Primary Syphilis and the Chancre
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Direct answer
Primary syphilis is the first stage of Treponema pallidum infection, presenting as a chancre, a painless, indurated, clean-based ulcer at the site of inoculation. The chancre appears about 10 to 90 days after exposure, usually around three weeks, with painless, rubbery regional lymphadenopathy. It heals spontaneously within three to six weeks, but the infection persists and can progress to secondary syphilis, so treatment is still needed.
What you must remember
- Causative organism: Treponema pallidum, a spirochaete.
- Incubation: about 10 to 90 days after exposure, typically around three weeks.
- Chancre: a single, painless, indurated ulcer with a clean base and raised, firm edges, also called a Hunterian chancre.
- Common sites: the genitals most often, but also the anus, lips, tongue, tonsils and fingers.
- Lymph nodes: bilateral, painless, firm and rubbery regional nodes, usually inguinal.
- Diagnosis: dark-field microscopy or direct fluorescent antibody testing of chancre exudate, with serology, which may be negative early.
- Treatment: a single intramuscular dose of benzathine penicillin G, per standard guidance.
- Key differentials: painful herpes simplex, chancroid with a ragged purulent ulcer, and lymphogranuloma venereum.
A stepwise approach to a genital ulcer
A patient with a single painless genital ulcer and firm inguinal nodes needs syphilis excluded early, since the chancre is infectious. The diagnosis combines the clinical picture with direct and serological tests, and the steps below follow that order.
- Take a focused history: Ask about the onset, pain, timing of exposure and sexual partners, since the incubation period is roughly three weeks.
- Examine the ulcer: Look for a single, painless, indurated ulcer with a clean base and firm edges, noting its site and any further lesions.
- Palpate the nodes: Check for bilateral, painless, rubbery inguinal lymphadenopathy, which supports syphilis over chancroid.
- Sample the lesion: Use dark-field microscopy or direct fluorescent antibody testing on serous fluid from the chancre.
- Send serology: Send a non-treponemal test and a treponemal test, remembering that early serology may be negative.
- Test for HIV and other STIs: Offer HIV testing and screen for other sexually transmitted infections, since co-infection is common.
- Treat and trace partners: Give benzathine penicillin and arrange notification and treatment of recent sexual partners.
Where the exam tries to catch you
Most traps involve confusing the chancre with painful genital ulcers or relying on a negative serology too early.
- Trap: calling a painful genital ulcer primary syphilis Correct: the chancre of syphilis is painless, whereas herpes and chancroid are typically painful.
- Trap: ruling out syphilis because serology is negative Correct: serology can be negative in early infection, so repeat testing or dark-field examination is needed.
- Trap: expecting the chancre to need only topical therapy Correct: the chancre heals on its own, but the infection still needs systemic penicillin.
- Trap: dismissing a single ulcer on the lip or anus Correct: extragenital chancres occur, so unusual sites must be examined and tested.
Frequently asked questions
What does a primary syphilis chancre look like?
A primary syphilis chancre is a single, painless, indurated ulcer with a clean base and firm raised edges. It is usually found on the genitals, though it may appear on the lips, tongue, anus or fingers.
How long after exposure does a syphilis chancre appear?
A syphilis chancre usually appears about three weeks after exposure, with an incubation period that ranges from about 10 to 90 days. The chancre heals within three to six weeks even without treatment, but the infection persists.
Which tests confirm primary syphilis?
Primary syphilis is confirmed by dark-field microscopy or direct fluorescent antibody testing of material from the chancre, along with serology. Early syphilis may give a negative serology, so repeat testing is needed when clinical suspicion is high.
How does primary syphilis differ from chancroid?
Primary syphilis gives a painless, indurated ulcer with a clean base and painless, rubbery nodes. Chancroid gives a painful, ragged ulcer with a purulent base and tender, suppurative nodes, and the organisms differ, with Treponema pallidum in syphilis and Haemophilus ducreyi in chancroid.