Which disease comes to mind when you read about sexually transmitted infection? AIDS for starters. Maybe syphilis and gonorrhoea and possibly the fungal infection, candida.

But not many will have heard of chlamydia trachomatis, even though it is the commonest curable sexually transmitted bacterial infection in the Western world.

With prevalence rates up to 29 per cent in some at-risk populations, chlamydia infection is a public health challenge, made even more difficult by the fact that it does not produce symptoms in 70 per cent of women and 50 per cent of men.

Unrecognised infection in women can have serious consequences; chlamydia carries with it an increased risk of infertility and ectopic pregnancy. Of the 6 per cent of couples who experience infertility, the cause in 15 per cent of cases will eventually be traced to Fallopian tube damage in the woman. Chlamydia is also the main preventable cause of ectopic pregnancy, again because of the inflammatory reaction it sets up in the Fallopian tubes.

So how can such a common infection, which is easily treated, be on the increase, leaving a destructive wake of reproductive misery in its path?

First, the higher levels of chlamydia infection reflect a general increase in sexually transmitted disease in the Republic. The latest AIDS figures show a significant increase in the number of people who have contracted HIV infection. There is also a parallel with the increasing incidence of syphilis and gonorrhoea.

Dr Colm Bergin, consultant in infectious diseases at the department of genito-urinary medicine and infectious diseases (GUIDE) at St James's Hospital, Dublin, says the increase in sexually transmitted infection (STI) is across all socio-economic groups and all levels of education. "Young people in particular perceive a lesser risk. Also, people's perception of what constitutes safe sex differs, and what may represent safe sexual practice for HIV may not prevent all STIs," he says.

Testing for chlamydia is carried out on all patients attending STI clinics in the Republic, regardless of symptoms. Figures from the GUIDE clinic at St

James's Hospital show a steady increase in the number of cases of chlamydia in the last 10 years, with levels of infection rising from 4 per cent in 1989 to 9.6 per cent in 1999.

Another reason for the unchecked rise in chlamydia levels is the difficulty in making a diagnosis. Because chlamydia is asymptomatic in most women, they remain unaware of the infection's presence. Even those women who do develop symptoms get only a very mild pain when passing urine or a slight, clear vaginal discharge.

While a higher percentage of men develop symptoms, many do not. Chlamydia causes nonspecific urethritis (NSU) - an inflammation of the lining inside the penis - which leads to discomfort when passing water and a slight discharge.

Women who have no symptoms do not attend for investigation and treatment. But even if they did, until recently there was no good test available to make a diagnosis.

The usual way of detecting genital infection is to take a swab. However, conventional swabs are useless at detecting chlamydia. A special type of swab, which has to be stored under particular conditions and analysed in a laboratory within hours of being taken, is the only way to test for the disease. Dr Mary Short, a Dublin GP with a special interest in women's health, says this situation effectively means she can only test for chlamydia during morning surgery hours. The special swab then has to be couriered to the virus reference laboratory in UCD for immediate analysis. If a woman presents with possible chlamydia at an evening surgery, it is too late in the day for analysis of the delicate swab to be done.

"A woman with a long-standing clear vaginal discharge which has not responded to anti-fungal treatment for thrush and which does not have the features of other possible causes of infection warrants investigation," she says. Recurrent bladder infection (cystitis) in younger, sexually active woman is another reason to suspect the presence of the bacteria, according to Dr Short. She sees a possible case of chlamydia three to four times a week among the relatively young, middle-class population who attend her practice.

A new test based on urine analysis has now been developed and is available in specialist units, but it will be some time before comprehensive chlamydia testing is available in GP surgeries.

GIVEN the serious consequences of the chlamydia trachomatis infection, should we be screening for the disease, at least in the female population? The medical consensus on this is a definite yes, from microbiologists to GPs to specialists in infectious disease.

Dr Mary Cafferky, consultant microbiologist at the Rotunda Hospital in Dublin, believes that: "Based on international studies and the experience of the STD clinic in St James's, an opportunistic screening programme should be considered. The Rotunda will be carrying out a pilot screening programme in the near future."

The pilot study will look at women attending antenatal clinics and those presenting to the hospital with symptoms of pelvic inflammatory disease. Dr Cafferky is confident that the latest urine test - a ligase chain reaction test, which detects chlamydial DNA - will be effective. Studies have shown that the new test is twice as sensitive as older tests, with a low rate of false negative results.

Dr Bergin says: "Whilst we are in favour of a national screening programme, it must be performed in tandem with a full sexual health screen for other diseases in addition to the development of an infrastructure to ensure partner notification and treatment. "You also need to ensure that those carrying out the screening would have adequate expertise. This infrastructure should incorporate sexually transmitted infection clinics, local microbiology departments, the National Disease Surveillance Centre and health board departments of public health. As an additional key element, we will require a chlamydia reference laboratory."

Family doctors are in favour of targeted screening based at general practices, especially now that a reliable and user-friendly test has been developed.

Case studies from Sweden and Wisconsin in the US have shown a reduction in the prevalence of chlamydia with the introduction of screening. Significantly, from an infertility perspective, a trial in the US has demonstrated a 56 per cent reduction in the incidence of pelvic inflammatory disease in women assigned to a chlamydia screening programme.

So who should be screened? A young age and a recent change in sexual partner are the most commonly quoted risk factors for infection. In the UK, a pilot study currently underway is targeting sexually active women under 25 and those over 25 with a new sexual partner or more than two sexual partners in the past year. Partner tracing is an important element of any programme.

Dr Bergin is also keen that we consider the introduction of sexual health clinics in this country. "I think this would be a good idea which would help change the focus in people's attitude to sex," he says. The clinics would offer proactive sexual health screening as a way of promoting preventative sexual health.

In the meantime, with up to 70 per cent of women suffering from infertility caused by Fallopian tube damage showing evidence of chlamydia infection, the sooner we start an effective national screening programme for chlamydia, the better.