The infection
Trichomoniasis is caused by Trichomonas vaginalis, a motile single-celled parasite that colonises the vagina, urethra and sometimes the prostate. By case numbers it is the most common curable sexually transmitted infection worldwide, yet it is rarely looked for in countries whose screening focuses on chlamydia and gonorrhoea. It is diagnosed far more often in women, simply because it is nearly always silent in men.
Key facts
| Organism | Parasite Trichomonas vaginalis |
|---|---|
| Incubation | 5 to 28 days |
| Asymptomatic cases | About 70% of infected people, women and men alike (CDC, 2025) |
| Test | NAAT; direct microscopy misses a substantial share |
| Curable | Yes, with metronidazole |
| Pregnancy | Associated with increased risk of preterm birth |
| Vaccine | None |
How it is transmitted
Transmission occurs through vaginal sex. Oral and anal sex are essentially not involved, as the parasite does not colonise those sites. Indirect transmission through damp towels or recently soiled objects is theoretically possible but rare, since the parasite survives poorly outside the body. The absence of symptoms in men keeps the infection circulating within couples.
Symptoms
In women the classic picture is copious frothy yellow-green discharge with an unpleasant smell, itching, burning on passing urine and pain during sex. In men the infection is almost always silent; when it declares itself it is as mild urethritis or irritation after passing urine. That imbalance explains why the male partner is so often the undiagnosed reservoir.
Testing
NAAT is the most reliable test and can be done on a vaginal swab or on urine. Direct microscopy, long the standard, detects only part of infections, and a negative result does not exclude the diagnosis. Screening is not generally routine: the test is requested for suggestive symptoms or as part of a full panel.
Treatment
Treatment is with metronidazole. In women a seven-day course is now preferred to a single dose, which fails more often. The partner must be treated at the same time, even without symptoms and even without testing, otherwise reinfection is near certain. Alcohol should be avoided during treatment and for the following days because of an unpleasant reaction.
If left untreated
In pregnancy, trichomoniasis is associated with premature rupture of membranes, preterm delivery and low birth weight. Like any genital inflammation, it increases the risk of acquiring and transmitting HIV. In men it can sustain chronic urethritis or prostatitis.
Prevention
- Condoms protect effectively during vaginal sex.
- Treat the partner as a matter of routine, with or without symptoms and with or without a test.
- Think of trichomoniasis where malodorous discharge persists despite antifungal treatment.
- Clean or cover sex toys between uses.
After a diagnosis
- Treat all current sexual partners without waiting for their results.
- Avoid sex until both sides have completed treatment.
- Retest about three months after treatment, even if partners were treated (CDC, 2025): reinfection is common.
- Add tests for other infections, which frequently coexist.
Worth remembering
Malodorous discharge that does not respond to antifungal treatment should raise the possibility of trichomoniasis: it is not thrush, and the treatment is entirely different.
A little history
Trichomonas vaginalis was described in 1836 in Paris by Alfred Donné, who observed it under the microscope in vaginal secretions and presented it to the Academy of Sciences: the first single-celled parasite of humans ever identified. For nearly a century, however, it was held to be a harmless guest; its role in vaginitis was only established in 1916 by Höhne, who reproduced the infection by inoculating volunteers.
No effective treatment existed before 1959: that year the Rhône-Poulenc chemists Cosar and Julou obtained metronidazole, derived from a natural antibiotic, azomycin. Marketed as Flagyl, it cures trichomoniasis in a single dose and remains, with its cousins of the same family, the only treatment available — which makes the resistance observed since the 2000s worrying.
Paradoxically, the most common non-viral STI in the world — the WHO counted 156 million new cases in 2020 — is also one of the least known to the public and to screening programmes, no doubt because it is benign in most cases and easily cured.
How this differs between countries
Same parasite, same treatment — but very uneven testing, which produces misleading statistics.
- Rarely sought in Europe. Trichomoniasis is not part of standard screening panels in most European countries, which makes it appear rare there.
- Frequently diagnosed elsewhere. In sub-Saharan Africa and the Americas it is looked for more systematically and ranks among the most common infections.
- Diagnostic method. Direct microscopy, still used in many countries, misses a substantial share of cases; the more sensitive NAAT is not available everywhere.
- Partner treatment. Universally recommended but unevenly applied, which explains how often reinfection occurs.
- Pregnancy. Routine screening of pregnant women is practised in some countries and not others, despite the association with preterm birth.