Trichomonas
Also known as: Trichomonas vaginalis, trichomoniasis
Trichomonas vaginalis is a flagellated protozoan parasite that causes trichomoniasis, a common sexually transmitted infection. It exists only as a motile trophozoite, with no cyst stage, and is treated with metronidazole.
T. vaginalis is a pear-shaped protozoan with four anterior flagella and an undulating membrane that give it a characteristic jerky, tumbling motility. Unlike Giardia and Entamoeba, it has no cyst form, so it cannot survive long outside a host and is transmitted almost exclusively through sexual contact rather than fecal-oral or waterborne routes. It colonizes the urogenital tract, adhering to squamous epithelium of the vagina and urethra.
In women, infection classically produces a frothy, malodorous, yellow-green discharge with vulvar itching and dysuria. Examination may show punctate cervical hemorrhages described as a strawberry cervix, and the vaginal pH rises above 4.5. Men are frequently asymptomatic but may develop urethritis or prostatitis and serve as a reservoir for reinfection. Diagnosis is traditionally made by observing motile trophozoites on a saline wet mount of vaginal secretions; nucleic acid amplification testing is substantially more sensitive and is now the preferred method.
Treatment is oral metronidazole or tinidazole, and sexual partners must be treated at the same time to prevent reinfection. Patients are counseled to avoid alcohol during and shortly after metronidazole therapy because of a disulfiram-like reaction. Untreated infection is associated with increased susceptibility to other sexually transmitted infections and, in pregnancy, with preterm birth and low birth weight.
On USMLE Step 1, Trichomonas appears in the parasitology and pharmacology sections. High-yield discriminators include the absence of a cyst stage, the wet-mount finding of motile trophozoites, the elevated vaginal pH that separates it from candidiasis, and the metronidazole–alcohol interaction. Contrast it with bacterial vaginosis, which shows clue cells and a positive whiff test, and with Candida, which shows pseudohyphae at normal pH.
Key takeaways
- Trichomonas vaginalis is a flagellated protozoan transmitted sexually, with a trophozoite stage but no cyst.
- Women may present with frothy yellow-green malodorous discharge, itching, and a strawberry cervix at pH above 4.5.
- Men are often asymptomatic carriers, which drives reinfection if partners are not treated.
- Diagnosis is by motile trophozoites on wet mount or, more sensitively, nucleic acid amplification testing.
- Metronidazole or tinidazole treats both patient and partners, and alcohol must be avoided due to a disulfiram-like reaction.