Mice tuberculosis quantified by the lung density technique

Groups of CF1 mice were respectively infected intravenously with four decreasing doses of the virulent H37Rv strain of Mycobacterium tuberculosis var. hominis and the progress of tuberculous disease was followed by three techniques yielding essentially objective results. Survival rates showed that 84% of the mice infected with the largest dose died within 30 days of infection but this technique was unable to differentiate among the other three groups of mice infected with smaller doses. Bacterial culture counts performed after increasing intervals of infection on spleen homogenates of similarly infected groups of mice showed that different degrees of tuberculous involvement had effectively been induced in the four groups. Lung density determinations performed on the same groups of animals gave results establishing that this technique was capable of distinguishing the four groups of mice one from another. The observation that the lung density technique reflects the degree of tuberculous disease accurately was further substantiated by experiments performed on groups of mice respectively injected intravenously with comparable numbers of viable cells of seven mycobacterial strains of different virulence, all harvested at the same "physiological age." Statistically significant differences existed between the experimental groups infected with the virulent strains on the one hand and those injected with the avirulent ones on the other; no differences existed among the groups infected with virulent strains or among those injected with avirulent ones. Of particular interest is the observation that the lung density technique can also differentiate groups of mice injected either with virulent or avirulent strains from those injected with the attenuated bovine BCG strain. The results of both the bacterial culture count and lung density techniques applied to the same groups of mice in the early and late phases of tuberculous disease are discussed.