Fluid support worsens outcome and negates the benefit of protective antigen-directed monoclonal antibody in a lethal toxin-infused rat Bacillus anthracis shock model

OBJECTIVE: The aim of this study was to test the effects of normal saline treatment either alone or in combination with protective antigen-directed monoclonal antibody in a lethal toxin-infused rat model of anthrax sepsis. DESIGN: Prospective controlled animal study. SETTING: Animal research laboratory. SUBJECTS: Sprague-Dawley rats (n = 539). INTERVENTIONS: We initially tested the efficacy of three normal saline doses (5, 10, or 20 mL/kg/hr intravenously for 24 hrs) or none (controls) started when rats were treated with either lethal toxin (24-hr infusion) or, for comparison, lipopolysaccharide (24-hr infusion) or Escherichia coli (intravenous bolus). We then investigated delaying normal saline for 6 hrs or combining it with protective antigen-directed monoclonal antibody following lethal toxin challenge. MEASUREMENTS AND MAIN RESULTS: Dose did not alter the effects of normal saline with any challenge (p not significant for all) or when combined with protective antigen-directed monoclonal antibody, so this variable was averaged in analysis. In initial studies, normal saline decreased mortality (mean hazards ratio of survival ± se) significantly with E. coli challenge (-0.66 ± 0.25, p = .009 averaged over normal saline dose) but not lipopolysaccharide (-0.17 ± 0.20). In contrast, normal saline increased mortality significantly with lethal toxin (0.69 ± 0.20, p = .001) in a pattern different from E. coli and lipopolysaccharide (p ≤ .002 for each). In subsequent studies, normal saline alone once again increased mortality (0.8 ± 0.3, p = .006), protective antigen-directed monoclonal antibody alone reduced it (-1.7 ± 0.8, p = .03), and the combination had intermediate effects that were not significant (0.04 ± 0.3). CONCLUSIONS: These findings raise the possibility that normal saline treatment may actually worsen outcome with anthrax lethal toxin. Furthermore, lethal toxin-directed therapies may not be as beneficial when used in combination with this type of fluid support.