occurred in unvaccinated animals about three weeks after aerosol infection but did not occur in BCG-vaccinated animals. Furthermore, the lungs of BCG-vaccinated animals failed to kill H37Rv that was introduced intravenously; however, evidence of mycobacteriostatic activity was found throughout the lungs. In view of the importance of hematogenous dissemination to the apex of the lungs in the establishment of pulmonary tuberculosis in man, the foregoing observations suggest a means by which vaccination with BCG may confer acquired resistance to tuberculosis.