Definition and epidemiology
Chlamydia trachomatis is a Gram-negative, obligate intracellular bacterium that causes the most commonly reported bacterial sexually transmitted infection (STI) worldwide. It infects columnar and transitional epithelium of the urethra, endocervix, rectum, pharynx, and conjunctiva.
- Highest prevalence in people aged 15-24 years
- Estimated well over 100 million new infections globally per year (WHO estimates)
- Up to 70-80% of infected women and a substantial proportion of infected men are asymptomatic
- Serovars D-K cause genital, ocular (adult inclusion conjunctivitis), and neonatal infections
- Serovars L1-L3 cause lymphogranuloma venereum (LGV), more common in men who have sex with men
- Serovars A-C cause trachoma, a leading infectious cause of blindness in endemic regions (not sexually transmitted)
- Most common bacterial STI; majority of infections are asymptomatic
- Obligate intracellular organism; cannot be seen on routine Gram stain
- First-line treatment is doxycycline 100 mg twice daily for 7 days
- Single-dose azithromycin is the preferred alternative in pregnancy
- Always treat for gonorrhea co-infection if not excluded and prevalence is high
- Reiter syndrome triad: urethritis, conjunctivitis, arthritis (HLA-B27 associated)
- Leading preventable cause of tubal factor infertility and ectopic pregnancy
- Serovars L1-L3 cause lymphogranuloma venereum with painful inguinal lymphadenopathy
- Annual screening recommended for sexually active women under 25
Etiology and risk factors
Transmission occurs through vaginal, anal, or oral sexual contact, and perinatally from an infected mother to neonate during vaginal delivery.
Risk factors
- Age under 25
- New or multiple sexual partners
- Inconsistent condom use
- Prior STI or current STI co-infection (especially gonorrhea)
- Men who have sex with men (higher risk of rectal and pharyngeal infection, and LGV)
- Lower socioeconomic status and limited access to healthcare/screening
Pathophysiology
Chlamydia trachomatis has a unique biphasic life cycle: an infectious, metabolically inactive elementary body attaches to and enters host epithelial cells, then converts intracellularly into a metabolically active reticulate body that replicates within a membrane-bound inclusion. Reticulate bodies convert back into elementary bodies, which are released to infect new cells.
The resulting inflammatory response, including recruitment of neutrophils and later a chronic lymphocytic/plasma cell infiltrate, causes the epithelial damage and scarring. Repeated or persistent infection drives fibrosis, which underlies key complications: tubal scarring (infertility, ectopic pregnancy) in women and epididymal scarring in men. The organism's intracellular lifestyle explains why it is not reliably seen on routine Gram stain and requires special diagnostic methods.
Clinical features
Women
- Often asymptomatic
- Mucopurulent cervical discharge, cervicitis, cervical friability
- Dysuria, urinary frequency (urethritis), often without pyuria on routine urinalysis culture
- Intermenstrual or postcoital bleeding
- Pelvic inflammatory disease (PID): lower abdominal pain, cervical motion tenderness, adnexal tenderness, fever
Men
- Urethral discharge (often scant, mucoid or clear) and dysuria
- Epididymo-orchitis: unilateral testicular pain and swelling in young men
- Reactive arthritis (part of the classic triad with urethritis and conjunctivitis) in genetically predisposed individuals (HLA-B27)
Other presentations
- Pharyngitis and proctitis (receptive oral/anal exposure), often asymptomatic
- Neonatal conjunctivitis (5-14 days after birth) and neonatal pneumonia (afebrile, staccato cough, 1-3 months of age)
- LGV: painless genital ulcer followed by painful inguinal/femoral lymphadenopathy ("groove sign"); proctocolitis in receptive anal exposure
- Fitz-Hugh-Curtis syndrome: right upper quadrant pain from perihepatic inflammation complicating PID
Diagnosis
Nucleic acid amplification testing (NAAT) is the diagnostic standard, performed on first-void urine, vaginal/endocervical swabs, or self-collected vaginal swabs; rectal and pharyngeal NAAT when relevant exposure.
Testing approach
- NAAT is more sensitive and specific than culture or antigen tests
- Culture is reserved for medicolegal cases (e.g., suspected abuse) or specialized settings
- Co-test for Neisseria gonorrhoeae, as co-infection is common
- Test for other STIs (HIV, syphilis) given shared risk factors
Screening recommendations
- Annual screening for all sexually active women under 25
- Screening in older women with risk factors (new/multiple partners)
- Screening in pregnant women at first prenatal visit (and repeat in third trimester if high risk)
- Routine screening not generally recommended in men unless symptomatic, MSM, or in high-prevalence settings
Differential diagnosis
| Condition | Distinguishing feature |
|---|---|
| Gonococcal urethritis/cervicitis | More purulent discharge; Gram-negative intracellular diplococci on Gram stain; frequent co-infection |
| Trichomoniasis | Frothy, malodorous discharge; motile trichomonads on wet mount; strawberry cervix |
| Bacterial vaginosis | Thin gray discharge, fishy odor, clue cells; not primarily an STI |
| Mycoplasma genitalium | Clinically similar urethritis/cervicitis; diagnosed by specific NAAT, often after chlamydia-negative result |
| Herpes simplex virus | Painful genital ulcers/vesicles rather than discharge |
| Candidiasis | Thick white curdy discharge, pruritus, pseudohyphae on microscopy |
Management
Doxycycline is first-line for uncomplicated urogenital, rectal, and pharyngeal chlamydia per current CDC and most international guidance, given superior efficacy for rectal infection compared with azithromycin. A single dose of azithromycin remains an acceptable alternative, particularly when adherence is a concern or in pregnancy (doxycycline is contraindicated in pregnancy).
