Chlamydia, Gonorrhea, Syphilis for USMLE Step 3
Chlamydia, Gonorrhea, Syphilis for the USMLE Step 3 Exam
Chlamydia
- Etiology:
- Caused by Chlamydia trachomatis, an obligate intracellular bacterium with serotypes D–K most commonly responsible for urogenital infections.
- Epidemiology:
- The most common bacterial STI in the U.S., affecting mainly individuals aged 15–24 years.
- Clinical Presentation:
- Men: Asymptomatic in up to 50% but can present with urethritis, dysuria, and mucoid or clear discharge.
- Women: Frequently asymptomatic; symptomatic infections include cervicitis (mucopurulent discharge) and urethritis. Complications include PID, with risks for ectopic pregnancy and infertility if untreated.
- Extragenital Sites: Pharyngeal and rectal infections may occur from oral and anal sex; neonatal conjunctivitis in infants born to infected mothers.
- Diagnosis:
- Nucleic Acid Amplification Test (NAAT): Gold standard, can be performed on urine or swabs from the genital tract.
- Treatment:
- Doxycycline 100 mg PO twice daily for 7 days or Azithromycin 1 g PO single dose.
- Partner notification and treatment are critical to prevent reinfection.
Gonorrhea
- Etiology:
- Caused by Neisseria gonorrhoeae, a gram-negative diplococcus.
- Epidemiology:
- High prevalence among young adults aged 15–24, with frequent co-infection with Chlamydia trachomatis.
- Clinical Presentation:
- Men: Often presents with urethritis (purulent discharge and dysuria) and can progress to epididymitis.
- Women: Frequently asymptomatic; may present with cervicitis, PID, and urethritis if symptomatic.
- Extragenital Infections: Pharyngeal and rectal infections from oral and anal sex; neonatal conjunctivitis (ophthalmia neonatorum) in infants.
- Disseminated Gonococcal Infection (DGI): Bacteremia leading to dermatitis-arthritis syndrome with tenosynovitis, migratory polyarthritis, and septic arthritis.
- Diagnosis:
- NAAT: Preferred for diagnosing N. gonorrhoeae from urine or swabs.
- Gram Stain: Helpful in symptomatic men, showing gram-negative intracellular diplococci in urethral discharge.
- Treatment:
- Ceftriaxone 500 mg IM single dose; if chlamydia co-infection is suspected, add doxycycline 100 mg PO twice daily for 7 days.
- Partner treatment is essential to prevent reinfection.
Syphilis
- Etiology:
- Caused by Treponema pallidum, a spirochete bacterium.
- Clinical Stages:
- Primary Syphilis:
- Appears ~3 weeks after infection as a painless chancre at the inoculation site, which heals spontaneously within weeks.
- Secondary Syphilis:
- Systemic symptoms, including a maculopapular rash (often involving palms and soles), condylomata lata, mucous patches, and generalized lymphadenopathy.
- Latent Syphilis:
- Asymptomatic stage following secondary syphilis, classified as early (within 1 year) or late (after 1 year).
- Tertiary Syphilis:
- Late-stage disease involving cardiovascular (aortitis), neurosyphilis (Tabes dorsalis, general paresis), and gummas (granulomatous lesions in soft tissues).
- Diagnosis:
- Nontreponemal Tests (Screening): RPR or VDRL, used for screening and monitoring treatment response.
- Treponemal Tests (Confirmatory): FTA-ABS or TPPA, which remain positive for life, confirming infection history.
- Treatment:
- Primary, Secondary, and Early Latent Syphilis: Benzathine penicillin G 2.4 million units IM single dose.
- Late Latent and Tertiary Syphilis (without neurosyphilis): Benzathine penicillin G 2.4 million units IM weekly for 3 weeks.
- Neurosyphilis: Aqueous crystalline penicillin G 18–24 million units/day IV for 10–14 days.
Key Points
- Chlamydia:
- Caused by Chlamydia trachomatis; often asymptomatic, especially in women, but can cause PID, ectopic pregnancy, and infertility.
- Diagnosed by NAAT, treated with doxycycline or azithromycin, with partner treatment essential.
- Gonorrhea:
- Caused by Neisseria gonorrhoeae, often co-infecting with chlamydia; presents with urethritis in men, often asymptomatic in women.
- Diagnosed by NAAT, treated with ceftriaxone plus doxycycline if chlamydia is co-infected, and partner treatment is crucial.
- Syphilis:
- Caused by Treponema pallidum, presenting in stages: primary (chancre), secondary (rash and systemic symptoms), latent, and tertiary (cardiovascular, neurosyphilis).
- Diagnosed with nontreponemal and treponemal tests, treated with penicillin, and follow-up serology is critical for monitoring treatment efficacy.