# Treponema > Treponema for FMGE Microbiology: dark-field microscopy, VDRL versus TPHA pairs, prozone, Jarisch-Herxheimer, yaws elimination and benzathine penicillin. - Canonical URL: https://prepelephant.com/topics/fmge/microbiology/treponema-fmge - Exam / course: FMGE · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Treponema", PrepElephant, https://prepelephant.com/topics/fmge/microbiology/treponema-fmge ## Direct answer Treponema pallidum, a fine spiral spirochaete too thin for the Gram stain, is demonstrated by dark-field microscopy of chancre or condyloma fluid and diagnosed serologically through two complementary families: non-treponemal tests (VDRL and RPR, flocculation card tests run quantitatively that fall after treatment) and treponemal-specific tests (TPHA/TPPA and fluorescent treponemal antibody absorption), which stay positive for life. Primary syphilis is the painless indurated chancre with painless inguinal adenopathy three weeks after exposure; secondary syphilis spreads as a palm-and-sole rash with mucous patches and condylomata lata; the gumma and aortitis of tertiary disease arrive years later. Benzathine benzylpenicillin 2.4 megaunits intramuscularly, single dose for early disease, remains the treatment everywhere — and the Jarisch–Herxheimer reaction of fever and rash within hours of the first dose is expected, not allergic. ## What you must remember - Test logic: VDRL/RPR for screening and for monitoring the response to treatment (a four-fold titre fall documents cure); TPHA/TPPA to confirm, persisting for life so a past-treated infection still reads positive. - The prozone phenomenon: antibody excess makes a neat VDRL read falsely negative in secondary syphilis and pregnancy — the laboratory dilutes the serum to unmask it. - Biological false-positive VDRLs (transient, low titre) occur in pregnancy, malaria, leprosy, hepatitis and other febrile or autoimmune states — exactly the diseases common in Indian practice, which is why a reactive VDRL alone never diagnoses syphilis. - Chancre versus chancroid is the classic genital comparison: syphilitic chancre is painless, indurated, non-purulent with painless nodes; Haemophilus ducreyi gives a painful, ragged, undermined ulcer with tender, sometimes suppurative buboes. - Condylomata lata (secondary syphilis, moist flat plaques teeming with spirochaetes) differ from condylomata acuminata (HPV papillomatous warts). - Neurosyphilis workup needs cerebrospinal fluid: VDRL on CSF is highly specific though insensitive; treat with intravenous aqueous crystalline penicillin for 10–14 days. - Congenital syphilis: early snuffles, desquamating rash, hepatosplenomegaly; late Hutchinson triad — notched teeth, interstitial keratitis, eighth-nerve deafness; prevented by maternal screening in the first antenatal visit. - Non-venereal treponematoses: yaws (T. pallidum subspecies pertenue), bejel and pinta spread by contact in childhood — India was declared yaws-free in 2016, a national programme milestone the exam likes. ## Reading a serology pair properly A pregnant woman at booking returns a reactive VDRL at 1:8. Do not treat the number as a diagnosis — order the TPHA. Both positive means syphilis, and the stage is set by history and examination; treatment with benzathine penicillin follows, with the VDRL titre repeated at three-month intervals to document a four-fold fall (1:2 or lower). VDRL positive but TPHA negative, in an afebrile woman with malaria or a history of leprosy, is a biological false positive to be rechecked after the intercurrent illness. VDRL negative but clinical secondary syphilis should prompt the prozone question — ask the laboratory to dilute the serum. In the newborn of a treated mother, a positive VDRL of the infant falling in parallel with passively transferred immunoglobulin G over months (less than the maternal titre and declining) needs no therapy, whereas a rising infant titre with snuffles does. Every branch of the algorithm rests on the same idea: the non-treponemal test tracks activity, the treponemal test tracks exposure. ## Where students slip The screening-versus-confirmatory roles get swapped — VDRL is the screening and monitoring tool, TPHA the confirmatory, and a stem asking “which test becomes negative after adequate treatment” wants VDRL/RPR, never TPHA. The prozone and the biological false positive are mirror-image traps: one hides true disease, the other fabricates it, and both are undone by the paired treponemal test. The Jarisch–Herxheimer reaction is misread as penicillin allergy; it is cytokine release from dying spirochaetes, managed with antipyretics, and treatment continues. Finally, the painless/painful genital ulcer table is answered backwards under time pressure — anchor the indurated, painless, single chancre to syphilis and the tender, ragged, multiple ulcer with bubo to chancroid. ## Frequently asked questions ### Which syphilis test is used to monitor treatment response? Quantitative VDRL or RPR — a four-fold titre decline over months documents cure; treponemal tests like TPHA persist for life and cannot follow activity. ### Why can a VDRL be negative in secondary syphilis? The prozone effect — antibody excess interferes with flocculation; diluting the serum unmasks a strongly positive reaction. ### What causes biological false-positive VDRL reactions? Conditions including pregnancy, malaria, leprosy, viral hepatitis and autoimmune disease, typically at low titre and transiently; treponemal-specific testing sorts them from true infection. ### How is neurosyphilis investigated? Cerebrospinal fluid examination — cell count, protein and VDRL, which is highly specific though insensitive; treatment is intravenous aqueous crystalline penicillin for 10–14 days. ### Which non-venereal treponematoses matter for Indian exams? Yaws (Treponema pallidum subspecies pertenue), bejel and pinta, transmitted by childhood contact; India was declared yaws-free in 2016 after national eradication efforts.