Syphilis is a sexually transmitted disease caused by the bacterium Treponema pallidum. In adults, the untreated disease can progress over the course of years or decades, causing a variety of health complications and death. Overall risk of placental transmission is 60-80%, rising during the second half of pregnancy. According to one estimate, roughly 30% of infected fetuses are stillborn or suffer early neonatal deaths.
Mechanism of Action
Syphilis diagnosis presents opportunity for innovation. Dark field microscopy can be used to characterize fluid from open lesions. More commonly, serological tests are used. Treponemal tests check for the specific antibodies to T. pallidum. These antibodies can remain active even after syphilis is cured, so a positive test does not confirm active infection. Non-treponemal confirm an active infection, but not necessarily syphilis. The two tests, therefore, are best used in combination. Syphilis treatment is relatively mature. For mothers, a single dose of 2.4M units Penicillin G IM is recommended. Erythromycin and tetracycline (which can be effective for non-pregnant adults) are second line therapy. For neonates, 50K units/kg/day is used.
Current Use in High-Resource Settings
Non-treponemal blood tests (RPR or VDRL) are typically a first screen for infection in a centralized laboratory setting where reagents, refrigeration, heating, and microscopes are available. If the patient’s non-treponemal test is reactive, then the more expensive treponemal blood test confirms the diagnosis. Several variations are used, but all require a microscope, centrifuge, water bath, plate shaker, and/or refrigerator.
Application in Low-Resource Settings
Antenatal syphilis screening is a national policy in many low-resource settings, but the majority of mothers do not have access to laboratory-based services. Rapid, point of care diagnostics performed on whole blood present a promising alternative. These tests are typically performed using whole blood taken from the patient’s finger tip and placed on immunochromatographic strip housed in a plastic cartridge. No power is required, and results visible to the naked eye are available in 20 minutes or less. While treatment is inexpensive, benzathine penicillin G requires refrigeration and at least one injection.