Parasites

Morphology

Cytology:
Trophozoites 10-20 µm length and 7-10 µm width, pear-shaped to ovoid, eccentrically placed nucleus, eosinophilic intracytoplasmatic granula, four anterior and one posterior flagella, and an undulating membrane. On PAP smears, often degenerated forms without visible flagella and with irregular outlines are seen; epithelial cells may show perinuclear clearing. Background often with abundant neutrophils; frequent mixed bacterial flora; small round parabasal epithelial cells may be increased. Trichomonas can co-occur with Leptothrix spp., long filamentous bacteria, which create a “spaghetti” appearance - the field may resemble a “spaghetti and meatballs” pattern.
Rapid, jerky motility in fresh wet mounts.

Histology:
Superficial mucosal inflammation with neutrophils and microabscesses in the epithelium; organisms are difficult to identify and are rarely seen in routine sections.

Differential Diagnosis:

• Sexually transmitted bacterial infections
• Giardiasis - anal Pap tests

Clinical notes

Transmission via sexual contact or during birth. Trichomoniasis is the most prevalent non-viral sexually transmitted disease worldwide, with humans as the only known reservoir for T. vaginalis.

In women: vaginitis with pruritus, burning, dysuria, malodorous discharge, and elevated vaginal pH; a “strawberry cervix” (petechial hemorrhages on the ectocervix) may be present.
In men: urethritis or prostatitis, mostly asymptomatic. 
Associated with adverse pregnancy outcomes such as preterm birth, infertility, and increased risk of HIV transmission

Ancillary Testing

Nucleic acid amplification test (NAAT), such as PCR or TMA, from vaginal swab, cervical swab, urethral swab, or first-void urine. Wet mount from fresh vaginal fluid has low sensitivity. Antigen rapid tests can be used as an adjunct. Consider testing for co-infections, especially BV and other STIs.

Additional Information

Trichomonads are often an incidental finding on cervical PAP smears. 
Conventional PAP is unreliable for diagnosing T. vaginalis, so confirmation is recommended. Liquid-based cytology (LBC) is more accurate, with reported sensitivity of 60–96% and specificity of 98–100%, so treating a clear LBC-positive result without additional testing may be reasonable. In men, microscopy of urethral swabs or urine sediment is unreliable due to low organism burden.
The sexual partner should always be treated concurrently, since the likelihood of prior transmission is high, and this helps prevent a ping-pong reinfection effect.