It is a spirochaete with endoflagella for motility and is arranged in regular spirals. It doesn’t grow in culture media. It has a cardiolipin antigen, and antibodies against cardiolipin are used to help diagnose treponemal infections. It is the causative agent of syphilis, which is an STD and a notifiable disease. Cases are steadily rising in the USA.
Syphilis: Syphilis presents in primary, secondary, and tertiary forms.
Primary syphilis is characterised by a painless, indurated ulcer called a chancre, typically seen at the point of entry of the bacteria (for example, the genital area or mouth). This is accompanied by regional, rubbery lymphadenopathy. This stage is contagious.
After haematogenous dissemination, secondary syphilis develops. It is characterised by:
Tertiary syphilis follows years later and may cause complications such as aortic aneurysms, chronic granulomas called “gumma”, tabes dorsalis, and general paralysis of the insane, etc. Typically, these lesions are not infectious.
Latent syphilis can be seen after the primary or secondary stage. The person has no disease manifestations but will be positive on serological tests for syphilis.
Congenital syphilis can be acquired across the placenta after the first trimester of pregnancy. It presents with hepatosplenomegaly and skin and bone lesions.
For diagnosis of syphilis, T. pallidum can be observed in samples taken from chancres or condylomas using dark field microscopy or a direct fluorescent antibody test. Gram stain is not used because the organism can’t be seen well. In biopsy samples, special stains such as silver impregnation by the Levaditi method or immunofluorescence can be used.
There are reagin (non-specific) tests as well as specific tests for detecting syphilis.
The reagin tests rely on detecting antibodies to nontreponemal antigens such as cardiolipin. They include:
Development of floccules is a positive test. A titre of 1:8 or more is considered significant. These tests are positive in primary and secondary syphilis, and the antibody titre decreases after effective treatment.
A drawback is false positive results in the presence of leprosy, SLE, malaria, infectious mononucleosis, hepatitis B, etc.
Remember: for CSF, the VDRL test is preferred over RPR because RPR gives more false negative results than VDRL.
When antibody titers are very high, the “prozone phenomenon” may occur and produce a false negative result. This can be overcome by diluting the initial sample.
The specific tests for syphilis include:
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