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Microbiology · Intestinal and urogenital protozoa

Trichomonas vaginalis !

  • Parasite
  • Exam priority: medium

A sexually transmitted parasite that causes frothy, foul-smelling vaginal discharge; in men it is usually silent.

Updated 2026-11-01

Also called: trich

At a glance

Shape
Pear-shaped trophozoite with four anterior flagella and undulating membrane
Family
Trichomonadidae
Type
Urogenital flagellated protozoan (trophozoite only)
Stain
Wet mount: jerky motility
Size
Trophozoite about 7 to 30 µm

Morphology

  • Pear-shaped trophozoite with four anterior flagella and one running along the edge of the undulating membrane.
  • Large nucleus at the front end and an axostyle sticking out at the back.
  • No cyst form; survives poorly outside the body.

Transmission

  • Sexual contact, mainly vaginal.
  • Rare perinatal transmission.
  • Humans are the only host.

Reservoir: Humans

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. The trophozoite passes between people during sexual contact.
  2. In women it colonizes the vagina, urethra and glands; in men, the urethra and prostate.
  3. It multiplies by longitudinal binary fission on the mucosa.
  4. There is no cyst: the cycle depends on direct person-to-person contact.

Virulence factors

  • Adhesins that anchor it to epithelium and proteases that damage the mucosa.
  • Contact-dependent cytotoxicity and an inflammatory response rich in neutrophils.
  • Raises the risk of acquiring and transmitting HIV.

Diseases it causes

  • Trichomoniasis: vaginitis with frothy yellow-green discharge and urethritis; in men it is usually asymptomatic. Trichomoniasis →

Clinical picture

  • Copious, frothy, yellow-green, foul-smelling discharge; itching and painful urination.
  • «Strawberry cervix» (punctate hemorrhages), seen mostly on colposcopy.
  • Raised vaginal pH.
  • Pregnancy: linked to preterm birth, premature rupture of membranes and low birth weight.

Diagnosis

  • Nucleic acid amplification tests (NAAT): the most sensitive and preferred.
  • Wet mount of discharge: motile trophozoites; cheap but insensitive, and must be read right away.
  • Culture: more sensitive than wet mount but takes days.
  • Testing for other sexually transmitted infections, including HIV.

Treatment

Nitroimidazoles: metronidazole (the drug of choice, also in pregnancy) or tinidazole. Sex partners are treated and sex is avoided until everyone has finished treatment. Metronidazole vaginal gel does not work for this infection.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Condoms during sex.
  • Simultaneous treatment of partners.
  • Screening of people with HIV and those with other STIs.

Vaccine: There is no vaccine.

Test yourself

Think of the answer, then tap the question to see it.

What does the cervix look like in trichomoniasis?

A «strawberry cervix», with small punctate hemorrhages, best seen on colposcopy.

What is the most sensitive test for Trichomonas vaginalis?

Nucleic acid amplification tests (NAAT); the wet mount picks up only a portion of cases.

Why is the sexual partner treated even without symptoms?

Because men are often silent carriers and reinfect the patient if left untreated.

Which life stage does Trichomonas lack compared with Giardia?

The cyst: Trichomonas exists only as a trophozoite, so it needs direct contact to spread.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Trichomoniasis (CDC, 2024); Trichomoniasis - STI Treatment Guidelines (CDC, 2021).

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Representative model, for educational purposes only.