Syphilis is one of those “looks-like-everything” infections that Step loves because it tests pattern recognition across multiple stages, plus a handful of classic associations (palms/soles rash, painless chancre, Argyll Robertson pupil, aortitis). If you can organize Treponema pallidum by timeline, pathology, and the right tests, you’ll pick up a lot of easy points.
Where Treponema pallidum fits (and why it’s “atypical”)
Treponema pallidum is a spirochete—a thin, helically coiled bacterium with motility from axial filaments (endoflagella). It’s “atypical” in practical terms because:
- It does not grow in routine culture (no standard artificial media).
- It’s too thin to see well on standard Gram stain → think dark-field microscopy (historically) and serology (modern boards/clinics).
First Aid cross-reference: Microbiology → Spirochetes (Treponema, Borrelia, Leptospira); plus Immunology/Medicine sections for congenital syphilis and tertiary manifestations.
Micro + pathophysiology (what Step expects you to understand)
Key virulence and immune evasion
- Outer membrane has few surface proteins → “stealth” organism; decreased immune recognition.
- Infection triggers a plasma cell–rich inflammatory response (a frequent histology clue).
- Chronic infection can lead to obliterative endarteritis (damage to vasa vasorum and small vessels), which underlies many tertiary findings.
Transmission
- Sexual contact (primary/secondary/early latent most infectious)
- Transplacental (congenital syphilis)
- Less commonly via blood exposure (rare with screening)
Timeline concept (board-friendly)
A common test style is matching symptoms to stage and infectiousness:
| Stage | Typical timing after exposure | Infectious? | Hallmark |
|---|---|---|---|
| Primary | ~3 weeks (range 10–90 days) | Yes | Painless chancre |
| Secondary | ~6 weeks–6 months | Yes | Diffuse rash incl palms/soles, mucous lesions |
| Latent | >1 year (or unknown duration) | Early: yes; Late: much less | Asymptomatic + positive serology |
| Tertiary | Years–decades | Generally no | Gummas, aortitis, neurosyphilis |
Clinical stages (what it looks like on test day)
1) Primary syphilis
Presentation
- Painless, indurated ulcer (chancre) at inoculation site
- Nontender regional lymphadenopathy
- Heals spontaneously in weeks (important trap: symptoms resolve but infection persists)
High-yield associations
- Painless genital ulcer differential: syphilis (vs HSV = painful vesicles/ulcers; chancroid = painful ragged ulcer)
- Dark-field microscopy can show spirochetes from chancre exudate (avoid oral lesions due to nonpathogenic treponemes)
2) Secondary syphilis
Presentation (the “systemic” stage)
- Diffuse maculopapular rash that classically involves palms and soles
- Condylomata lata: broad-based, moist, wart-like lesions (very infectious)
- Mucous patches (oral/genital)
- Constitutional symptoms: fever, malaise, sore throat
- Generalized lymphadenopathy
High-yield associations
- Rash on palms/soles differential includes:
- Secondary syphilis
- RMSF
- Coxsackie (hand-foot-mouth)
- Drug eruptions (varies)
3) Latent syphilis
Definition
- Positive serology with no symptoms
- Subdivided into:
- Early latent: within 1 year of infection (more infectious; higher relapse to secondary)
- Late latent: >1 year (less infectious, but vertical transmission can still occur)
Board focus
- Latent syphilis is where Step asks which test is positive, how to treat, and when to evaluate for neurosyphilis.
4) Tertiary syphilis
Years later, untreated infection can cause destructive inflammatory disease.
A) Cardiovascular syphilis
- Aortitis (classically ascending thoracic aorta) due to endarteritis obliterans of the vasa vasorum
- Consequences:
- Aortic root dilation
- Aortic regurgitation
- Aneurysm (ascending)
High-yield buzzwords: “tree-barking” of aorta (gross pathology), vasa vasorum involvement.
B) Neurosyphilis (can occur at any stage, but classically tertiary)
Classic board entities include:
- Tabes dorsalis (posterior column/dorsal root degeneration)
- Sensory ataxia, lightning pains
- Loss of vibration/proprioception
- Positive Romberg
- General paresis (meningoencephalitis)
- Personality changes, dementia
- Argyll Robertson pupil
- Accommodates but does not react (light-near dissociation)
Step tip: If the question says neuro symptoms + syphilis serologies, think CSF testing and IV penicillin G.
