Syphilis transmission occurs mainly through sexual contact. The bacteria that causes syphilis, Treponema pallidum, is often present in mouth sores or on the genitals during certain stages of the disease. This bacteria can be passed on, therefore transmitting syphilis infection, to others through direct contact with tiny breaks in their skin or through mucous membranes. Though less common, syphilis transmission may also occur congenitally or via blood transfusions. Rather than being caused by direct contact with sores as in sexual transmission, these types of transmission occur through contact with blood that has been infected with the Treponema pallidum bacteria.
There are four main stages of infection with syphilis. Sexual syphilis transmission occurs primarily in the first and second stages, when an infected person often has a sore or lesions on his or her genitals or in his or her mouth. These sores and lesions often contain high amounts of Treponema pallidum bacteria, which can be passed to someone whose broken or irritated skin or mucous membranes, such as those found in the mouth and genitals, comes in contact with them. The latter stages of the disease, known as latent and tertiary syphilis, are less likely to be associated with sexual transmission, as infectious sores and lesions are less common.
While most syphilis transmission occurs through the sexual route, it can sometimes occur congenitally, or before birth. This form of syphilis transmission occurs when a pregnant mother has Treponema pallidum bacteria in her blood, a condition that can be present during all stages of syphilis infection. While a pregnant mother and her unborn baby don’t share the same blood, their circulatory systems are connected via the placenta, the organ that supplies the baby with the oxygen and nutrients it needs. In addition to being permeable to nutrients and oxygen, the placenta is also permeable to Treponema pallidum bacteria, so it can pass from the mother’s blood into the developing baby’s.
Syphilis transmission can also occur via the bloodborne route when a person receives a blood transfusion from an infected donor. As with congenital syphilis transmission, this can occur at any stage of infection. In many developed countries, this is relatively rare, as donated blood is usually tested for syphilis before it is transfused into another person. Still, since there is some risk, individuals who are at high risk for or who are being treated for syphilis are typically advised not to donate blood until they have had a blood test confirming the absence of infection.