Microbiology · Year 2 · from Microbiology
Case 2: Primary Syphilis
Presentation
A 28-year-old man presents to a sexual health clinic with a painless sore on his penis that appeared about 2 weeks ago. He reports unprotected sexual contact with multiple male partners in the past 2 months. Examination reveals a single, round ulcer approximately 1.5 cm in diameter on the penile shaft with raised, indurated edges and a clean base. The lesion is non-tender. Bilateral inguinal lymph nodes are palpable but non-tender. He has no other symptoms.
Clinical Image
Primary syphilis chancre - a painless ulcer with indurated (firm) edges and a clean base, the hallmark of primary syphilis.
Image Source: Lecture image - Treponema pallidum infection
Questions
- What is the diagnosis, and what organism causes this disease?
- How does this lesion differ from other causes of genital ulcers?
- What serologic tests should be ordered, and how are they interpreted?
- What is the appropriate treatment, and what follow-up is required?
Answers
- Diagnosis and organism: This is primary syphilis, the first stage of infection caused by Treponema pallidum, a spirochete. The chancre is the hallmark of primary syphilis, appearing 10-90 days (average 21 days) after exposure at the site of inoculation. T. pallidum cannot be cultured in vitro, making the diagnosis dependent on clinical recognition and serology.
- Differentiation from other genital ulcers:
- Syphilis chancre: Painless, single ulcer with indurated edges and clean base, non-tender lymphadenopathy
- Herpes simplex: Painful, multiple vesicles/ulcers, tender lymphadenopathy
- Chancroid (H. ducreyi): Painful ulcer with ragged undermined edges and purulent base, tender/suppurative lymph nodes
- Lymphogranuloma venereum: Small, often-unnoticed primary lesion followed by prominent lymphadenopathy
- Serologic testing: A two-tier algorithm is used:
- Non-treponemal tests (RPR, VDRL): Detect antibodies to cardiolipin; become positive 1-4 weeks after chancre appears; titers correlate with disease activity and decline after treatment
- Treponemal tests (FTA-ABS, TP-PA): Detect antibodies to T. pallidum antigens; confirm reactive non-treponemal tests; remain positive for life regardless of treatment
In primary syphilis, serologic tests may still be negative (window period), so dark-field microscopy of lesion fluid can provide immediate diagnosis if available. Testing for HIV is essential given shared risk factors.
- Treatment and follow-up:
- Treatment: Single dose of benzathine penicillin G 2.4 million units IM is curative for primary syphilis
- For penicillin-allergic non-pregnant patients: Doxycycline 100 mg twice daily for 14 days
- Follow-up: Clinical and serologic (RPR/VDRL titers) at 6 and 12 months; titers should decline 4-fold by 6-12 months indicating adequate treatment
- Partner notification and treatment is essential
- The chancre heals spontaneously in 3-6 weeks regardless of treatment, but without treatment the infection progresses to secondary and tertiary stages