Key management points
- Treat all sexual partners from the preceding 60 days (expedited partner therapy where permitted)
- Advise abstinence from sex until 7 days after completing treatment and until partners are treated
- Co-treat empirically for gonorrhea if local prevalence is high or if gonorrhea is not excluded
- Pregnant patients: azithromycin (or amoxicillin as an alternative); doxycycline avoided
- LGV requires a longer course of doxycycline (typically 21 days) due to deeper tissue invasion
- Retest (test of cure not routinely needed with recommended regimens, but repeat testing at 3 months is recommended due to high reinfection rates)
- Report to public health authorities as required (notifiable disease in most jurisdictions)
- Neonatal conjunctivitis/pneumonia requires systemic (oral) treatment of the infant, not topical therapy, since infection is systemic
Complications and prognosis
Untreated or recurrent chlamydia can ascend and cause significant reproductive morbidity, but prognosis is excellent with timely antibiotic treatment.
- Pelvic inflammatory disease, with risk of tubo-ovarian abscess
- Infertility due to tubal scarring (a leading preventable cause of female infertility)
- Ectopic pregnancy from tubal damage
- Chronic pelvic pain
- Fitz-Hugh-Curtis syndrome (perihepatitis)
- Epididymitis and, rarely, male infertility
- Reactive arthritis
- Increased susceptibility to HIV acquisition and transmission
- Vertical transmission causing neonatal conjunctivitis or pneumonia
- LGV can cause chronic lymphatic obstruction, strictures, and fistulas if untreated
"CHLAMYDIA": Cervicitis, Hidden (asymptomatic), Life-cycle intracellular, Azithromycin/doxycycline, Men get urethritis, Young people at risk, Doxycycline is first-line, Infertility risk, Arthritis (reactive)
Practice questions
1. A 19-year-old sexually active woman presents for a routine wellness visit with no complaints. She has had two new partners in the past year and uses condoms inconsistently. Pelvic exam is unremarkable. Which of the following is the most appropriate next step?
2. A 24-year-old pregnant woman at 10 weeks gestation tests positive for chlamydia on routine prenatal screening. She is asymptomatic. Which treatment is most appropriate?
3. A 3-week-old infant presents with a staccato cough, tachypnea, and bilateral diffuse infiltrates on chest X-ray, without fever. The infant was born vaginally to a mother who did not receive prenatal care. Which organism is the most likely cause?
Frequently asked questions
What causes chlamydia?
Chlamydia is caused by Chlamydia trachomatis, an obligate intracellular bacterium transmitted through vaginal, anal, or oral sexual contact, or from mother to infant during childbirth. It infects the epithelial lining of the urethra, cervix, rectum, pharynx, or conjunctiva. Risk is highest in sexually active people under 25 with new or multiple partners.
What are the first symptoms of chlamydia?
Most people have no symptoms at all, which is why screening matters. When symptoms occur, women may notice abnormal vaginal discharge, burning with urination, or bleeding between periods, while men may notice urethral discharge or testicular pain. Symptoms, when present, usually develop one to three weeks after exposure.
How is chlamydia diagnosed?
Chlamydia is diagnosed with a nucleic acid amplification test (NAAT), the most sensitive and specific method available. Samples can be collected from urine, or from vaginal, endocervical, rectal, or pharyngeal swabs depending on exposure site. Testing for gonorrhea and other STIs is usually done at the same time.
Is chlamydia curable?
Yes, chlamydia is fully curable with a short course of antibiotics, typically doxycycline for seven days or a single dose of azithromycin. Cure rates are high when the medication is taken as prescribed and sexual partners are also treated. Reinfection is common, so retesting around three months later is recommended.
Chlamydia vs gonorrhea — what is the difference?
Both are bacterial STIs that can cause similar urethritis or cervicitis, and they frequently occur together, but they are caused by different organisms and treated differently. Gonorrhea (Neisseria gonorrhoeae) tends to cause more purulent discharge and is treated with ceftriaxone, while chlamydia tends to be milder or silent and is treated with doxycycline. Co-testing and often empiric co-treatment are standard practice.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: A55
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.