Progressive pulmonary tuberculosis is not due to increasing numbers of viable bacilli in rabbits, mice and guinea pigs but is due to a continuous host response to mycobacterial products

Tuberculosis (TB) kills more people in the world today than any other infectious disease. A better vaccine to prevent clinical tuberculosis is greatly needed. Candidate vaccines are often evaluated by infecting rabbits, mice and guinea pigs by an aerosol of virulent tubercle bacilli and culturing their lungs for viable bacilli at various times thereafter. In all three species, however, the number of viable bacilli usually does not continuously increase until the host succumbs. The number of viable bacilli increases logarithmically for only about 3 weeks. Then, the host develops delayed-type hypersensitivity (DTH) and cell-mediated immunity (CMI), which keep the number of viable bacilli rather constant during the subsequent weeks. In the immunized host, DTH and CMI stop the logarithmic increase sooner than in the unimmunized controls, so that the stationary bacillary levels that follow are lower. This review analyzes host-parasite interactions in the lungs of rabbits, mice and guinea pigs. All three species cannot prevent inhaled fully virulent tubercle bacilli from establishing an infection, but they differ markedly in the type of the disease produced once it is established.