How Syphilis Spreads
Syphilis is transmitted through direct contact with infectious mucocutaneous lesions during vaginal, anal, or oral sex, and vertically from mother to fetus through the placenta during pregnancy. 1
Primary Routes of Transmission
Sexual Transmission
Sexual contact with active syphilitic lesions is the primary mode of transmission in adults. 2 The spirochete Treponema pallidum enters through mucous membranes or microscopic breaks in skin during sexual activity. 3
Transmission occurs specifically through contact with infectious lesions present in primary and secondary syphilis stages, which are the most contagious periods. 3 These lesions can appear on genitals, anus, rectum, lips, or in the mouth. 1
Sexual transmission only occurs when mucocutaneous syphilitic lesions are present; such manifestations are uncommon after the first year of infection. 2 This means late-stage syphilis is rarely sexually transmitted.
Vaginal, anal, and oral sex all represent effective routes of transmission when infectious lesions are present. 1
Vertical (Congenital) Transmission
Syphilis transmits from infected pregnant women to their fetuses through the placenta (transplacental transmission). 2, 4 This can occur at any stage of pregnancy.
Up to 40% of fetuses with in-utero exposure to syphilis are stillborn or die from infection during infancy, highlighting the severity of vertical transmission. 1
Congenital transmission can result in stillbirth, miscarriage, preterm birth, birth defects, and lifelong physical or neurologic changes in surviving infants. 4
Non-Sexual Direct Contact
While rare, transmission can theoretically occur through direct contact with infectious lesions outside of sexual activity, though this is not a common route. 3
Healthcare providers examining patients with syphilis should use gloves when examining lesions or handling potentially infectious materials to prevent occupational transmission. 5
Important Transmission Characteristics
Timing and Infectiousness
Primary and secondary stages are the most infectious periods. 3 During these stages, lesions contain high concentrations of treponemes.
The CDC recommends presumptive treatment for sexual partners exposed within 90 days of a diagnosed case, even if seronegative, because early infection may not produce detectable antibodies. 6, 7
For partner notification purposes, the lookback periods are: 3 months plus duration of symptoms for primary syphilis, 6 months plus duration of symptoms for secondary syphilis, and 1 year for early latent syphilis. 2, 6
High-Risk Populations
Men who have sex with men (MSM) comprised one-third (32.7%) of all males with primary and secondary syphilis in 2023. 1
Individuals at increased risk include people with HIV, those engaging in condomless sex with multiple partners, and MSM. 1
Common Pitfalls to Avoid
Do not assume syphilis cannot be transmitted during latent stages. While less common, transmission can still occur during early latent syphilis (first year after infection). 2
Do not overlook the need for partner treatment. Partners exposed within 90 days should receive presumptive treatment with benzathine penicillin G 2.4 million units IM, even with negative serology. 6, 7
Do not forget that all pregnant women require routine serologic screening at the first prenatal visit, at 28 weeks gestation, and at delivery in high-risk populations to prevent congenital transmission. 2
All patients diagnosed with syphilis should be tested for HIV due to high co-infection rates and because syphilitic lesions increase HIV transmission risk. 6, 7, 8