The Efficacy of Antibiotic Treatment to Cure Guinea Pigs Intranasally Infected with Bacillus anthracis Spores

Abstract

Respiratory anthrax is a fatal disease in the absence of early therapy with antibiotics. Guinea pigs are highly sensitive to infection with Bacillus anthracis spores by intranasal spore instillation succumbing within 2 to 4 days post infection. Post exposure prophylaxis that initiated 24 h post infection with tetracycline, doxycycline, ciprofloxacin, ofloxacin, erythromycin, gentamicin or imipenem protected the animals, but after termination of treatment the animals developed a fatal anthrax disease. Combined treatment with antibiotics and active immunization with PA based vaccine conferred an efficient immune response that prevented the development of a fatal anthrax disease after termination of antibiotic administration. The CDC recommended in case of a major bioterrorist attack a mass distribution of antibiotics within 48 h. As the onset of symptoms in humans were reported to start on day 1–7 post exposure, this delay in initiation of treatment may result in treatment of sick populations. We evaluated the efficacy of late antibiotic administration starting from 30 h post infections and every 6 h thereafter, to cure respiratory anthrax sick animals. Administration of ciprofloxacin provided efficient protection up to 44 h post infection, at which time 60% of the sick animals survived. Administration of doxycycline cured up to 48 h post infection, when up to 75% of the sick animals survived. The antibiotic administration protected most animals with bacteremia level up to 10⁵ CFU/ml, but failed to protect animals exhibiting higher levels of bacteremia. In conclusion these results demonstrate the efficacy of antibiotic treatment to cure animals even after the onset of the disease.