Spirochetes: Syphilis, Lyme Disease, and Leptospirosis

Q: How do condyloma lata differ from condyloma acuminata?

Condyloma lata are smooth, moist, broad-based papules/plaques seen in secondary syphilis, whereas condyloma acuminata are HPV-associated genital warts. Confusing the two is a classic exam trap.

Q: Which serologic tests screen for syphilis versus confirm it?

Non-treponemal tests (RPR, VDRL) are used for screening and to follow treatment response, while treponemal tests (FTA-ABS, TP-PA) are confirmatory and remain positive for life.

Q: What is erythema migrans and how often does it appear in Lyme disease?

Erythema migrans is a pathognomonic target/bull's-eye rash of early localized Lyme disease, appearing in 70–80% of patients along with flu-like symptoms.

Q: How is Lyme disease treated?

Early disease is treated with doxycycline (amoxicillin in pregnancy); neurologic involvement or high-grade cardiac block warrants IV ceftriaxone.

Q: What clinical features point to leptospirosis?

Suspect it in a febrile patient with freshwater exposure, calf myalgias, and conjunctival suffusion. The severe form, Weil's disease, is defined by the triad of jaundice, acute kidney injury, and hemorrhage, often with elevated transaminases.

Q: What are the classic neurosyphilis findings?

Tabes dorsalis (posterior column degeneration), general paresis (dementia), and Argyll Robertson pupils, which accommodate but do not react to light. Neurosyphilis can occur at any stage.

Spirochetes are a group of medically important bacteria unified by their spiral morphology but distinguished by very different transmission routes and clinical syndromes. The three key pathogens are Treponema pallidum (syphilis, sexually and vertically transmitted), Borrelia burgdorferi (Lyme disease, tick-borne), and Leptospira interrogans (leptospirosis, from water/animal urine). Master each by its mode of transmission, its signature clinical stage or feature, and its diagnostic and treatment approach.

Treponema pallidum — Syphilis (The Great Imitator)

Transmitted sexually and vertically, syphilis progresses through defined stages. **Primary:** a painless chancre with a clean base and indurated borders at the inoculation site — highly infectious and heals spontaneously in 3–6 weeks. **Secondary** (4–10 weeks later): systemic dissemination with a maculopapular rash classically involving the palms and soles, condyloma lata (smooth, moist, broad-based papules/plaques in intertriginous/anogenital areas — not to be confused with HPV-associated condyloma acuminata), generalized lymphadenopathy, and constitutional symptoms. **Latent:** no clinical manifestations; early latent (<1 year) remains infectious while late latent (>1 year) is less infectious. **Tertiary** (years to decades later): gummas (granulomatous lesions in any organ), cardiovascular disease (aortitis, aortic root dilation, 'tree-bark' aorta), and neurosyphilis (tabes dorsalis from posterior column degeneration, general paresis/dementia, and Argyll Robertson pupils that accommodate but do not react to light). Neurosyphilis can occur at any stage. **Diagnosis:** dark-field microscopy of the primary chancre; non-treponemal tests (RPR, VDRL) for screening and to follow treatment response; treponemal tests (FTA-ABS, TP-PA) for confirmation and stay positive for life. **Treatment:** penicillin G (benzathine penicillin for most stages).

Borrelia burgdorferi — Lyme Disease

Transmitted by the Ixodes tick (deer tick); the reservoir is the white-footed mouse, while deer are important for the tick life cycle but do not transmit the spirochete. Endemic to the Northeast, Upper Midwest, and Pacific coast US. **Stage 1 (Early Localized):** erythema migrans — a target/bull's-eye rash that is pathognomonic and appears in 70–80% of patients — plus flu-like symptoms. **Stage 2 (Early Disseminated), days to weeks later:** multiple erythema migrans lesions, cardiac involvement (Lyme carditis, classically high-grade or complete third-degree AV block, though any degree of block can occur), and neurologic involvement (Bell's palsy, meningitis). **Stage 3 (Late), months later:** arthritis of large joints (especially the knee) and chronic neurologic disease. **Treatment:** doxycycline for early disease (amoxicillin in pregnancy); IV ceftriaxone for high-grade cardiac block or neurologic involvement.

