Trichomonas vaginalis
On this page
Direct answer
Trichomonas vaginalis is a flagellated protozoan that causes sexually transmitted vaginitis and urethritis, and it has no cyst stage, existing only as a trophozoite. Transmission occurs through sexual contact, so both partners must be treated together. The most-tested features are motile trophozoites on a saline wet mount and metronidazole as the drug of choice.
What you must remember
- Stage: Only the trophozoite exists, with no cyst form, so cysts are never sought in a specimen.
- Flagella: Four anterior flagella and one recurrent flagellum that forms an undulating membrane.
- Wet mount: Saline preparation shows pear-shaped, jerky motile trophozoites, but its sensitivity is limited.
- Strawberry cervix: Colpitis macularis with punctate haemorrhages gives the cervix a strawberry appearance on speculum examination.
- Discharge: Frothy, yellow-green discharge with a vaginal pH above 4.5 supports the diagnosis, and a positive whiff test with KOH can occur, though it is classic for bacterial vaginosis.
- Drug of choice: Metronidazole, with a single 2 g oral dose as the traditional Indian regimen (a 7-day course is also used, and is preferred for women in CDC guidance), is given to both partners.
- Culture and NAAT: Diamond medium culture is more sensitive than wet mount, and nucleic acid amplification testing is the most sensitive.
- Male infection: Men are often asymptomatic carriers, which explains reinfection after treating only the woman.
From a wet mount to a cured partner
Trichomoniasis is quick to suspect at the bedside but often missed when a single test is run. The pathway below shows why partner treatment is part of the cure rather than an optional step.
- Presentation: Women report frothy, yellow-green discharge, vulvovaginal itching and dysuria, while men are usually asymptomatic. Symptoms can wax and wane, so a history of recurrent discharge is important.
- Speculum examination: The clinician looks for the strawberry cervix and checks the pH of the vaginal fluid, both of which help point toward trichomoniasis.
- Wet mount: A drop of saline mixed with discharge shows motile trophozoites. A negative result does not exclude infection, because the test is insensitive.
- Confirmation: Culture in Diamond medium or nucleic acid amplification testing improves sensitivity, and NAAT is the most sensitive method available.
- Treatment: Metronidazole is given to the patient and her sexual partners at the same time, and sexual activity is avoided until both are treated and symptoms have resolved.
- Follow-up: Retesting after treatment and screening for other sexually transmitted infections such as HIV, syphilis and gonorrhoea are recommended.
Where the exam tries to catch you
Most errors come from applying a bacterial or fungal approach to a flagellate that spreads sexually.
- Trap: Searching for cysts in a vaginal smear Correct: Trichomonas has no cyst stage, so only motile trophozoites are seen.
- Trap: Treating only the symptomatic woman Correct: partners must be treated together, otherwise reinfection follows successful treatment.
- Trap: Choosing fluconazole for a flagellate causing vaginal discharge Correct: fluconazole treats candidiasis, while metronidazole is the drug for trichomoniasis.
- Trap: Calling a negative wet mount conclusive Correct: wet mount sensitivity is limited, so culture or NAAT should be used when available.
Frequently asked questions
What is the drug of choice for Trichomonas vaginalis?
Metronidazole is the drug of choice for Trichomonas vaginalis, and it is given to both sexual partners at the same time. A single 2 g oral dose is the traditional regimen, a 7-day course is an accepted alternative, and persistent infection may need a longer course or repeat treatment.
Why does the cervix look like a strawberry in trichomoniasis?
The strawberry cervix, or colpitis macularis, results from punctate haemorrhages on the cervix and vaginal wall caused by trichomonal inflammation. It is seen on speculum examination and is a classic clinical sign tested in exams.
How is Trichomonas vaginalis diagnosed in the laboratory?
Trichomonas vaginalis is diagnosed by saline wet mount, culture in Diamond medium, or nucleic acid amplification testing. NAAT is the most sensitive method and is preferred when it is available.
Does Trichomonas vaginalis have a cyst stage?
Trichomonas vaginalis has no cyst stage and exists only as a trophozoite, so transmission is direct by sexual contact. This is why cysts are never sought in a wet mount or other specimen.
Why are men treated for Trichomonas vaginalis even without symptoms?
Men with trichomoniasis are often asymptomatic, yet untreated partners cause reinfection of the woman after her successful treatment. Treating both partners together is therefore the standard approach.