Trichomoniasis

Trichomonas vaginalis is a flagellated protozoan parasite and the causative agent of trichomoniasis, one of the most prevalent non-viral sexually transmitted infections in the United States. This extracellular parasite primarily colonizes the lower genitourinary tract in women—particularly the vagina—and in men, the urethra and prostate. Its structural and functional adaptations enable its survival, motility, and pathogenicity within the host environment.

Structural Features and Host Entry

T. vaginalis measures approximately the size of a white blood cell and possesses four anterior flagella and a single recurrent flagellum associated with an undulating membrane. These structures grant the organism a characteristic jerky motility observable under wet mount microscopy. The parasite adheres to epithelial cells in the urogenital tract via surface proteins and lectin-like adhesins, facilitating its colonization. Entry into the host typically occurs through direct sexual contact. Although T. vaginalis can survive for a few hours in moist environments, almost all transmissions are venereal.

Life Cycle and Reproductive Strategy:

Unlike many other protozoan parasites, T. vaginalis does not have a cyst form. It exists solely in its trophozoite stage, replicating through binary fission. Once introduced into the host, it remains localized to the mucosal surfaces of the vagina, urethra, or prostate, without systemic dissemination. The absence of a cyst stage limits environmental survival but ensures effective transmission via sexual activity.

Cellular Pathogenesis and Host Response

At the cellular level, T. vaginalis releases cytotoxic proteins, such as pore-forming proteins and cysteine proteases, which degrade host epithelial cells. This cytotoxic activity disrupts the epithelial barrier, triggering inflammation and elevating vaginal pH above 4.5. The destruction of epithelial integrity facilitates bacterial overgrowth and promotes increased susceptibility to other sexually transmitted infections, particularly HIV. In women, these changes manifest as frothy, malodorous discharge, erythema, and in some cases, a characteristic “strawberry cervix.” In men, the organism is often asymptomatic but can occasionally cause urethritis, prostatitis, or epididymitis.

The parasite's ability to evade host immune defenses and cause epithelial damage underscores the need for effective diagnosis, treatment, and partner management to disrupt its life cycle and prevent reinfection.