Syphilis
Syphilis:
Good old syphilis! Syphilis is caused by the spirochete Treponema pallidum. It is transferred by person to person contact and is a well-known sexually transmitted disease.
Syphilis broadly breaks down into primary, secondary, latent, and tertiary syphilis. Let’s break down each one:
Primary syphilis:
This occurs about 2 weeks to 3 months after infection and presents as a localized sore or chancre where the bacteria came in contact with the person. This stage usually lasts between 1-6 weeks if left untreated.
Secondary syphilis:
If primary syphilis is left untreated, new symptoms will arise in about 25% of people about 1-2 months after the primary sore or chancre has disappeared (the sore or chancre does sometimes persist). The most notable symptom of secondary syphilis is a rash on the palms of the hands and/or soles of the feet. This rash is indicative that the organism has begun to spread through the person’s body. This stage usually lasts between 1-8 weeks.
Latent syphilis:
This stage occurs after secondary syphilis and is characterized by a lack of symptoms but the person is still carrying the bacteria. Latent syphilis is not contagious but pregnant women can still pass the bacteria to their child.
Tertiary syphilis:
If still untreated, about 33% of people will develop tertiary syphilis. Symptoms here are severe and include neurological and cardiovascular issues that can result in death. This stage occurs 10-30 years after initial infection.
Screening and testing for syphilis in the clinical lab is generally done using serology tests because of efficiency and cost. There are seven syphilis tests to be aware of and know the basics of:
1. Rapid plasma reagin test (RPR)
a. There are two syphilis tests, RPR and VDRL, that involve testing for “nontreponemal” antigens. Nontreponemal antigens are nonspecific for the causative bacteria Treponema pallidum but are associated with syphilis. The RPR reagent antigen is cardiolipin attached to charcoal particles (the charcoal particles are for visualization). The term reagin generally means antibody, so the name just indicates it’s a quick antibody test. The RPR test also utilizes a silicon needle for dispensing the antigen onto the testing card.
b. Malaria, rheumatoid arthritis, infectious mononucleosis, leprosy, hepatitis, and systemic lupus erythematosus (SLE) can all cause a false positive RPR or VDRL test.
2. Venereal disease research laboratory (VDRL)
a. The VDRL test is very similar to RPR except it uses the reagent antigen cardiolipin and lecithin (lecithin enhances sensitivity). VDRL serum must also be heat inactivated to inactivate complement. VDRL is less sensitive than the RPR test. Both RPR and VDRL are associated with false positives so any positive in either of these tests must be confirmed by a specific Treponema pallidum test. A modified VDRL test can be used in cases of syphilis in the cerebral spinal fluid (CSF) and is generally diagnostic.
3. Fluorescent treponemal antibody absorption test (FT-ABS)
a. The FT-ABS test is an indirect immunofluorescence test. The test utilizes a specific Treponema pallidum strain called the Nichols strain. FT-ABS is more specific than RPR and VDRL but is more time consuming.
4. Microhemagglutination (MHA-TP)
a. The MHA-TP uses a sheep red cell line that is sensitized with Treponema pallidum. It is about equal strength to the FT-ABS test. This test has been replaced by the TP-PA test.
5. Treponema pallidum immobilization test (TPI)
a. The TPI test uses live Treponema pallidum. Patient serum is added to the live treponemes and if there is antibody in the patient’s serum, the treponemes will be immobilized. Dark field microscopy is used to visualize the organisms. The test is expensive and seldom used.
6. Dark field microscopy
a. Primary and secondary syphilis can be diagnosed by looking at skin lesions under dark field microscopy. The presence of the corkscrew shaped Treponema pallidum is diagnostic.
7. Treponema pallidum particle agglutination test (TP-PA)
a. The TP-PA test is very similar to the MHA-TP test; however, the TP-PA test uses gelatin particles sensitized with Treponema pallidum instead of sheep red cells. TPPA is generally positive for life.
Study Tip
It is important to know which tests are more sensitive than others in primary, secondary, and tertiary syphilis. Use the image below to keep them straight.
