Pelvic Inflammatory Disease Etiology
Pelvic inflammatory disease is primarily caused by sexually transmitted infections, most commonly Chlamydia trachomatis and Neisseria gonorrhoeae, which ascend from the lower genital tract to infect the upper reproductive organs. 1, 2
Primary Causative Organisms
The etiology of PID is polymicrobial in most cases, with multiple organisms implicated simultaneously 3:
Sexually Transmitted Pathogens
- Chlamydia trachomatis is recovered from the cervix in 5-39% of women diagnosed with PID 3, 2
- Neisseria gonorrhoeae is isolated from the cervix in 27-80% of PID cases 3, 2
- Together, these two organisms account for 30-50% of PID cases 4
- Between 10-40% of women with untreated gonococcal or chlamydial cervicitis will develop clinical symptoms of acute PID 3, 2, 5
Additional Bacterial Pathogens
- Anaerobic bacteria (Bacteroides, Peptostreptococcus, Peptococcus species) are isolated from the upper genital tract in 25-50% of acute PID cases 3, 5
- Aerobic/facultative bacteria including Gardnerella vaginalis, Streptococcus species, Escherichia coli, and Haemophilus influenzae 3, 2
- Bacterial vaginosis-associated organisms serve as an antecedent to polymicrobial PID 3, 2, 5
- Mycoplasmas (including Mycoplasma genitalium) have been recovered, though their role is less clear 3
Pathogenesis
The infection spreads via direct canalicular ascent from the endocervix through the endometrium to the fallopian tube mucosa 3, 5:
- Organisms initially cause endocervicitis in the lower genital tract 3
- Direct upward spread occurs through the cervical canal to the upper reproductive organs 1, 5
- Noncanalicular spread via parametrial lymphatics has also been documented 5
Factors Facilitating Ascent
- Uterine instrumentation (such as IUD insertion) can mechanically introduce organisms into the upper tract 5
- Hormonal changes during menstruation alter the cervical mucus barrier and facilitate bacterial ascent 5
- Retrograde menstruation can carry organisms from the lower to upper genital tract 5
- Individual organism virulence factors determine the likelihood and severity of ascending infection 5
Clinical Significance
The polymicrobial nature of PID is critical to understand: most cases involve more than one organism, not just the initial sexually transmitted pathogen 3, 5. This explains why broad-spectrum antibiotic coverage is essential for treatment 2.
Common Pitfall to Avoid
Do not assume PID is caused by a single organism—even when N. gonorrhoeae or C. trachomatis is identified, anaerobic and facultative bacteria are frequently co-pathogens requiring coverage 3, 5. The shift from viewing PID as a monoetiology infection (primarily gonorrhea prior to the mid-1970s) to recognizing its polymicrobial nature is fundamental to appropriate management 6.