There is uncertainty about the answer to this. Some data are described below, however the burden of CT-related disease is difficult to estimate, due partly to the multiply causes of these disease, and there is debate around the use of different data sources and assumptions. General practice data have suggested that 165 000 cases of PID (according to the definition used) are diagnosed amongst reproductive age women each year, a prevalence of 1.7 per cent. The prevalence of disease was 2.2 per cent and 2.5 per cent within 16-19 and 20-24 year age groups respectively1. This study was based on a sample of 70 791 women attending general practice

The frequency of ectopic pregnancy treated in hospitals in England appears to be stable at around 1/100 conceptions (approximately 8000 cases per annum). This is similar to other European countries. If around 40 per cent of these cases are caused by chlamydial infection, this equates to approximately 3200 cases of ectopic pregnancy/annum.

The literature suggests that between 10-20 per cent of couples will experience problems with conceiving2. If around 10-20 per cent are due to tubal damage, and 40-60 per cent is caused by C. trachomatis , 0.5-2 per cent of couples may experience difficulty in conceiving because of Chlamydia (1990s figures). Very little is known about the epidemiology of complications seen in men and their burden within England.

Here are estimates from 3 sources: the National Survey of Sexual Attitudes and Lifestyles3 (NATSAL), the Chlamydia Screening Studies (ClaSS)⁴, and studies undertaken on clinical service attenders⁵.

NATSAL stratified probability sample survey of 11161 men and women aged 16 – 44 years in Britain in 20003.

Prevalence amongst participants in this study was 2.2 per cent (95 per cent CI 1.5-3.2) in men: 1.5 per cent (95 per cent CI 1.11-2.14) in women.

Age-specific prevalence was highest among men aged 25-34 (3.0 per cent: 95 per cent CI 1.7-5.1) and women aged 18-24 years (3.0 per cent: 95 per cent CI 1.7-5.0).

The prevalence in men aged 18-24 years was 2.7 per cent (95 per cent CI 1.2-5.8).

Risk factors: non-married status, age, and reporting partner concurrency or two or more sexual partners in the past year.

ClaSS A cross-sectional study based on general practice patient lists (14 382 patients included)⁴.

In people aged 16 to 24 participating in this study the prevalence was 5.1 (95 per cent CI 4.4-6.3) in men: and 6.2 per cent (95 per cent CI 4.9-7.8) in women.

Increased prevalence was associated with: having one or more new sexual partners in the past 12 months, single marital status, and there was weak association with more deprived areas.

Prevalence studies A systematic review of estimates of the prevalence of genital chlamydial infection in various population groups (women only) undertaken by Adams et al⁵.

This systematic review shows a clear pattern in the prevalence of infection in patients attending difference clinical settings. The results are summarised in table 1.

Table 1 Estimates of prevalence of genital chlamydial infection amongst attendees of different services: UK⁵*

Setting	<20 yrs	20-24 yrs	25-29 yrs	30+ yrs
Population	5.8	3.7	2.0	1.2
General practice	7.9	5.1	2.7	1.7
Family planning clinic	9.9	6.5	3.5	2.1
Youth clinic	10.7	7.0	-	-
Termination of pregnancy	12.3	8.1	4.4	2.7
Antenatal	14.1	9.4	5.2	3.1
Genitourinary medicine	16.8	11.2	6.2	3.8

* Women only, insufficient data available for men.

Some information comes from the Incidence/re-infection study undertaken by LaMontagne et al.⁶.

This UK study was an 18-month prospective cohort study of women aged 16-24 years recruited from general practices (GPs), family planning (FP) and genitourinary medicine (GUM)

Chlamydia incidence was 4.9 (2.7-8.8)/100 person-years among women recruited from GPs; 6.4 (4.2-9.8) FP; 10.6 (7.4-15.2) GUM. Incidence associated with young age, history of chlamydial infection, and new sex partner acquisition. Sexual behaviours determined incidence and re-infection, regardless of healthcare setting.

For PID, the observational studies are very small. In Randomised Control Trials (RCTs) attributable risks of 14 per cent and 40 per cent per year have been seen in the control populations^7-10. Again this is a very wide estimate and underlines the lack of evidence-base in this area of the literature.

Some studies are summarised in table 2.

