How Syphilis Spreads
Syphilis is transmitted through direct contact with infectious mucocutaneous lesions during vaginal, anal, or oral sex, and vertically from infected pregnant women to their fetuses through the placenta. 1
Primary Routes of Transmission
Sexual Transmission
- Sexual transmission occurs only when mucocutaneous syphilitic lesions are present, which are most common during the primary and secondary stages of infection 2
- Direct contact with active chancres, skin rashes, or mucosal lesions during sexual activity (vaginal, anal, or oral) transmits the spirochete bacterium Treponema pallidum 1, 3
- These manifestations are uncommon after the first year of infection, meaning latent syphilis is not sexually transmitted 2
Vertical (Congenital) Transmission
- Infected pregnant women can transmit syphilis to their fetuses through the placenta during pregnancy 1
- Up to 40% of fetuses with in-utero exposure to syphilis are stillborn or die from their infection during infancy 1
Infectious Stages
High-Risk Periods
- Primary and secondary syphilis are the most infectious stages 4, 5
- Primary syphilis is characterized by painless anogenital chancres at the site of inoculation 6, 1
- Secondary syphilis presents with diffuse rash, mucocutaneous lesions, and lymphadenopathy 6, 1
- Early latent syphilis (within the first year of infection) may still pose transmission risk 2
Non-Infectious Periods
- Late latent syphilis (>1 year duration) and tertiary syphilis are not sexually transmitted 2
- The latent period is asymptomatic but serologically reactive 2
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
- People with HIV infection are at increased risk 1
- Individuals engaging in condomless sex with multiple partners 1
- From 2019 to 2023, US syphilis cases increased by 61% overall, with diagnoses among females increasing by 112% and congenital syphilis cases increasing by 106% 1
Important Clinical Caveat
Syphilis lesions increase the risk of both acquisition and transmission of HIV infection, making co-infection a major public health concern 7, 4