Ceftriaxone is highly effective clinically in patients with Lyme disease. We studied a representative invasive human isolate of Borrelia burgdorferi for which the MBC of ceftriaxone was 0.050 g/ml. A once-per-day dosage regimen of ceftriaxone (50 mg/kg/dose) administered intramuscularly for 5 days was 100% effective in sterilizing tissue samples of C3H mice infected with this strain of B. burgdorferi, regardless of whether the mice were being treated concomitantly with a corticosteroid. Administration of the same five doses of ceftriaxone at 6-h intervals over just 24 h was also 100% effective. These experiments suggest that shorter courses of antibiotics than those currently recommended should be considered for study in patients with early uncomplicated Lyme disease.Lyme disease caused by the spirochete Borrelia burgdorferi is the most common vector-borne infection in North America. In vitro, B. burgdorferi is susceptible to the lethal action of several different antibiotic classes. For example, the MBC of the expanded-spectrum cephalosporin ceftriaxone is usually Յ0.4 g/ml and this agent is also effective clinically (2, 6). While most authoritative sources recommend treatment regimens for patients with Lyme disease similar to that employed for other spirochetal diseases, duration of therapy is nonetheless a topic of controversy (20,21). The treatment durations recommended in the Infectious Diseases Society of America Practice Guidelines on the Treatment of Lyme Disease range from 14 to 28 days, which is considerably longer than those used successfully to treat patients with infection due to relapsing-fever borrelia (one dose to 10 days of therapy) (4,19).Unlike most other spirochetal microorganisms, B. burgdorferi can be readily cultured in vitro and a variety of rodents and other laboratory animals are suitable for experimental studies. In animal experiments, 5 to 10 days of treatment of rodents (7-9) and 30 days of therapy of dogs (14) using the appropriate dosage and type of antibiotic cure the infection on the basis of the inability to recover B. burgdorferi by culture of postmortem tissue samples. Shorter courses of treatment, however, have not been systematically studied.The purpose of this study was to determine if ceftriaxone therapy of mice could be shortened to a time period estimated to achieve a drug level in serum in excess of the MBC for B. burgdorferi for a time period shorter than that which would follow a single parenteral dose given to humans. The second objective was to determine if immunosuppressive doses of corticosteroids affect the activity of short-course therapy with ceftriaxone in vivo.
MATERIALS AND METHODSAnimals. Female C3H mice (4 to 5 weeks old) were obtained from Charles River Laboratories (Wilmington, Mass.). All mice were housed in a filtered-air environment maintained at 20 Ϯ 2°C. Antibiotic-treated and non-antibiotictreated (control) mice were kept in separate cages for the infectivity experiments.Bacteria and bacterial cultures. A strain of B. burgdorferi designated BL206 w...