Chlamydia

• 1. Introduction

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  Chlamydia is the most common bacterial sexually transmitted infection in the United Kingdom, and it affects roughly 4.2% of women and 2.7% of men worldwide. In 2015 alone, some 61 million new cases occurred across the globe. Yet most of the people carrying it have no idea. At least 70% of genital infections in women produce no symptoms at all. Half that number in men are similarly silent. This is why clinicians have given it an informal name: the silent epidemic. How does a bacterium spread so widely while announcing itself so rarely? What happens inside the body when it goes undetected? And what does it take to stop it? Those questions run through everything that follows.

• 2. The Silent Epidemic

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  Chlamydia trachomatis, the bacterium responsible for the infection, only infects humans. Its name comes from the Greek word for cloak, khlamyda, and that origin is fitting. The organism conceals itself effectively inside host cells, establishing long-term associations that can persist for months or years. Long-term cohort studies show that approximately 50% of those infected clear the infection within a year, 80% within two years, and 90% within three years. During all that time, the infection can go unnoticed and untreated.

  When symptoms do appear, they tend to surface only several weeks after the initial exposure. The incubation period between exposure and the ability to infect others is thought to be on the order of two to six weeks. In women, signs can include abnormal vaginal discharge or bleeding, abdominal pain, painful intercourse, fever, or a burning sensation when urinating. In men, roughly half of those infected will show signs of urethral inflammation, and symptoms can include discharge from the penis, testicular pain or swelling, or fever.

  The bacterium is capable of entering what researchers call a persistent growth state when the host cell is starved of nutrients such as amino acids, iron, or vitamins. In this state, the organism stops dividing and grows larger than usual. It remains viable, however, and can return to normal activity once conditions improve. Some researchers believe these persistent bacteria are connected to chronic chlamydial disease, though the exact relevance is still debated.

• 3. Paths of Transmission

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  Chlamydia spreads through vaginal, anal, oral, or manual sex, and through direct contact with infected tissue such as the conjunctiva of the eye. It can also pass from an infected mother to her baby during vaginal childbirth. As many as half of all infants born to mothers with chlamydia will be born with the disease.

  Conjunctivitis due to chlamydia in newborns typically appears around one week after birth, a timing that helps distinguish it from conjunctivitis caused by chemical irritation, which appears within hours, or by gonorrhea, which appears within two to five days. Beyond newborns, eye infections can also spread in communities with poor sanitation through personal contact, shared towels, flies, and coughing or sneezing.

  Recent research using droplet digital PCR and viability assays found evidence of high-viability Chlamydia trachomatis in the gastrointestinal tract of women who had abstained from receptive anal intercourse. Distinct bacterial strains were detected in rectal versus endocervical samples, suggesting the gastrointestinal colonization may have been acquired through prior vaginorectal or oral routes rather than direct anal exposure. The probability of becoming infected is generally thought to be proportionate to the number of bacteria a person is exposed to.

• 4. Complications Left Untreated

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  Approximately half of women with an asymptomatic chlamydia infection that goes undetected will develop pelvic inflammatory disease, or PID. PID is a broad term for infection of the uterus, fallopian tubes, or ovaries. Scarring from that inflammation can lead to chronic pelvic pain, difficulty becoming pregnant, ectopic pregnancy, and other dangerous pregnancy complications. Chlamydia causes between 250,000 and 500,000 cases of PID every year in the United States alone. Women infected with chlamydia are up to five times more likely to acquire HIV if they are exposed to it.

  In men, untreated infections can spread to the testicles, causing epididymitis. Chlamydia causes more than 250,000 cases of epididymitis in the United States each year. In rare cases, this can lead to sterility. The bacterium is also a potential cause of prostatic inflammation, though the exact link is difficult to establish because of possible contamination from urethral infections.

  Chlamydia may also trigger reactive arthritis, a triad of arthritis, conjunctivitis, and urethral inflammation. About 15,000 men in the United States develop reactive arthritis from chlamydia each year, and about 5,000 of those are permanently affected. The condition can occur in both sexes but is more common in men.

