Doxycycline or ciprofloxacin prophylaxis and therapy against experimental Yersinia pestis infection in mice

The efficacy of doxycycine and ciprofloxacin against an experimental plague infection was assessed by comparing the median lethal dose (MLD) of Yersinia pestis in antibiotic-treated and untreated mice. The MLD of Y. pestis GB strain in untreated mice by the intra-peritoneal route was 23 cfu. If ciprofloxacin dosage (20 or 40 mg/kg twice daily) was initiated 48 h before infection, it afforded complete protection against an intra-peritoneal challenge of 5.24×10⁷ cfu. Ciprofloxacin therapy initiated 24 h post-challenge was less protective, the MLD was raised to 2.0×10⁵ and 2.2×10⁵ cfu for 40 and 20 mg/kg respectively. Doxycycine dosage (40 mg/kg twice daily) initiated 48 h prior to infection raised the MLD to 1.6×10 cfu, but other prophylactic and therapeutic regimes were ineffective against challenges greater than 6.76×10² cfu. Ciprofloxacin may therefore be a useful antibiotic to consider for the treatment of plague.