C) Gummatous syphilis
- Gummas = granulomatous lesions that can affect skin, bone, organs
- Can be destructive but are typically noninfectious
Congenital syphilis (always high-yield)
Occurs via transplacental transmission (especially with untreated maternal infection).
Early congenital syphilis (often symptomatic in infancy)
- Snuffles (copious infectious rhinitis)
- Diffuse rash (can include palms/soles)
- Hepatosplenomegaly, jaundice
- Anemia, thrombocytopenia
Late congenital syphilis (classically after age 2)
Remember the triad:
- Hutchinson teeth (notched, peg-shaped incisors)
- Interstitial keratitis
- Sensorineural deafness
Also:
- Saddle nose deformity
- Sabre shins (anterior bowing of tibia)
First Aid cross-reference: Congenital infections section (TORCH-like comparisons) + Micro spirochetes.
Diagnosis: how Step wants you to test
Because T. pallidum can’t be cultured easily, diagnosis is primarily serologic, often as a two-step algorithm.
Nontreponemal tests (screening and monitoring)
- VDRL and RPR
- Detect anti-cardiolipin (reagin) antibodies (against lipid released from damaged cells)
- Useful for:
- Screening
- Tracking treatment response (titers fall with successful therapy)
False positives (high-yield):
- Pregnancy
- Autoimmune disease (e.g., SLE, antiphospholipid syndrome)
- Viral infections, older age (test-specific contexts)
Treponemal tests (confirmation)
- FTA-ABS, TP-PA, treponemal EIAs
- More specific; typically remain positive for life → not ideal for monitoring treatment response.
Quick comparison table
| Test type | Examples | What it detects | Best use | False positives? |
|---|---|---|---|---|
| Nontreponemal | RPR, VDRL | Anti-cardiolipin antibodies | Screen + monitor titers | Yes |
| Treponemal | FTA-ABS, TP-PA | Antibodies to T. pallidum | Confirm diagnosis | Less common |
Neurosyphilis testing (Step 1/2 favorite)
If neuro/ocular symptoms, HIV coinfection with concerning symptoms, or treatment failure is suspected:
- Evaluate CSF
- CSF-VDRL: highly specific (if positive, strong evidence)
- CSF pleocytosis/protein elevation support diagnosis
Treatment (memorize the “one drug” rule + exceptions)
First-line: penicillin
- Penicillin G is the treatment of choice for all stages
- Early syphilis (primary/secondary/early latent): typically IM benzathine penicillin G
- Late latent/unknown duration: longer course (often weekly IM doses)
- Neurosyphilis: IV aqueous penicillin G
If penicillin allergy
- Nonpregnant, non-neuro disease: doxycycline may be used (board nuance varies by stage)
- Pregnancy: desensitize and give penicillin (key high-yield rule)
Jarisch–Herxheimer reaction (testable adverse event)
- Acute febrile reaction after starting therapy (fever, chills, headache, myalgias)
- Due to cytokine release from dying spirochetes
- Treat supportively (antipyretics); do not stop antibiotics
High-yield “Step stems” and classic traps
1) Painless genital ulcer + nontender nodes
- Primary syphilis until proven otherwise → confirm with serology (or dark-field if available)
2) Rash involving palms/soles + condylomata lata
- Secondary syphilis → highly infectious → treat and counsel partners
3) Positive RPR in pregnancy
- Confirm with treponemal test; treat with penicillin (desensitize if allergic)
4) Dementia/psychiatric changes or sensory ataxia years later
- Think tertiary neurosyphilis (general paresis/tabes dorsalis) → CSF testing, IV penicillin G
5) Aortic regurgitation + aneurysm of ascending aorta
- Syphilitic aortitis due to vasa vasorum endarteritis
Rapid review: one-page mental map
- Primary: painless chancre
- Secondary: palms/soles rash, condylomata lata, systemic symptoms
- Latent: asymptomatic, serology positive
- Tertiary: gummas, aortitis, neurosyphilis (Argyll Robertson, tabes dorsalis, general paresis)
- Dx: RPR/VDRL screen (anti-cardiolipin), FTA-ABS confirm
- Tx: penicillin; Jarisch–Herxheimer; pregnancy = penicillin no matter what