Clinical overview
Each sexually transmitted infection produces a characteristic pathological lesion, and the lesion explains the clinical presentation. This chapter walks through the major STIs by pathogen, linking pathology to symptoms, signs, and the implications for treatment failure and complications. The clinical management of discharge and ulcer syndromes is in Discharge and ulcers — here we cover what each organism does to tissue, why it presents the way it does, and how the pathology shapes diagnostic and treatment choices.
Core knowledge
Chlamydia trachomatis (serotypes D–K — urogenital)
Cervicitis can ascend through the endometrium to the fallopian tube, leaving tubal scarring that explains infertility and ectopic risk.
Pathology:
- Obligate intracellular bacterium; infects columnar and transitional epithelium.
- Two-phase life cycle (elementary body — extracellular infectious form; reticulate body — intracellular replicating form).
- Causes follicular cervicitis with intense lymphoid infiltration.
- Ascends to endometrium → endometritis (often silent), then tubes → silent salpingitis.
Clinical correlate:
- Often asymptomatic (60–70% of women) — pathology proceeds silently.
- Cervicitis with mucopurulent discharge, contact bleeding, friable cervix.
- Intermenstrual or postcoital bleeding.
- PID with often relatively mild symptoms (vs gonorrhoea) but significant tubal damage.
- Long-term sequelae from indolent tubal damage — infertility, ectopic, chronic pelvic pain.
Diagnosis: NAAT/PCR (endocervical swab or self-collected vaginal swab); urine for men. Culture obsolete.
Neisseria gonorrhoeae
Pathology:
- Gram-negative diplococcus; pili attach to columnar epithelium.
- Intracellular survival in neutrophils — paradoxically not killed by neutrophil response in the early stages.
- Causes acute purulent inflammation with neutrophil infiltrate.
- Ascends similarly to chlamydia but tends to produce more acute clinical syndromes.
Clinical correlate:
- More acute presentation: dysuria, frequent purulent cervical discharge, severe pelvic pain in PID.
- Disseminated gonococcal infection (DGI): septic arthritis, dermatitis (small pustules), tenosynovitis. Septic arthritis often monoarticular.
- Pharyngitis, conjunctivitis (neonatal ophthalmia in vertical transmission).
Diagnosis: NAAT/PCR + culture (essential for sensitivity given resistance patterns).
Mycoplasma genitalium
Pathology:
- Smallest free-living bacterium; lacks cell wall (so beta-lactams ineffective).
- Causes cervicitis and urethritis with chronic inflammation.
Clinical correlate:
- Persistent or recurrent cervicitis/urethritis after standard treatment.
- Associated with PID and tubal damage.
- Increasingly recognised in non-gonococcal, non-chlamydial PID.
Diagnosis: PCR; macrolide resistance testing where available.
Treatment: azithromycin (resistance rising), moxifloxacin, pristinamycin — challenging.