Other references that report information on this include:
Morré S, van den Brule A, Rozendaal L, Boeke AJ, Voorhorst F, De Blok S et al. The natural course of asymptomatic Chlamydia trachomatis infections: 45% clearance and no development of clinical PID after one-year follow-up. Int J STD AIDS 2002; 13(Suppl 2):12-18.

van Valkengoed IG, Morré SA, van den Brule AJ, Meijer CJ, Bouter LM, Boeke AJ. Overestimation of complication rates in evaluations of Chlamydia trachomatis screening programmes--implications for cost-effectiveness analyses. Int J Epidemiol. 2004 Apr;33(2):416-25. Review.

Risser WL, Risser JM. The incidence of pelvic inflammatory disease in untreated women infected with Chlamydia trachomatis: a structured review. Int J STD AIDS. 2007 Nov;18(11):727-31. Review.

No longitudinal cohort studies have been undertaken to answer this question. All published data are derived from a combination of available data from surveillance and studies, and assumptions concerning disease progression. Although known to cause infertility, there is considerable uncertainty about how often this happens. There are other causes of infertility, and it is often difficult to know the cause for any case.

The retrospective population-based cohort study conducted by Low et al (2006) estimated the cumulative incidence of infertility by the age of 35 years to be 6.7% (5.7% to 7.9%)in women who ever tested positive for chlamydia, and 4.7% (4.4% to 5.1%), in those with negative tests.

Low N, Egger M, Sterne JA, Harbord RM, Ibrahim F, Lindblom B, Herrmann B. Incidence of severe reproductive tract complications associated with diagnosed genital chlamydial infection: the Uppsala Women's Cohort Study. Sex Transm Infect. 2006 Jun;82(3):212-8.

Chlamydial infection in men can lead to: pharyngitis, rectal infection urethritis, epididymitis, prostatitis, proctitis, Reiter's syndrome (which can be found in men & women).

However, infection is largely asymptomatic which suggests that they are less likely to consult clinical services and be diagnosed and treated.

It has been estimated that 5 per cent of men with genital chlamydial infection (symptomatology not specified) will develop epididymitis. The incidence and prevalence of Reiter's syndrome is unknown.

In its ‘Core Requirements’, the NCSP recommends annual screening for women aged 16-24 years who are sexually active, or sooner if partner change occurs.

Some data relevant to these recommendations come from the Incidence/re-infection study⁶.

Patients should be retested on partner change. There are no data on how soon after sex a test may become positive. In its ‘Core Requirements’, the NCSP recommends opportunistic screening at any time after sex with a new partner but if the change of partner occurred less than two weeks before screening, then the screen should be repeated two weeks later.

Treatment for chlamydia is effective and reduces the risk of PID. Mathematical modelling suggests that the prevalence of chlamydial infection could be reduced by approximately 30 per cent after 1 year and 70 per cent after 5 years if there is around 26 per cent coverage and 20 per cent partner notification in both men and women. Continuous screening at around 46 per cent coverage (20 per cent partner notification) could reduce prevalence by 40 per cent after 1 year and 80 per cent after 7 years¹³. Models are based on a number of assumptions about how infection behaves and how screening works, so there is uncertainty in the accuracy of their findings. Cost effectiveness analyses have suggested that chlamydia screening could be cost effective under certain conditions.

Evidence comes from two sources: randomised control trials and other screening programmes. The findings of some studies are shown in Table 3. Both RCTs showed a 50 per cent reduction in the incidence of PID after 1 year.

In Wisconsin, USA, intervention based on screening for genital chlamydial infection was followed by a reduced incidence of PID and ectopic pregnancy by 33 per cent and 20 per cent respectively over a five year period¹¹. However, it is not clear how much screening caused these declines.

A Swedish intervention programme also found screening for genital chlamydial infection was followed by rapidly reduced incidence of ectopic pregnancy amongst 20 to 24 year olds no evidence to support these associations is available from RCTs¹².It is not clear how much screening caused these declines.

Monitoring of chlamydia infection, of PID and other outcomes of chlamydia infection over a number of years is needed to show the results of the English programme.

Basically, if all else is equal, the higher the coverage the greater the expected benefit, over plausible ranges of coverage. At low levels of coverage the benefit may be difficult to see (too small to detect). Factors other than coverage also play a part in determining the benefits of screening.

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