Trichomonas vaginalis

Trichomonas vaginalis is an anaerobic, flagellated protozoan responsible for the sexually transmitted infection trichomoniasis. It is the most common pathogenic protozoan infecting humans in industrialized nations. Infection rates are comparable between men and women, but women tend to develop symptoms, whereas men usually remain asymptomatic. Transmission occurs through direct skin-to-skin contact, most often during vaginal intercourse. Worldwide, around 160 million new infections are acquired each year; in North America alone, 5 to 8 million new cases occur annually, with up to half of those being asymptomatic. Standard treatment involves metronidazole or tinidazole. Recent research has clarified the parasite’s evolution, genomic complexity, and pathogenic mechanisms. Population studies and genomic sequencing highlight genetic variability and emerging treatment resistance. Understanding host-pathogen interactions and improving prevention remain key public health goals.

**Clinical**

**History** In 1836, Alfred Francois Donné first described a diagnostic procedure for trichomoniasis, based on microscopic observation of motile protozoa in vaginal or cervical secretions. He published this in *Comptes rendus de l'Académie des sciences* under the title “Animalcules observés dans les matières purulentes et le produit des sécrétions des organes génitaux de l'homme et de la femme,” and gave the parasite its binomial name, *Trichomonas vaginalis*. Eighty years later, in 1916, Hohne recognized trichomoniasis as a distinct clinical entity.

**Signs and Symptoms** Most infected women (85%) and men (77%) are asymptomatic. Among women, half may develop symptoms within six months, including vaginal erythema, dyspareunia, dysuria, and a diffuse, malodorous, yellow-green vaginal discharge, along with genital itching. Strawberry cervix appears in about 5% of women. In men, the infection can cause urethritis, epididymitis, and prostatitis.

**Complications** In women, complications include preterm delivery, low birth weight, increased mortality, and a predisposition to HIV infection and cervical cancer. The parasite can be found in the urinary tract, fallopian tubes, and pelvis, and may lead to pneumonia, bronchitis, and oral lesions.

**Diagnosis** On a cervical smear, infected women sometimes show a transparent “halo” around the superficial cell nucleus; more typically, the organism appears with a slight cyanophilic tinge, faint eccentric nuclei, and fine acidophilic granules. Because of low sensitivity, diagnosis via genital discharge or cervical smear is unreliable. A wet mount, where motility is observed, is also used. The most common current method is overnight culture, with 75–95% sensitivity. Newer approaches, such as rapid antigen testing and transcription-mediated amplification, offer higher sensitivity but are not yet widespread.

**Prevention and Treatment** Infection is cured with metronidazole or tinidazole. The CDC recommends a single 2-gram dose of either drug as first-line treatment; if that fails, 500 milligrams of metronidazole twice daily for seven days is an alternative. Sexual partners should also be treated, as they may be asymptomatic carriers. Trichomoniasis is the most prevalent non-viral sexually transmitted disease, with about 156 million new cases reported worldwide in 2020. It is curable and transmitted through unprotected intercourse. Consistent condom use and screening of sexually active individuals help prevent infection. Symptoms include urethral or vaginal irritation, unusual discharge, genital itching, frequent urination, and dysuria. Antigen detection and nucleic acid amplification tests can aid diagnosis. Although resistance rates are low, they remain a growing public health concern. No vaccine exists, making prevention and early treatment essential.

**Pathogenesis** *T. vaginalis* causes infection by adhering to and breaking down vaginal epithelial cells. It secretes extracellular vesicles that disrupt immune function and enhance adhesion. Some strains carry intracellular viruses or bacteria that worsen infections. The parasite is linked to disruptions in vaginal microflora. Neutrophils are the primary immune cells involved; they can kill the parasite through trogocytosis.

**Morphology** Unlike other metamonads such as *Giardia duodenalis*, *T. vaginalis* exists only as a trophozoite and cannot form cysts. Its shape is variable: in culture away from the host, it is typically pear- or oval-shaped, but on vaginal epithelial cells it becomes more amoeboid. It is slightly larger than a white blood cell, measuring about 9 × 7 μm. In both forms, it has five flagella—four at the anterior end and one at the posterior.