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 infec tious 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 th eir lungs for viable bacilli at various times thereafter. In all three spec ies, however, the number of viable bacilli usually does not continuously in crease until the host succumbs. The number of viable bacilli increases loga rithmically for only about 3 weeks. Then, the host develops delayed-type hy persensitivity (DTH) and cell-mediated immunity (CMI), which keep the numbe r of viable bacilli rather constant during the subsequent weeks. In the imm unized host, DTH and CMI stop the logarithmic increase sooner than in the u nimmunized controls, so that the stationary bacillary levels that follow ar e lower. This review analyzes host-parasite interactions in the lungs of ra bbits, 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. (C) 20 01 Harcourt Publishers Ltd.