Merck Manual

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Sheldon R. Morris

, MD, MPH, University of California San Diego

Last full review/revision Dec 2020| Content last modified Sep 2022
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Trichomoniasis is infection of the vagina or male genital tract with Trichomonas vaginalis. It can be asymptomatic or cause urethritis, vaginitis, or occasionally cystitis, epididymitis, or prostatitis. Diagnosis is by direct microscopic examination, dipstick tests, or nucleic acid amplification tests of vaginal secretions or by urine or urethral culture. Patients and sex partners are treated with metronidazole or tinidazole.

T. vaginalis is a flagellated, sexually transmitted protozoan that more often infects women (about 20% of women of reproductive age) than men. Infection may be asymptomatic in either sex, but asymptomatic is the rule for men. In men, the organism may persist for long periods in the genitourinary tract without causing symptoms and may be transmitted unwittingly to sex partners. Trichomoniasis may account for up to 5% of nongonococcal, nonchlamydial urethritis in men in some areas.

Symptoms and Signs of Trichomoniasis

In women, symptoms of trichomoniasis range from none to copious, yellow-green, frothy vaginal discharge with a fishy odor, and soreness of the vulva and perineum, dyspareunia, and dysuria. Asymptomatic infection may become symptomatic at any time as the vulva and perineum become inflamed and edema develops in the labia. The vaginal walls and surface of the cervix may have punctate, red “strawberry” spots. Urethritis and possibly cystitis may also occur.

Men are usually asymptomatic; however, sometimes urethritis results in a discharge that may be transient, frothy, or purulent or that causes dysuria and frequency, usually early in the morning. Often, urethritis is mild and causes only minimal urethral irritation and occasional moisture at the urethral meatus, under the foreskin, or both. Epididymitis and prostatitis are rare complications.

Diagnosis of Trichomoniasis

  • Microscopic examination of vaginal secretions, dipstick tests, or nucleic acid amplification tests (NAATs)

  • Culture of urine or urethral swabs from men

In women, diagnosis is based on clinical criteria and point of care (POC) testing. One of the following POC tests may be done:

  • Direct microscopic examination of vaginal secretions

  • Immunochromographic flow dipstick tests

Microscopic examination is the simplest method and enables clinicians to test for trichomoniasis and bacterial vaginosis Bacterial Vaginosis (BV) Bacterial vaginosis is vaginitis due to a complex alteration of vaginal flora in which lactobacilli decrease and anaerobic pathogens overgrow. Symptoms include a gray, thin, fishy-smelling vaginal... read more Bacterial Vaginosis (BV) at the same time. Tests for both infections should be done because they cause similar symptoms and/or may coexist. Vaginal secretions are obtained from the posterior fornix. The pH is measured. Secretions are then placed on 2 slides; they are diluted with 10% potassium hydroxide on one slide (KOH wet mount) and with 0.9% sodium chloride on the other (saline wet mount). For the whiff test, the KOH wet mount is checked for a fishy odor, which results from amines produced in trichomonas vaginitis or bacterial vaginosis. The saline wet mount is examined microscopically as soon as possible to detect trichomonads, which can become immotile and more difficult to recognize within minutes after slide preparation. (Trichomonads are pear-shaped with flagella, often motile, and average 7 to 10 micrometers—about the size of white blood cells—but occasionally reach 25 micrometers.) If trichomoniasis is present, numerous neutrophils are also present. Trichomoniasis is also commonly diagnosed by seeing the organism when a Papanicolaou (Pap) test is done.

Alternatively, immunochromographic flow dipstick tests or NAATs, which are available from some laboratories, may be done. In women, these tests are more sensitive than microscopic examination or culture. Also, NAATs can be configured to simultaneously detect other organisms or other STIs such as chlamydial infection or gonorrhea.

Culture of urine or urethral swabs is the only validated test for detecting T. vaginalis in men. In men, microscopy of urine is insensitive, and NAATs and dipstick tests have not been rigorously validated; however, epidemiologic studies suggest that for NAATs, urethral swabs are better than urine.

As with diagnosis of any STI, patients with trichomoniasis should be tested to exclude other common STIs such as gonorrhea and chlamydial infection.

Treatment of Trichomoniasis

  • Oral metronidazole or tinidazole

  • Treatment of sex partners

Metronidazole or tinidazole 2 g orally in a single dose cures up to 95% of women with trichomoniasis if sex partners are treated simultaneously. Effectiveness of single-dose regimens in men is not as clear, so treatment is typically with metronidazole or tinidazole 500 mg orally twice a day for 5 to 7 days.

If infection persists in women and reinfection by sex partners has been excluded, women are retreated first with metronidazole or tinidazole 2 g orally once or metronidazole 500 mg twice a day for 7 days. If the initial retreatment regimen fails, metronidazole or tinidazole 2 g once a day for 5 days may be effective.

Metronidazole may cause leukopenia, disulfiram-like reactions to alcohol, or candidal superinfections. It is relatively contraindicated during early pregnancy, although it may not be dangerous to the fetus after the 1st trimester. Tinidazole has not been established as safe during pregnancy and so is not used.

Sex partners should be seen and treated for trichomoniasis with tinidazole 2 g in a single dose or metronidazole 500 mg twice a day for 5 days and should be screened for other STIs. If poor adherence to follow-up by sex partners is likely, treatment can be initiated in sex partners of patients with documented trichomoniasis without confirming the diagnosis in the partner.

Key Points

  • Trichomoniasis can be asymptomatic, particularly in men, or cause vaginitis or sometimes urethritis.

  • In women, diagnose by microscopic examination of vaginal secretions, dipstick tests, or NAATs.

  • In symptomatic men, diagnose by culture of urine, urethral swab, or possibly NAATs.

  • Treat patients and their sex partners with oral metronidazole or tinidazole.

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