Objective: To determine the prevalence of in vitro resistance to metronidazole among unselected isolates of Trichomonas vaginalis and correlate in vitro findings with response to metronidazole therapy.
Study design: Vaginal fluid from women attending a gynecology clinic at an urban hospital was cultured, isolates were tested for in vitro resistance to metronidazole, and these results were correlated with therapeutic outcome.
Results: Among 911 women, T vaginalis was detected by culture in 82 (9.0%). Of the 82 isolates, 2 (2.4%; 95% CI, 0.3-8.5%) had low-level in vitro resistance (minimum lethal concentration, 50 micrograms/mL). Women with positive wet mount examinations were treated with metronidazole, 2 g, once and asked to return in one week. Of the 42 infected women agreeing to return for a repeat examination and culture, 26 (61.9%) did, and all (including one woman with a resistant isolate) were cured.
Conclusion: Isolates of T vaginalis resistant to metronidazole occur widely throughout the United States. Although the in vitro susceptibility of T vaginalis to metronidazole has been very poorly studied, our study is consistent with a decade-old prevalence estimate of in vitro resistance (5%), and suggests that high-level resistance is uncommon. This study confirmed, in the absence of reinfection, the continuing clinical effectiveness of single-dose metronidazole for the large majority of trichomoniasis cases.