Trichomonas vaginalis

Introduction

Trichomoniasis is caused by Trichomonas vaginalis, a flagellated, motile protozoan parasite that infects the vagina, urethra and paraurethral glands.¹ It is one of the few common STIs caused by a protozoan rather than a bacterium or virus, which has practical consequences for both diagnosis (it cannot be detected by tests designed for bacterial NAAT panels unless specifically included) and treatment (antibiotics that work against bacteria, such as doxycycline, have no effect on it).

It is less commonly diagnosed in the UK than chlamydia or gonorrhoea, partly because testing is not as routinely bundled into standard STI screens, and partly because it disproportionately affects women, with infection in men often asymptomatic and self-limiting.²

Transmission and pathophysiology

T. vaginalis is transmitted almost exclusively through vaginal intercourse; it does not typically infect the rectum, pharynx or, in men, the deeper genital tract, colonising instead the vagina, urethra and Skene's/paraurethral glands. The organism does not survive for long outside the human body, so transmission via fomites (towels, toilet seats) is negligible despite persistent public misconceptions.¹

The incubation period ranges from 5 to 28 days. Infection provokes a local inflammatory response, which accounts for the discharge and vulvovaginal irritation seen in symptomatic cases, and can cause punctate epithelial haemorrhages on the cervix - the 'strawberry cervix' appearance.

Clinical features

In women

Around 70% of women with trichomoniasis are symptomatic, more than with chlamydia or gonorrhoea. Classic features are a frothy, yellow-green, offensive-smelling vaginal discharge, vulval itching and soreness, dysuria, and dyspareunia. On speculum examination, a minority of patients show 'strawberry cervix' (colpitis macularis) - erythematous punctate haemorrhagic spots on the cervix - which is highly specific but seen in only around 2% of cases, so its absence does not exclude the diagnosis.¹

In men

Infection in men is more often asymptomatic and typically self-limiting. When symptomatic, it presents as non-gonococcal urethritis: mild dysuria and a scant urethral discharge, occasionally with balanoposthitis (inflammation of the glans and foreskin).

Differential diagnosis

• Bacterial vaginosis: thin, grey, fishy-smelling discharge, but typically without itching or inflammation
• Candidiasis: thick, white, 'cottage cheese' discharge with itching, but not classically frothy or malodorous
• Chlamydia/gonorrhoea: can co-exist; distinguished on NAAT
• Atrophic vaginitis: in postmenopausal women, a non-infective differential for discharge and irritation

Investigations

NAAT is now the preferred diagnostic test in the UK where available, with substantially higher sensitivity than microscopy.³ Where NAAT is not available, wet mount microscopy of a vaginal swab, examined promptly, can show motile flagellated trophozoites, but sensitivity is low (around 50-60%) and falls further the longer the sample sits before examination.

Vaginal pH testing (raised above 4.5, similar to bacterial vaginosis) can support the diagnosis but is non-specific. In men, urethral swab or first-catch urine NAAT is used. As trichomoniasis is exclusively sexually transmitted, a positive result should always prompt a full STI screen and partner notification.

Management

First-line: metronidazole 400-500 mg orally twice daily for 5-7 days, or a single 2 g oral dose.⁴ The multi-day regimen is generally preferred as it is associated with slightly better cure rates, but the single dose improves adherence and can be given as directly observed therapy.

Patients should avoid alcohol during treatment with metronidazole and for 48 hours afterwards, because of a disulfiram-like reaction (flushing, nausea, palpitations). Sexual abstinence is advised until the patient and all partners have completed treatment.

Partner notification is essential given the exclusively sexual transmission of this organism: current partners should be treated empirically, as the sensitivity of testing in men is limited and reinfection from an untreated partner is common.

Pregnancy

Trichomoniasis in pregnancy is associated with preterm birth, premature rupture of membranes and low birth weight, although randomised trial evidence has not shown that treating asymptomatic infection in pregnancy reduces these outcomes.¹ Symptomatic infection should still be treated with metronidazole, which is considered safe in pregnancy, to relieve symptoms and reduce the (small) risk of vertical transmission causing neonatal respiratory or urogenital infection.

Complications

Trichomoniasis increases the risk of acquiring and transmitting HIV, likely through the inflammatory response it causes at the genital mucosa, which is one reason prompt treatment matters beyond symptom relief. In pregnancy it is associated with adverse outcomes as above. Untreated infection can persist for months to years, acting as a source of ongoing transmission and inflammation.

Red flags

Prognosis

With appropriate treatment and partner notification, trichomoniasis is readily cured with an excellent prognosis and no lasting sequelae in the individual. Population-level control depends heavily on partner treatment, since reinfection from an untreated partner is the most common reason for apparent treatment failure.