Syphilis is caused by the bacterium Treponema pallidum, a fragile organism that cannot survive long outside a human host. While the infection is classified as a sexually transmitted infection and is most commonly spread through direct contact with an infectious sore during sexual activity, it can be transmitted in other ways. These non-sexual routes of transmission are important to understand, as they carry significant health risks for certain populations. Non-sexual spread often centers on two primary mechanisms: transmission from a pregnant person to their fetus and exposure through blood. Understanding how this bacterium spreads is the first step toward effective prevention and treatment.
Vertical Transmission During Pregnancy
The most significant non-sexual route is vertical transmission, which occurs when Treponema pallidum crosses the placental barrier from a pregnant individual to their developing fetus. This results in congenital syphilis. The risk of transmission is highest during the early stages of maternal infection, such as the primary and secondary stages, but it can occur at any point during gestation.
If a pregnant person has untreated syphilis, the bacterium can severely harm the fetus, leading to adverse outcomes. These risks include premature birth, low birth weight, miscarriage, stillbirth, and death shortly after birth. Untreated maternal syphilis results in infant death in a high percentage of cases.
This outcome is entirely preventable through routine prenatal screening and early treatment. Standard care involves testing for syphilis at the first prenatal visit, and sometimes again later in the third trimester for those at higher risk. The infection is effectively cured in both the pregnant person and the fetus with a specific regimen of penicillin, which remains the only antibiotic proven to prevent the infection. Prompt treatment, especially when administered more than 30 days before delivery, is highly successful in preventing congenital syphilis.
Transmission Through Blood Exposure
Syphilis can be transmitted when the bacteria are present in the bloodstream, creating a potential route for blood-borne spread. Treponema pallidum circulates in the blood during the primary, secondary, and sometimes the early latent stages of infection. One recognized non-sexual route is through the sharing of injection equipment, such as needles used for intravenous drug use.
When needles are shared, residual blood carrying the spirochete can be directly introduced into another person’s bloodstream, facilitating transmission. This risk is elevated among individuals who engage in sharing contaminated equipment.
Historically, syphilis was a concern for transmission through blood transfusions. However, the risk of acquiring syphilis via donated blood is now extremely low due to modern safety protocols. Rigorous screening of all blood donors and laboratory testing of blood products are standard practices. Furthermore, the Treponema pallidum bacteria are fragile and generally cannot survive the standard refrigeration temperature and storage duration required for donated blood products.
Clarifying Non-Infectious Contact
The fear of non-sexual transmission often stems from misconceptions about the bacteria’s ability to survive in the environment. Syphilis is not spread through casual contact, including hugging, shaking hands, or using the same bathroom facilities. The bacterium is fragile and requires specific conditions—moisture, warmth, and a living host—to remain viable and cause infection.
The organism dies rapidly when exposed to air or drying. For an infection to occur, there must be direct physical contact between broken skin or a mucous membrane and an active, infectious lesion, such as a chancre or mucous patch. These lesions are concentrated sources of the bacteria.
Because of this fragility, it is impossible to contract syphilis from inanimate objects. Toilet seats, doorknobs, swimming pools, hot tubs, shared clothing, or eating utensils do not pose a risk of transmission. The bacteria cannot survive long enough on these surfaces to establish an infection. While rare cases of non-sexual transmission have been reported through prolonged, close contact with active lesions, such as a human bite or mouth-to-mouth transfer of pre-chewed food, these are unusual exceptions that confirm the need for direct contact with an open sore.