Ineffectiveness of erythromycin for treatment of Haemophilus vaginalis-associated vaginitis: possible relationship to acidity of vaginal secretions.

To assess the efficacy of oral erythromycin in the treatment of nonspecific vaginitis (NSV), conducted a nonrandom, unblinded pilot study among 17 women with symptoms and signs of NSV. At the completion of treatment, 10 of 13 patients had persistent symptoms, 9 of 13 had persistent abnormal discharge, and 11 of 13 had persistently positive cultures for Haemophilus vaginalis. Ten patients with persistent or relapsing NSV and four who did not complete erythromycin treatment were retreated with oral metronidazole, and 14 of 14 showed clinical improvement and eradication of H. vaginalis. The susceptibility of 27 clinical isolates of H. vaginalis to erythromycin was determined at pH 5.5, 6.0, 6.5, and 7.0. The minimal inhibitory concentration of erythromycin for H. vaginalis was approximately 10-fold higher at pH 5.5 than at pH 7.0. Erythromycin is not effective for the treatment of H. vaginalis-associated NSV; this may be partly attributable to the reduced activity of this drug in acidic vaginal secretions.