Treating Anthrax-Induced Meningitis in Rabbits

Abstract

Treatment of anthrax is challenging, especially during the advanced stages of the disease. Recently, the Centers for Disease Control and Prevention (CDC) updated its recommendations for postexposure prophylaxis and treatment of exposed populations (before and after symptom onset). These recommendations distinguished, for the first time, between systemic disease with and without meningitis, a common and serious complication of anthrax. The CDC considers all systemic cases meningeal unless positively proven otherwise. The treatment of patients suffering from systemic anthrax with suspected or confirmed meningitis includes the combination of three antibiotics, i.e., a fluoroquinolone (levofloxacin or ciprofloxacin), a β-lactam (meropenem or imipenem), and a protein synthesis inhibitor (linezolid or clindamycin). In addition, treatment with an antitoxin (anti-protective antigen antibodies) and dexamethasone should be applied. Since the efficacy of most of these treatments has not been demonstrated, especially in animal meningitis models, we developed an anthrax meningitis model in rabbits and tested several of these recommendations. We demonstrated that, in this model, ciprofloxacin, linezolid, and meropenem were ineffective as single treatments, while clindamycin was highly effective. Furthermore, combined treatments of ciprofloxacin and linezolid or ciprofloxacin and dexamethasone failed in treating rabbits with meningitis. We demonstrated that dexamethasone actually hindered blood-brain barrier penetration by antibiotics, reducing the effectiveness of antibiotic treatment of anthrax meningitis in this rabbit model.

Keywords: Bacillus anthracis; anthrax; antibiotic; ciprofloxacin; clindamycin; dexamethasone; linezolid; meningitis; meropenem; rabbits.

FIG 1

Histology of brains from rabbits that had been infected by IN instillation, IC injection, or direct injection into the CSF in the cisterna magna (ICM injection), showing hemorrhagic areas (yellow dashed arrows) and inflamed meninges (black arrows). H&E staining of the meninges and cortical region of the brain from rabbits that succumbed to B. anthracis infection via the specified route of infection is shown. Magnification, ×10.

FIG 2

Experimental design.

FIG 3

Efficacy of ciprofloxacin treatment of rabbits infected IC with the VollumΔTox strain. The bacterial levels at treatment initiation are indicated. Animals that survived are indicated by blue circles, and animals that succumbed to infection are indicated by red Xs.

FIG 4

Effect of adding dexamethasone to ciprofloxacin treatment of IC VollumΔTox infections in rabbits. (A) Survival curves for the two treatments. The highlighted region is the treatment period. (B) Percentages of successful (blue) or failed (red) treatments, of the total number of rabbits infected IC.

FIG 5

Serum and CSF inhibitory concentrations of ciprofloxacin in naive rabbits and rabbits infected IC with VollumΔTox. The infected rabbits were inoculated IC and treated for 3 days. Serum (red) and CSF (green) samples were collected 30 min after the last treatment with ciprofloxacin or ciprofloxacin and dexamethasone. The inhibitory concentrations were determined by a growth inhibition test and are presented as the maximal dilution of serum (or CSF) to inhibit growth.

FIG 6

Quantitative histology detecting CNS infiltration of immune cells in brains from animals treated with ciprofloxacin (Cipro) or ciprofloxacin and dexamethasone (Dexa).

FIG 7

(A) Efficacy of ciprofloxacin, linezolid, meropenem, and clindamycin as a single treatment in rabbits infected IC/ICM with the fully virulent, wild-type Vollum strain. (B) Efficacy of combined treatment with ciprofloxacin and linezolid, meropenem, or dexamethasone in rabbits infected IC/ICM with the fully virulent, wild-type Vollum strain. (C) Additive effect of administering the antibiotics directly to the CSF (ICM injection) with systemic treatment with ciprofloxacin or meropenem in rabbits infected ICM with the fully virulent, wild-type Vollum strain. (Left) Percentages of surviving (blue) and deceased (red) animals, out of the infected animals. (Right) Survival curves for the treated animals.

FIG 8

Additive effect of protective anti-PA immunity with systemic treatment with ciprofloxacin or meropenem in rabbits infected ICM with the fully virulent, wild-type Vollum strain. (A) Percentages of surviving (blue) or deceased (red) animals, of the infected animals. (B) Survival curves for the treated animals.