Measuring the impact of the Chlamydia trachomatis epidemic: new serological assays to assess disease burden (London)

In the last few years, there has been a considerable increase of public awareness about genital infection with ?chlamydia?. Many young women now appreciate that they can be infected without having any symptoms and that untreated infection can prejudice their fertility in later life. There is also evidence that this infection is becoming more common due to increasing high risk behaviour among the young. However we have no reliable way of knowing this. In response to the problem, the Department of Health are currently putting in place a nationwide programme to screen women for chlamydial infection.

However, there are still many aspects of chlamydial biology that we do not understand. Critically, although we know that chlamydial infection can cause damage to the womb (pelvic inflammatory disease (PID)) and subsequent infertility, ectopic pregnancy and chronic pelvic pain, we do not know how often this occurs. This is because it can be difficult to sometimes tell sometimes whether a woman has PID as some do not have symptoms ? so called ?silent PID?.

Infection with chlamydia stimulates the production of antibodies, often detectable in the blood, to help protect us. Examination of women with high antibody levels has shown that the majority have damage to their womb and antibody levels are linked to the degree of damage. The measuring of chlamydial antibodies in the blood could thus give us a way to measure how many women have been infected with chlamydia and what proportion have developed upper genital tract infection. However, there are a number of problems to overcome. The best tests for chlamydial antibody used at present are unsuitable for testing large numbers of blood samples and many also detect antibodies to a similar infection - Chlamydia pneumoniae which is a common respiratory infection. We propose to develop two new test that will utilise established technology for automation. The application of these tests to large numbers of blood samples will have three important outcomes:

1. When used over time, it will allow us to assess the effectiveness of the chlamydial screening programme in reducing not only the number of people infected but also in preventing chlamydial PID.
2. It will identify women who have had chlamydial PID at an early stage, who may need subsequent assistance to conceive.
3. It will identify some women who may have ongoing chlamydial infection with ?silent? PID and who might benefit from treatment.

Chlamydia trachomatis is the most common curable sexually transmitted infection in Britain. In opportunistic screening programmes 5-10% of sexually active women under 25 are ?currently? infected. These programmes recruit a selected fraction of the population and largely exclude men. C. trachomatis is frequently asymptomatic and if untreated is a major cause of pelvic inflammatory disease (PID) and tubal infertility, conditions which are difficult to diagnose non-invasively. While screening gives some indication of current infection, little is known about the proportion of the population ever infected with C. trachomatis (cumulative incidence) and of these who have ever developed pelvic disease.

Antibody assays, if sufficiently accurate, could be used in epidemiological studies as a measure of population disease burden, and thus as a measure of the population impact of screening programmes over time (e.g. antibody prevalence by age 25); as a measure of previous undiagnosed infection; and, through the use of quantitative antibody assays, as a potential clinical tool for screening of PID and tubal adhesions and therefore subfertility. However no practical assays, which have been sufficiently rigorously assessed, exist for these purposes.

We propose to develop and characterise sensitive, specific, and high throughput chlamydia antibody tests for use as sero-epidemiological tools to assess disease burden and clinical outcome associated with C. trachomatis genital tract infection:

The two tests will be:
1) An enzyme linked immunosorbent assay, using a C. trachomatis-specific recombinant protein.
2) A modification of the whole immunofluoresence test (WIF) based on flow cytometry (WIF-FC).

There is some evidence that WIF antibody titres correlates with tubal damage, but interpretation of the assay is problematic as it also detects cross reactive C. pneumoniae antibodies. To address this, we shall preadsorb test sera with C. pneumoniae lipopolysaccharide, derived from an Escherichia coli expression system.

Specificity and sensitivity of these novel antibody assays as markers of prior infection will be assessed using sera from patients with C. trachomatis diagnosed by standard clinical methods (e.g. PCR), at differing time periods following infection, and anonymised sera of C. trachomatis unexposed controls aged 10-14yrs from a serum bank. These will be compared to one of the best performing commercially available assays. We will then extend the evaluation to establish whether quantitative modified WIF antibody ?titres? can be utilised as a serological test predicting pelvic disease by testing sera from patients with serological evidence of prior C. trachomatis infection with and without laparoscopic evidence of pelvic disease.