18.9
Trichomoniasis is a sexually transmitted infection caused by Trichomonas vaginalis.
It is a single-celled, anaerobic protozoan that lacks mitochondria and contains hydrogenosomes, which generate energy under anaerobic conditions within the human host.
Transmission occurs through direct sexual contact, after which the parasite colonizes the lower reproductive tract.
This parasite exists only in the trophozoite stage, which replicates asexually by binary fission.
In females, trophozoites adhere to the vaginal epithelium using surface adhesins.
Adhered trophozoites secrete cytotoxic proteins that degrade cellular junctions.
This results in cellular apoptosis and sloughing of epithelial cells, leading to mucosal inflammation and tissue damage.
The parasite also disrupts the normal vaginal microbiota.
Damage to epithelial cells increases the host’s susceptibility to other sexually transmitted infections, including HIV.
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.
Trichomoniasis is a sexually transmitted infection caused by Trichomonas vaginalis.
It is a single-celled, anaerobic protozoan that lacks mitochondria and contains hydrogenosomes, which generate energy under anaerobic conditions within the human host.
Transmission occurs through direct sexual contact, after which the parasite colonizes the lower reproductive tract.
This parasite exists only in the trophozoite stage, which replicates asexually by binary fission.
In females, trophozoites adhere to the vaginal epithelium using surface adhesins.
Adhered trophozoites secrete cytotoxic proteins that degrade cellular junctions.
This results in cellular apoptosis and sloughing of epithelial cells, leading to mucosal inflammation and tissue damage.
The parasite also disrupts the normal vaginal microbiota.
Damage to epithelial cells increases the host’s susceptibility to other sexually transmitted infections, including HIV.
From Chapter 18:
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