This sexually transmitted infection affects women more than men, and is not considered a dangerous condition. Treatment is simple and usually very effective

If you haven't heard of this infection before, don't worry. It is the 'stepchild' of sexually transmitted infection - one that is hard enough to pronounce, let alone find information about why it is important to avoid.

Around 120 million women worldwide are diagnosed with trichomoniasis each year. It affects men and women but is diagnosed more frequently in women. In the UK in 2003, 6,152 women were diagnosed, compared to 283 cases in men, with the incidence staying fairly constant year on year. It is most prevalent in people aged 20-45.

Left untreated, it usually causes no symptoms in most cases ? especially in males - but it can cause vaginitis in some women and non-gonococcal urethritis in men.

If trichomoniasis is so common and causes significant risks to women's health, one would expect to find more interest in its prevention. There are several reasons why this STI has been ignored. First, its symptoms - primarily discharge and irritation of the vagina and urethra - have been seen as troubling but are not considered as serious as other STIs. And second, the most common treatment - a drug called metronidazole - is highly effective and has not been threatened by the development of resistant organisms that have caused problems in treating other STIs.

The causes
The organisms that cause trichomoniasis are protozoan (the simplest, single-cell organism in the animal kingdom) called trichomonads, the most common being Trichomonas vaginalis. This parasite resides primarily in the genitourinary tract, where it finds the warmth and moisture needed to grow and multiply.

Trichomoniasis is the least common of three types of vaginitis. The other two are bacterial vaginosis (BV), which is an overgrowth of normal bacteria, and vulvovaginal candidiasis, more commonly known as yeast infection. Trichomoniasis makes up only about five per cent of vaginal infections.

Complications
As with BV, recent research has clearly shown that trichomoniasis can be associated with pre-term delivery complications. Research also links trichomoniasis with an increased risk of HIV transmission (HIV is the virus that leads to AIDS). Although more study is needed, experts estimate that women infected with this STI are two to four times more likely to contract HIV than uninfected women. The reason for this increased risk is that vaginitis increases a response from the immune system, producing more of the cells that HIV targets for infection. Trichomonads can also harbour infective bacteria such as those causing chlamydia and gonorrhoea.

Trichomoniasis is transmitted primarily through sexual intercourse. It is most common in people who are at their peak of sexual activity. Poor hygiene, infection with other STIs and having multiple sexual partners are primary factors that put women at higher risk.

Symptoms
Around 50 per cent of women infected with trichomoniasis have no noticeable symptoms. Most symptoms occur within three weeks after being exposed to an infected partner. However, in some cases the symptoms may be delayed for years. The most common symptoms are vaginal discharge, irritation itching and a burning sensation during urination.

Trichomoniasis most often manifests itself in women as a yellowish-green, frothy, vaginal discharge. The discharge may sometimes have a bad odour, but usually does not. Itching and soreness of the vagina and vulva also are common. Some women experience abdominal pain due to cervicitis (inflammation of the uterine cervix), which is caused by trichomoniasis. This can be coupled with other infections, particularly pelvic inflammatory disease (PID).

The majority of infected men never have symptoms; of those presenting as the sexual partner of an infected woman, 15-50 per cent are symptom free. The most common symptom is discharge from the penis or a burning sensation during urination. The foreskin of the penis also may become inflamed. When men have symptoms, they often subside without treatment after a week or more.

Diagnosis
For both men and women, making a diagnosis on signs and symptoms alone is inadequate, particularly because of the broad range of symptoms that are shared with other inflammatory conditions of the vagina and cervix. The most common way to diagnose trichomoniasis is by looking at specimens under a microscope. These 'wet mounts,' as they are called, are magnified and clinicians look for the protozoa moving around.

Treatment
For more than 30 years one drug, metronidazole (Flagyl or generic), has been the standard to treat trichomoniasis with a success rate of 95 per cent. In most cases, infection is cured in women with a single oral dose of two grams. Pregnant women may be treated at any time. There is no reason for a follow-up visit if you do not have symptoms after the treatment or if you had no symptoms before treatment. For those who fail treatment, longer courses of metronidazole are recommended, usually for three to five days.

It is very important not to drink alcohol when taking metronidazole and for 48 hours after the drug treatment is stopped. This is because the two interact, causing the patient to be extremely unwell.

Tinidazole is a 'one-off' treatment. It is the same class of drug as metronidazole, but it is not recommended during pregnancy or when breastfeeding.

< It is important that all sexual partners are treated, and that you avoid sexual contact until given the all-clear from your prescribing doctor.

Prevention
Prevention is similar to that for other STIs, and includes:

• Use a latex condom each and every time you have sex
• Limit your number of sexual partners
• Get tested if you think you have an STI and have your partner tested, too

You can reduce your risk of acquiring vaginitis if you:

• Avoid douching
• Take antibiotics only as necessary

Prognosis
Metronidazole will cure 95 per cent of cases. In the remaining five per cent, failure to respond to metronidazole can be attributed to failure to comply with treatment or re-infection by an untreated partner. A repeat course of antibiotics usually cures the patient if treatment was not initially successful. In a minority of cases, co-existent streptococcal infection reduces efficacy of metronidazole and needs to be treated first before further treatment with the drug.