Syphilis Transmission Risk Through Vaginal Intercourse
Syphilis has a moderate to high probability of transmission during vaginal intercourse when mucocutaneous lesions are present, with sexual transmission occurring only when infectious lesions exist on the partner. 1
Transmission Dynamics and Risk Factors
Sexual transmission of Treponema pallidum occurs exclusively when mucocutaneous syphilitic lesions are present, which are uncommon after the first year of infection. 2 This means the highest transmission risk occurs during:
- Primary syphilis (characterized by painless anogenital chancres) 3
- Secondary syphilis (characterized by diffuse rash, mucocutaneous lesions, and lymphadenopathy) 3
- Early latent syphilis (within the first year of infection) 2
The transmission probability is moderate to high during contact between susceptible and infectious sexual partners, which helps explain the persistence of syphilis within high-risk populations. 1 This contrasts with some other sexually transmitted infections that have lower per-contact transmission rates.
Clinical Context for Risk Assessment
The early stages of syphilis (primary, secondary, and early latent within the first year) are potentially infectious, while late latent and tertiary stages rarely transmit infection. 4, 5 This temporal pattern is critical for understanding transmission risk:
- Primary syphilis: Partners exposed within 3 months plus duration of symptoms are at risk 2, 6, 7
- Secondary syphilis: Partners exposed within 6 months plus duration of symptoms are at risk 2, 6, 7
- Early latent syphilis: Partners exposed within 1 year before diagnosis are at risk 2, 6, 7
Partner Management Based on Transmission Risk
Persons exposed within 90 days preceding diagnosis of primary, secondary, or early latent syphilis might be infected even if seronegative; therefore, such persons should be treated presumptively with benzathine penicillin G 2.4 million units IM. 2, 8, 7 This recommendation reflects the significant transmission risk during early infection, even before serologic tests become positive.
For partners exposed more than 90 days before diagnosis, presumptive treatment should still be administered if serologic results are not immediately available and follow-up is uncertain. 2
Important Clinical Caveats
The presence of syphilitic lesions dramatically increases HIV transmission risk, making co-infection screening essential for all patients with syphilis. 2, 6, 8 This bidirectional relationship between syphilis and HIV amplifies the public health importance of identifying and treating syphilis promptly.
Congenital transmission represents a separate high-risk scenario, with up to 40% of fetuses with in-utero exposure being stillborn or dying from infection during infancy. 3 This underscores the critical importance of screening pregnant individuals three times during pregnancy (at first prenatal visit, third trimester, and delivery). 3
Prevention Strategies
Routine screening is recommended at least annually for sexually active individuals at increased risk, including men who have sex with men, people with HIV, and those engaging in condomless sex with multiple partners. 2, 3 More frequent screening every 3-6 months is appropriate for those with multiple partners or high-risk behaviors. 2
Doxycycline postexposure prophylaxis (200 mg taken within 72 hours after sex) is recommended for men who have sex with men and transgender women with a history of sexually transmitted infection in the past year. 3