Efficacy of combinations of doxycycline and rifampicin in the therapy of experimental mouse brucellosis.

Mice infected with Brucella melitensis were treated with doxycycline and rifampicin in daily dosages of: 1.5, 3 and 6 mg/kg separately and in combination, for a period of 14 days. Doxycycline alone in all dosages did not result in a cure rate superior to controls. A significant reduction in the viable count of B. melitensis recovered from the spleens of mice (which was the criterion for cure) was achieved with 6 and 3 mg/kg/day of doxycycline. Rifampicin at a dose of 6 mg/kg/day gave significantly better cure rates and reduction of mean viable counts of B. melitensis than untreated controls or doxycycline in similar doses. Low dose therapy with rifampicin (1.5 and 3 mg/kg) resulted in a further reduction in viable counts from spleens but with a cure rate no better than untreated animals. Rifampicin was more potent than doxycycline in all low dose regimens. When rifampicin was given in combination with doxycycline, the effect observed was similar to that of rifampicin alone. No synergy between rifampicin and doxycycline was demonstrable in the mouse model of brucellosis.