T. vaginalis (Trichomonas vaginalis) is a single-celled parasite that causes trichomoniasis, the most common curable sexually transmitted infection worldwide. The CDC estimated more than two million infections in the United States in 2018 alone, making it more prevalent than gonorrhea and chlamydia combined. Despite those numbers, most people have never heard of it, partly because the infection often produces no symptoms at all.
The Parasite Itself
T. vaginalis is a protozoan, a microscopic organism far larger and more complex than a bacterium but still invisible to the naked eye. A single organism measures roughly 9 by 7 micrometers, about the size of a white blood cell. It has a pear-shaped body with four whip-like flagella at the front that propel it through fluid and an undulating membrane along one side that ripples as it moves. Under a microscope, you can see it darting around or beating its flagella in place.
Unlike many parasites, T. vaginalis does not form cysts, the dormant, hard-shelled stage that helps other organisms survive outside a host. This means it is fragile in the environment and dies quickly on dry surfaces, in water, or in extreme temperatures. It survives by living exclusively in the warm, moist lining of the human urogenital tract. It has no other natural host.
One unusual feature is its energy-producing organelles called hydrogenosomes, which replace the mitochondria found in most other cells. These allow T. vaginalis to thrive in the low-oxygen environment of the vagina and urethra. When the parasite attaches to tissue, it shifts from its pear shape to a flattened, amoeba-like form that clings to the vaginal or urethral lining.
How It Spreads
Trichomoniasis spreads through sexual contact. The parasite passes between partners during vaginal intercourse and can infect the vagina, vulva, urethra, or the opening of the penis. Because T. vaginalis cannot survive long outside the body and does not form protective cysts, transmission through towels, toilet seats, or swimming pools is extremely unlikely. Infection is more common in women than in men, and older women are more likely to carry it than younger women.
Symptoms in Women
More than half of women infected with T. vaginalis develop vaginal discharge. The discharge is often frothy, yellow-green, and accompanied by a strong or unpleasant odor. Other common symptoms include itching, burning, redness, and soreness around the genitals, along with pain during urination or intercourse.
One distinctive sign that clinicians look for is called “strawberry cervix,” a pattern of tiny red spots on the cervix caused by pinpoint hemorrhages where the parasite has inflamed and damaged the tissue. This appearance is not something you would notice yourself, but it is a hallmark finding during a pelvic exam. The remaining women with trichomoniasis have no symptoms at all and can unknowingly carry and transmit the infection for months or even years.
Symptoms in Men
The picture in men is different. The World Health Organization estimates that only about 10% of infected men develop urethritis or urethral discharge, though some research suggests the symptomatic rate could be higher. When symptoms do appear, they typically include a clear or whitish discharge from the penis, burning during urination, and sometimes irritation or itching at the tip of the penis. Because so many men remain asymptomatic, they often serve as undiagnosed carriers, reinfecting partners after treatment.
How Trichomoniasis Is Diagnosed
The traditional method is a wet mount: a clinician places a drop of vaginal fluid on a slide with a bit of saline and looks for the parasite swimming under a microscope. It is cheap and fast, but it catches only 44% to 68% of infections. Many cases are missed simply because there are not enough organisms in the sample or the parasites have stopped moving by the time the slide is examined.
Culture testing, where a sample is placed in a growth medium and checked over several days, was long considered the most sensitive non-molecular method. Its sensitivity ranges from 44% to 75%. Today, molecular tests that detect the parasite’s genetic material (nucleic acid amplification tests) are the most accurate option and are increasingly used in clinical labs. These can pick up infections that both wet mount and culture would miss.
Link to HIV and Other Complications
T. vaginalis does more than cause discomfort. The infection triggers an aggressive inflammatory response in the vaginal and cervical tissue. This inflammation disrupts the protective mucosal barrier in several ways: it causes tiny hemorrhages in the tissue, alters the normal vaginal pH and bacterial balance, and suppresses local immune factors that normally have antiviral properties. All of these changes create easier entry points for HIV. People with trichomoniasis face a measurably higher risk of both acquiring and transmitting HIV.
In pregnancy, trichomoniasis has been associated with preterm birth and low birth weight. The infection can also increase susceptibility to other sexually transmitted infections and may contribute to pelvic inflammatory disease in women.
Treatment and Reinfection
Trichomoniasis is curable with oral antiparasitic medication, typically taken either as a single large dose or as a shorter course of smaller doses over several days. The treatment is effective in most cases, but reinfection is common. Studies show that reinfection rates are high, largely because sexual partners are not treated at the same time. If only one partner takes medication, the untreated partner simply passes the parasite back.
For this reason, health guidelines recommend that sexual partners be treated simultaneously, even if they have no symptoms. Retesting is advised within three months of treatment to catch reinfection early. You should avoid sexual contact until both you and your partner have completed treatment and any symptoms have resolved.
Why It Often Goes Undetected
Trichomoniasis falls through the cracks of routine healthcare more than most STIs. It is not part of standard STI screening panels in many clinical settings, and many providers do not test for it unless a patient specifically reports symptoms. The high rate of asymptomatic infection in both men and women means millions of people carry T. vaginalis without knowing it. Combined with the relatively low sensitivity of traditional diagnostic methods, this creates a large reservoir of undiagnosed infections circulating in the population. If you are sexually active and concerned about STIs, requesting a specific test for trichomoniasis is the most reliable way to know your status.