What are the modes of transmission for syphilis?

Modes of Transmission for Syphilis

Syphilis is transmitted through direct contact with infectious mucocutaneous lesions during sexual activity (vaginal, anal, or oral sex), transplacentally from mother to fetus during pregnancy, and rarely through blood transfusion or non-sexual contact with active lesions. 1, 2

Primary Transmission Routes

Sexual Transmission

• Sexual contact is the predominant mode of transmission in adults, occurring only when mucocutaneous syphilitic lesions are present on the infected partner. 3, 1
• Transmission occurs through vaginal, anal, or oral sexual contact with infectious lesions, which are most common during primary and secondary stages of disease. 1, 2
• Sexual transmission becomes uncommon after the first year of infection as active lesions resolve. 3
• The per-act probability of transmission varies by route: male-to-male receptive anal intercourse carries a risk of 1 in 10 to 1 in 1600, male-to-female vaginal intercourse 1 in 200 to 1 in 2000, and female-to-male vaginal intercourse 1 in 700 to 1 in 3000. 3

Vertical (Congenital) Transmission

• Transplacental transmission from infected mother to fetus during pregnancy is the second major route, with up to 40% of exposed fetuses resulting in stillbirth or neonatal death. 1, 4
• Transmission can occur at any stage of pregnancy when the mother has untreated or inadequately treated syphilis. 5, 4
• In the absence of antiretroviral therapy (note: this appears to be an error in the source referring to penicillin treatment), the probability of perinatal transmission is approximately 1 in 4 (25%). 3
• Transmission can also occur through direct contact with maternal lesions at the time of delivery. 4

Blood-Borne Transmission

• Transmission through contaminated blood transfusion carries an estimated probability of 95 in 100, though this is now rare due to blood screening protocols. 3
• Needle sharing among injection drug users carries a transmission risk of approximately 1 in 150. 3

Occupational and Non-Sexual Contact

• Occupational needlestick exposure carries a transmission risk of approximately 1 in 300. 3
• Non-sexual contact with active lesions can occasionally transmit infection, though this is uncommon. 2

Clinical Context for Transmission Risk

High-Risk Populations

• Men who have sex with men (MSM) comprised 32.7% of all males with primary and secondary syphilis in 2023, representing a particularly high-risk group. 1
• Individuals with HIV infection, those engaging in condomless sex with multiple partners, and people with a history of sexually transmitted infections are at increased risk. 1

Infectious Period

• Persons are most infectious during primary and secondary syphilis when mucocutaneous lesions are present. 3
• For partner notification purposes, the infectious period extends 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. 3

Critical Prevention Considerations

Healthcare Provider Precautions

• Healthcare providers should always wear gloves when examining patients with suspected or confirmed syphilis, especially when examining lesions, as primary and secondary lesions are highly infectious. 6
• Particular care is needed when examining oral, genital, or perianal lesions, which contain high concentrations of treponemes. 6
• The risk of occupational transmission is extremely low when standard precautions are followed. 6

Common Pitfall

A critical error is failing to recognize that persons exposed to a patient with primary, secondary, or early latent syphilis within the preceding 90 days should be treated presumptively, even if seronegative, as they may be infected before seroconversion occurs. 3, 7