The infection
Chlamydia is caused by the bacterium Chlamydia trachomatis, which infects the genital, anal, throat and eye mucous membranes. It is the most frequently reported bacterial STI in high-income countries, peaking between 15 and 25. Its defining feature is silence: most infected people have no idea they are carrying it, continue to pass it on, and only discover it through a complication or routine screening.
Key facts
| Organism | Bacterium Chlamydia trachomatis |
|---|---|
| Incubation | One to three weeks, often with no sign at all |
| Asymptomatic cases | Around 70% in women, 50% in men |
| Reliable testing from | About two weeks after exposure |
| Sample | Self-taken vaginal swab or first-void urine; throat and rectal swabs if exposed |
| Curable | Yes, with a one-week course of antibiotics |
| Vaccine | None |
How it is transmitted
It passes through unprotected vaginal, anal and oral sex, by contact between mucous membranes. Ejaculation is not required. It can also be passed from mother to baby during birth, causing conjunctivitis or pneumonia in the newborn. Sharing unwashed or uncovered sex toys transmits it too.
Symptoms
Where symptoms appear, they do so one to three weeks after exposure: discharge, burning on passing urine, pelvic pain, bleeding between periods or after sex in women; urethral discharge and discomfort in men. Rectal and throat infections are almost always silent. The essential point is that most infections produce nothing at all, and feeling well is never evidence of being uninfected.
Testing
Testing uses a nucleic acid amplification test, which is highly accurate. For women, a self-taken vaginal swab performs as well as a clinician-taken sample and is more comfortable. For men, first-void urine is sufficient, collected at least an hour after last passing urine. If you have anal or oral sex, ask specifically for rectal and throat swabs: a urine test alone misses those sites entirely. The result is reliable about two weeks after exposure.
Treatment
The first-line treatment is doxycycline, 100 mg twice daily for seven days, which outperforms single-dose azithromycin, particularly for rectal infection. Azithromycin remains an option in pregnancy or where adherence is a problem. Avoid sex for the full seven days and until partners have been treated, or reinfection is immediate.
If left untreated
Untreated, chlamydia ascends to the uterus and tubes and causes pelvic inflammatory disease, often as silently as the original infection. The consequences are chronic pelvic pain, ectopic pregnancy and tubal infertility: it is the leading preventable cause of infertility in women. In men it can cause epididymitis. Particular strains cause lymphogranuloma venereum, with severe rectal disease.
Prevention
- Condoms, including for oral and anal sex, remain the main protection.
- Annual screening is recommended for every sexually active person under 25, with or without symptoms and even with a single partner.
- Test at every change of partner, and before stopping condoms in a new relationship.
- Clean sex toys between uses and between partners, or cover them with a condom.
After a diagnosis
- Notify every partner from the last two months, and the most recent partner beyond that if there has been none since.
- Partners must be treated even if their test is negative and they have no symptoms.
- Retest three months after treatment: reinfection is far more common than treatment failure.
- Use the diagnosis as a prompt for a full screen: gonorrhoea, syphilis, HIV and hepatitis.
Worth remembering
A negative test taken too early means nothing. If the exposure was less than two weeks ago, repeat the test rather than relying on the first result.
A little history
Chlamydia trachomatis was first known through another disease it causes: trachoma, the blinding eye infection described in the Ebers papyrus in Egypt 3,500 years ago. In 1907, in Java, Ludwig Halberstaedter and Stanislaus von Prowazek observed characteristic "inclusions" in conjunctival cells and proposed the name Chlamydozoa, from the Greek for "cloak". For half a century it was taken for a virus: it does not grow on ordinary culture media.
The trachoma agent was isolated in 1957 by the Chinese microbiologist Tang Fei-fan, in embryonated eggs. In the 1960s it was established that it is a bacterium after all — but one that has to live inside cells — and it was understood that it also causes the "non-gonococcal" urethritis for which no microbe had been found. Diagnosis long remained difficult, confined to laboratories equipped for cell culture.
Everything changed in the 1990s with nucleic acid amplification tests, which can be run on a simple urine sample or a self-taken swab: the scale of a mostly silent infection came to light, and screening programmes for young adults were set up — England launched its own in 2003. Single-dose azithromycin, which arrived in the 1990s, and doxycycline remain the two reference treatments.
How this differs between countries
What differs between countries is neither the organism nor the treatment, but how hard anyone looks for it.
- Organised screening or none. Several European and English-speaking countries recommend annual testing for sexually active people under 25, sometimes with kits posted to the home. Elsewhere nothing is offered without symptoms.
- Syndromic management. Across much of sub-Saharan Africa and South Asia, laboratory diagnosis is not routinely available: treatment follows standardised algorithms based on symptoms. That works for the symptomatic person but misses the majority of infections, which are silent.
- First-line antibiotic. Seven days of doxycycline has replaced single-dose azithromycin in most recent guidance, but national protocols have not all been updated.
- Cost. Testing is free in dedicated services in many countries and entirely self-funded in others.
- Extragenital swabs. Routine throat and rectal testing is recommended in some countries and absent in others, even though those sites are almost always symptomless.