Treponema
On this page
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.