Syphilis is a bacterial infection caused by Treponema pallidum. The disease progresses through distinct stages if left untreated, starting with the appearance of a sore at the site of infection. While syphilis is a well-known sexually transmitted infection (STI), a common public health concern revolves around whether the bacteria can spread through less intimate contact like kissing. The risk of transmission depends entirely on the presence of infectious lesions and the nature of the contact.
Transmission Risk from Oral Contact
Casual kissing, such as a brief kiss on the cheek or closed-mouth contact, carries a negligible risk of transmitting syphilis. The bacteria, Treponema pallidum, is fragile and cannot survive long outside the human body or pass through intact skin. Transmission requires direct physical contact with an active, infectious lesion, characteristic of the primary and secondary stages of the disease.
If a person has an active chancre (sore) or a mucous patch in or around their mouth, deep or open-mouth kissing can facilitate bacterial transfer. These highly contagious lesions contain a high concentration of bacteria. Transmission occurs when the infectious material contacts a mucous membrane or a minor break in the skin of the uninfected partner. The risk is related to the direct exchange of infectious fluid from a sore, not the act of kissing itself.
If no infectious lesions are present, transmission through kissing is not possible. This is why transmission through inanimate objects like drinking glasses or doorknobs is not a concern. While rare compared to sexual activity, transmission via kissing is a documented route when deep kissing involves contact with infectious oral lesions.
Syphilis: Primary Routes of Infection
The overwhelming majority of syphilis infections occur through sexual contact involving the genitals, anus, or mouth. The most common route is through unprotected vaginal, anal, or oral sex, where the bacteria gains entry into the body. Transmission happens when there is direct contact between the mucous membranes or abraded skin of one person and an infectious sore on the other person.
The infectious sore, or chancre, most frequently appears on the genitals, anus, or rectum, as these are the most common sites of bacterial entry during sexual activity. Chancres can be painless and located internally, meaning they often go unnoticed by the infected person. The risk of acquiring the disease from a partner with primary or secondary syphilis is estimated to be between 30% and 60% per exposure.
Another serious route of infection is vertical transmission, where the infection is passed from a pregnant person to their baby during gestation. This is known as congenital syphilis. The risk of transmission to the fetus is highest if the pregnant person has primary or secondary syphilis and goes untreated. Less common routes of transmission include exposure through blood products, though this is now rare due to widespread blood screening practices.
Identifying Syphilis Symptoms in the Mouth
When the bacteria enters the body through the oral cavity, the initial sign is the development of a primary stage chancre at the site of inoculation. This chancre is typically a single, firm, round ulceration that can appear on the lips, tongue, gums, or tonsils. A distinctive feature of this sore is that it is often painless, which allows it to easily go unnoticed.
Oral chancres are present in a smaller percentage of cases, occurring in about 4% to 12% of patients with primary syphilis. These sores may be difficult to differentiate from other mouth ulcers or lesions. Even without treatment, the primary chancre will spontaneously heal within three to six weeks, but the bacteria remains in the body and the infection progresses.
If the infection is not treated, the secondary stage can present with highly infectious lesions inside the mouth, known as mucous patches. These patches are grayish-white, slightly raised areas found on the tongue, tonsils, or the inner lining of the cheek. They are considered extremely infectious due to the high concentration of bacteria they contain. Approximately 30% of individuals in the secondary stage will develop these contagious oral lesions.
Testing and Treatment
Diagnosis of syphilis typically begins with a physical examination of any suspicious sores, followed by a blood test. The standard diagnostic approach involves using two types of blood tests: a non-treponemal test (such as RPR or VDRL) and a treponemal test. Non-treponemal tests measure antibodies that are not specific to the syphilis bacteria, while treponemal tests look for antibodies that directly target Treponema pallidum.
If an active sore is present, a health care provider may also take a fluid sample from the lesion to look for the bacteria directly. In the early stages, non-treponemal tests may not be fully sensitive, so a combination of tests is necessary to confirm the diagnosis and determine the stage of infection. Staging is important because it dictates the appropriate treatment regimen.
Syphilis is fully treatable and curable, especially when caught in the early stages. The preferred and most effective treatment is an injection of Benzathine penicillin G. For primary, secondary, and early latent syphilis, a single intramuscular dose of penicillin is usually curative. Patients with an allergy may be treated with alternative antibiotics, although penicillin remains the only recommended option for treating syphilis during pregnancy or cases involving the nervous system.