Urogenital Protozoa
Trichomonas vaginalis is a sexually transmitted flagellate protozoan and the causative organism of trichomoniasis, one of the most common non-viral sexually transmitted infections worldwide. Unlike the intestinal flagellate Giardia, it colonises the urogenital tract and has a life cycle restricted to a single morphological form.
Trichomonas vaginalis exists only as a trophozoite; it has no cyst stage at any point in its life cycle.
The trophozoite is pear-shaped to ovoid, measuring roughly 7 to 23 micrometres, with a single anterior nucleus. Four anterior flagella provide forward motility. A fifth flagellum is recurved along the outer margin of an undulating membrane that runs partway down the body.
This produces a characteristic jerky, twitching motility on wet mount microscopy, which aids rapid presumptive diagnosis.
An axostyle, a rigid rod-like structure, runs longitudinally through the cell and protrudes posteriorly. It is thought to contribute to attachment and possibly to mechanical damage of host epithelium.
The organism is anaerobic and possesses hydrogenosomes rather than classical mitochondria. These organelles generate hydrogen and ATP under anaerobic conditions. They are the biochemical basis for susceptibility to nitroimidazole drugs, which require reduction by hydrogenosomal enzymes to become active.
The complete absence of a cyst stage has direct implications for transmission biology and organism survival. Without a resistant cyst wall, Trichomonas vaginalis cannot survive prolonged desiccation, gastric acid exposure, or significant environmental persistence. The trophozoite itself is relatively fragile outside a moist, warm environment.
This rules out the faecal-oral and waterborne transmission routes seen with cyst-forming intestinal flagellates and amoebae. Effective transmission is confined almost entirely to direct mucosal contact, principally sexual contact.
The organism can survive for limited periods on moist fomites (damp towels, shared moist surfaces), which accounts for rare instances of apparent non-sexual transmission. In young children, sexual abuse must nonetheless always be considered and excluded.
Trichomonas vaginalis is transmitted almost exclusively through vaginal intercourse, with the organism colonising the vaginal epithelium in women and the urethra (and occasionally prostate) in men. Vertical transmission from an infected mother to a neonate during vaginal delivery is recognised, producing transient neonatal vaginal or respiratory colonisation that is usually self-limited as maternal oestrogen effects wane.
As with other sexually transmitted infections, concurrent testing for other STIs is standard practice given shared risk factors and frequent co-infection, and trichomoniasis itself is associated with an increased risk of HIV acquisition and transmission due to the mucosal inflammation it produces.
In women, the trophozoite colonises the vaginal epithelium and endocervix, adhering to epithelial cells and producing a range of host responses from asymptomatic carriage to marked inflammation. Symptomatic infection classically produces a profuse, frothy, malodorous, yellow-green vaginal discharge, vulvovaginal irritation and pruritus, dysuria, and dyspareunia.
On speculum examination, a "strawberry cervix" (colpitis macularis), punctate haemorrhagic lesions of the cervix and vaginal wall, is a classic though relatively insensitive finding when present. Vaginal pH is characteristically elevated (above 4.5), overlapping with bacterial vaginosis and complicating diagnosis on pH alone.
A substantial proportion of infected women, commonly cited as up to half, are asymptomatic or have minimal symptoms, serving as an important reservoir for ongoing transmission.
In men, infection is more often asymptomatic (a majority of infected men have no symptoms), but Trichomonas vaginalis can produce urethritis (dysuria, mild discharge), and less commonly epididymitis or prostatitis. Because male infection is frequently mild or silent, men often serve as an unrecognised source of ongoing transmission to sexual partners, which has implications for partner treatment strategy.
Wet mount microscopy of vaginal or urethral secretions examines for motile trophozoites with characteristic jerky movement. It is rapid and inexpensive but has only moderate sensitivity (roughly 50 to 70 percent).
Sensitivity declines further if the specimen is not examined promptly, because motility, the key feature relied upon, is lost as the organism cools and dies outside the body. Immediate bedside examination is therefore important when wet mount is used.
Culture, historically considered a reference standard, has higher sensitivity than wet mount. It requires special media (such as Diamond's medium) and takes several days for results, which limits its practical use in routine care.
Nucleic acid amplification testing (NAAT, including PCR-based assays) is now the most sensitive and specific diagnostic method available. It detects infection missed by wet mount and culture. It is increasingly the preferred test where available, including on self-collected vaginal swabs or urine samples, which improves accessibility and screening uptake.
Point-of-care antigen tests offer rapid results with sensitivity intermediate between wet mount and NAAT. They are useful where molecular testing is not accessible.
Papanicolaou (Pap) smear can incidentally detect Trichomonas vaginalis during routine cervical cytology. However, it has poor sensitivity and a relevant false-positive rate. A Pap smear finding of Trichomonas should generally be confirmed with a dedicated diagnostic test before treatment, particularly in low-prevalence populations.
