Natural History of Francisella tularensis in Aerosol-Challenged BALB/c Mice

The objective of this study was to evaluate the natural history and pathogenesis of Francisella tularensis in a murine model of inhalational tularemia with the SchuS4 strain. Before the efficacy of antimicrobials could be assessed in this model, further model development was required to determine the optimal time to start therapy. This study helped define the time course of infection after aerosol challenge by quantifying the presence of bacteria in lung, blood, and spleen at multiple harvest points. In this study, mice were infected via a targeted inhaled dose of 100 50% lethal doses (LD50s) (LD50 = 300 CFU) of F. tularensis by whole-body aerosol. At 1, 24, 36, 48, 60, 72, 75, 78, 81, 84, 87, and 90 h postchallenge, groups of 15 animals were sacrificed and blood, lung, and splenic tissue samples were harvested, homogenized, plated, and incubated to evaluate the bacterial load in those tissues. It was determined that of the 3 sample types harvested, splenic tissue provided the most consistent bacterial counts, which steadily increased with the progressing infection. Further, it was determined that lung samples from all (15/15) animals were positive for infection at 75 h postaerosolization and that 14/15 animals had positive splenic tissue counts. Bacterial levels in blood were not predictive of treatment initiation. For future therapeutic evaluation studies in this model using F. tularensis (SchuS4), it was determined that therapy should be initiated at 75 h postchallenge and validated by spleen involvement.