Leptospira interrogans — Leptospirosis

Transmitted through water or animal urine — classically freshwater exposure. Leptospirosis presents with high fevers, rigors, headache, and severe myalgias (especially of the calves). **Conjunctival suffusion** (redness without exudate) is a characteristic clue. The severe form, **Weil's disease**, is defined by the triad of jaundice, acute kidney injury, and hemorrhage. It belongs on the differential for a febrile returning traveler with freshwater exposure, often accompanied by elevated transaminases; empiric doxycycline is often started while awaiting serologic confirmation.

High-yield

Spirochetes have a spiral shape; the three USMLE pathogens are Treponema pallidum, Borrelia burgdorferi, and Leptospira interrogans.
Syphilis = 'the Great Imitator' with a painless chancre (primary) → palm-and-sole maculopapular rash (secondary) → gummas/cardiovascular/neurosyphilis (tertiary).
Condyloma lata (secondary syphilis) are smooth, moist, broad-based papules/plaques — distinct from HPV condyloma acuminata.
Argyll Robertson pupils accommodate but do not react to light ('prostitute's pupil') — a neurosyphilis clue.
Erythema migrans (bull's-eye rash) is pathognomonic for Lyme disease and appears in 70–80% of patients.
Lyme reservoir is the white-footed mouse; the vector is the Ixodes tick.
Lyme carditis classically causes high-grade or complete (third-degree) AV block, prompting hospitalization and IV ceftriaxone.
Lyme arthritis favors large joints, especially the knee.
Weil's disease triad = jaundice + acute kidney injury + hemorrhage; conjunctival suffusion is a classic clue; think freshwater exposure.
Penicillin G treats syphilis; doxycycline treats early Lyme; IV ceftriaxone for neurologic/high-grade cardiac Lyme.
Non-treponemal tests (RPR/VDRL) screen and follow treatment; treponemal tests (FTA-ABS/TP-PA) confirm and remain positive for life.

Pitfalls

Assuming the syphilitic chancre is painful — it is characteristically painless with a clean base and indurated borders.
Confusing condyloma lata of secondary syphilis (smooth, moist, broad-based papules) with condyloma acuminata (HPV genital warts).
Forgetting that the secondary syphilis rash involves the palms and soles.
Believing neurosyphilis only occurs in tertiary disease — it can occur at any stage.
Thinking treponemal tests can be used to monitor treatment response — non-treponemal tests (RPR/VDRL) follow treatment; treponemal tests stay positive for life.
Calling first-degree AV block the usual finding in Lyme carditis — the classic/hallmark presentation is high-grade or complete (third-degree) heart block.
Naming the deer as the Lyme reservoir — the white-footed mouse is the reservoir; deer maintain the tick population.
Using doxycycline for early Lyme in pregnancy — amoxicillin is preferred, and IV ceftriaxone is used for neurologic/high-grade cardiac involvement.
Listing hepatosplenomegaly as a hallmark of leptospirosis — the classic severe (Weil's) picture is jaundice, acute kidney injury, and hemorrhage.
Overlooking freshwater exposure as the clue for leptospirosis in a febrile traveler.

Clinical pearls

A painless genital ulcer with an indurated, clean base is a syphilitic chancre until proven otherwise.
A maculopapular rash on the palms and soles should prompt consideration of secondary syphilis.
A bull's-eye rash after outdoor exposure in an endemic area is early Lyme disease — treat with doxycycline.
New high-grade or complete AV block plus a recent tick-endemic exposure suggests Lyme carditis warranting IV ceftriaxone.
Fever, calf myalgias, and conjunctival suffusion after freshwater exposure suggest leptospirosis; jaundice, acute kidney injury, and hemorrhage define Weil's disease.

Frequently asked

How do the transmission routes differ among the three key spirochetes?

Treponema pallidum spreads sexually and vertically; Borrelia burgdorferi is transmitted by the Ixodes tick; Leptospira interrogans is acquired through water or animal urine (classically freshwater exposure).

What are the four stages of syphilis and their hallmark findings?

Primary: painless chancre. Secondary: maculopapular rash on the palms and soles, condyloma lata (smooth, moist, broad-based papules/plaques), lymphadenopathy. Latent: asymptomatic (early latent <1 year is infectious). Tertiary: gummas, cardiovascular aortitis with 'tree-bark' aorta, and neurosyphilis.

How do condyloma lata differ from condyloma acuminata?

Condyloma lata are smooth, moist, broad-based papules/plaques seen in secondary syphilis, whereas condyloma acuminata are HPV-associated genital warts. Confusing the two is a classic exam trap.

Which serologic tests screen for syphilis versus confirm it?