Three metronidazole-resistant strains of Trichomonas vaginalis from the United States.

Trichomonas vaginalis isolates were obtained in the United States from three women with trichomoniasis refractory to routine treatment with metronidazole. Cure was obtained in these cases with increased dosage of metronidazole. In experimental mouse infections, all three strains showed a significantly (about 10 to 20 times) lower susceptibility to metronidazole than did a number of control strains. In vitro, the lowered susceptibility could be detected only in an aerobic assay but not in the routine anaerobic assay. The conclusion is that T. vaginalis strains with lowered metronidazole susceptibility do exist. Such strains might be responsible for a certain, but as yet entirely unknown, number of failures in routine metronidazole therapy of human trichomoniasis.