  A different strain of Chlamydia trachomatis causes lymphogranuloma venereum, an infection of the lymph nodes and lymphatic system. It typically presents with genital ulceration and swollen lymph nodes in the groin, but it can also manifest as rectal inflammation or swollen nodes elsewhere in the body.

• 5. Trachoma and Global Blindness

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  Trachoma is a chronic conjunctivitis caused by Chlamydia trachomatis, and it was once the leading cause of blindness worldwide. Between 1995 and 2002 its share of global blindness cases fell from 15% to 3.6%, a significant reduction but not an elimination.

  The infection spreads through fingers, shared towels or cloths, coughing, sneezing, and flies that seek out the eye. Symptoms include mucopurulent discharge, irritation, redness, and lid swelling. Repeated infections that go untreated can cause permanent damage to sight.

  The World Health Organization pursued a strategy known by the acronym SAFE, which stands for surgery for in-growing or in-turned lashes, antibiotics, facial cleanliness, and environmental improvements. The organization aimed for global elimination of trachoma by 2020 under its GET 2020 initiative, but that target was not met. The updated World Health Assembly road map for neglected tropical diseases, covering 2021-2030, now sets 2030 as the new target date for elimination.

• 6. Diagnosis and Screening

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  Diagnostic methods for genital chlamydial infections evolved rapidly from the 1990s through 2006. Nucleic acid amplification tests, known as NAATs, are now the primary approach. These include polymerase chain reaction, transcription mediated amplification, and DNA strand displacement amplification. NAATs have an estimated sensitivity of approximately 90% and a specificity of approximately 99%, whether the sample comes from a cervical swab or a urine specimen.

  Because of that accuracy, NAATs have largely replaced culture, which was once the gold standard for chlamydia diagnosis, and have also displaced non-amplified probe tests. The older probe tests detected only 60-80% of infections in asymptomatic women and often produced false positives. Culture remains useful in specific circumstances and is currently the only assay approved for testing non-genital specimens. It is also the only method that allows a test of cure after antibiotic treatment. As of 2020, rapid point-of-care tests are not considered effective for diagnosing chlamydia in men of reproductive age and non-pregnant women because of high false-negative rates.

  Screening guidelines reflect the infection's high rate among younger people. Yearly screening is recommended for sexually active women under 25, and for others at higher risk. Studies have found that up to 9% of women aged under 25 attending for emergency contraception tested positive for chlamydia. The U.S. Preventive Services Task Force and the Centers for Disease Control and Prevention both recommend screening at the first prenatal visit. In the United Kingdom, the National Health Service aims to test at least 25% of the sexually active under-25 population annually.

• 7. Treatment and Prevention

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  Chlamydia can be cured with antibiotics. Azithromycin and doxycycline are the treatments most commonly used. In men, doxycycline at 100 milligrams twice daily for seven days is probably more effective than a single one-gram dose of azithromycin, though evidence for which antibiotic works better in women is described as very uncertain. During pregnancy, erythromycin or amoxicillin are the recommended agents. In babies, erythromycin or azithromycin is preferred.

  Sexual partners of infected individuals should also be treated. An approach called patient-delivered partner therapy allows a clinician to provide prescriptions or medication to the patient, who then delivers them to their partner without the partner needing to see the healthcare provider directly. Infected people are advised not to have sex for seven days after treatment and until they are free of symptoms. Because chlamydia commonly co-occurs with other infections, testing for gonorrhea, syphilis, and HIV is also recommended for anyone who has been infected.

  Following treatment, guidelines call for retesting after three months to check for reinfection. However, a test of cure can produce a false positive result because the NAAT method may continue to detect genetic material from the bacterium even after all viable organisms have been cleared.

  Prevention rests on abstinence, consistent condom use, or mutually monogamous or tested-polyamorous relationships. In the United States, about 1.6 million cases were reported in 2023, and the CDC estimates the true annual total including unreported cases runs to about 2.9 million. Infections are most common among people aged 15-25, and African Americans face disproportionately higher rates compared to other racial and ethnic groups.