Merck Manual

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Sheldon R. Morris

, MD, MPH, University of California San Diego

Full review/revision Jan 2021 | Modified Sep 2022

Trichomoniasis is a sexually transmitted infection of the vagina or urethra that is caused by the protozoa Trichomonas vaginalis and that causes vaginal irritation and discharge and sometimes urinary symptoms.

  • Women may have a greenish yellow, frothy, fishy-smelling vaginal discharge with irritation and soreness in the genital area.

  • Men usually have no symptoms, but a few have a frothy discharge from the penis and mild pain or discomfort during urination.

  • Examination of a sample of the discharge under a microscope usually enables doctors to identify trichomoniasis.

  • A single dose of an antibiotic cures most women, but most men need to take an antibiotic for 5 to 7 days.

Trichomonas vaginalis commonly causes a sexually transmitted infection (STI) of the vagina in women and an STI of the urinary tract in men and women. Women are much more likely to develop symptoms. About 20% of women develop trichomoniasis of the vagina (trichomonas vaginitis Trichomonas Vaginitis Trichomonas vaginitis is a vaginal infection due to the protozoa Trichomonas vaginalis. Trichomonas vaginitis is usually sexually transmitted. It can cause a green or yellow... read more ) during their reproductive years.

Symptoms of Trichomoniasis

In women, the infection usually starts with a greenish yellow, frothy, fishy-smelling discharge from the vagina. In some women, the discharge is slight. The genital area may be irritated and sore, and sexual intercourse may be painful. In severe cases, the genital area and surrounding skin may be inflamed, and the tissues around the vagina's opening (labia) may be swollen. Urination may be painful or frequent, as occurs in a bladder infection. Urinary and vaginal symptoms may occur alone or together.

Most men with trichomoniasis of the urethra (the tube carries urine from the bladder out of the body) have no or only mild symptoms, but they can still infect their sex partners. Some men have a frothy discharge from the penis, pain during urination, and an urge to urinate frequently.

Did You Know...

  • Most men with trichomoniasis have no symptoms, but they can still infect their sex partners.

Diagnosis of Trichomoniasis

  • Examination and sometimes culture of a sample of the discharge or urine

Doctors suspect trichomoniasis in the following people:

The organism is much more difficult to detect in men than in women. In women, the diagnosis can usually be made quickly by examining a sample of the vaginal discharge with a microscope and identifying the organism. If results are unclear, the sample is cultured for several days. In men, a sample of the discharge from the end of the penis (obtained in the morning, before urination) may be examined under a microscope and sent to the laboratory for culture. Alternatively, doctors sometimes do tests to identify the organism’s unique genetic material (DNA or RNA). Sometimes techniques that increase the amount of the bacteria's genetic material are used. These tests are called nucleic acid amplification tests (NAATS). These techniques make the organisms easier to detect,

Occasionally, microscopic examination of the urine detects Trichomonas, but identification is more likely if a urine culture is done.

Prevention of Trichomoniasis

The following general measures can help prevent trichomoniasis (and other STIs):

Not having sex (anal, vaginal, or oral) is the most reliable way to prevent STIs but is often unrealistic.

Treatment of Trichomoniasis

  • The antibiotic metronidazole or tinidazole

  • Simultaneous treatment of sex partners

A single dose of metronidazole or tinidazole (which are antibiotics), taken by mouth, cures up to 95% of infected women. However, their sex partners must be treated simultaneously or women may be reinfected. Sometimes, if a woman's partner is unlikely to return for follow-up, the woman is given drugs or a prescription to deliver to her sex partner (called expedited partner therapy Sex partners Sex partners ).

Metronidazole has side effects. If taken with alcohol, metronidazole may cause nausea and flushing of the skin. The drug may also cause a metallic taste in the mouth, nausea, or a decrease in the number of white blood cells. Women who take the drug may be more susceptible to vaginal yeast infections Vaginal Yeast Infection (Candidiasis) The vagina is infected by a yeast called Candida, usually Candida albicans, resulting in a yeast infection called candidiasis. Being pregnant or having diabetes or a weakened immune... read more (vaginal candidiasis).

Infected people should abstain from sexual intercourse until the infection is cured, or they can infect their partners.

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