Syphilis is an infectious disease caused by the bacterium Treponema pallidum. This spirochete has a distinct spiral or corkscrew shape.
The Microbe’s Unique Characteristics
Treponema pallidum is a fragile, spiral-shaped bacterium, measuring about 6 to 15 micrometers long and 0.1 to 0.2 micrometers wide. Its unique corkscrew shape, facilitated by internal flagella, allows it to move effectively and penetrate tissues. This bacterium is classified as gram-negative, but it notably lacks lipopolysaccharides in its outer membrane, which are common in other gram-negative bacteria.
The microbe is also known for its fastidious nature, meaning it has strict requirements for survival. Treponema pallidum cannot be cultured in artificial laboratory media. It can only survive briefly outside the human body, which is why transmission almost always requires direct contact with an infectious lesion. The bacterium’s outer membrane has a very low concentration of surface-exposed proteins, allowing it to evade immune detection in the early stages of infection.
How the Microbe Causes Syphilis
Treponema pallidum typically enters the body through mucous membranes or tiny breaks in the skin, often during sexual contact. Once inside, it rapidly penetrates the lymphatic system and bloodstream, becoming a systemic infection within hours. The incubation period, from exposure to the appearance of the first lesion, averages about three weeks but can range from 10 to 90 days.
The disease progresses through distinct stages if left untreated. The primary stage is marked by the appearance of a chancre, a firm, painless ulcer that develops at the site of inoculation. This sore, typically 1-2 cm in diameter, often appears on the genitals, mouth, or rectum and usually heals within 3 to 10 weeks, even without treatment. However, the microbe continues to multiply and spread throughout the body during this time.
The secondary stage usually emerges 2 to 8 weeks after the chancre disappears. This stage is characterized by a widespread, non-itchy rash that can cover the entire body, including the palms of the hands and soles of the feet. Other symptoms can include fever, fatigue, swollen lymph nodes, sore throat, muscle aches, and hair loss. These symptoms can resolve spontaneously, leading into the latent stage.
During the latent stage, there are few or no noticeable symptoms, but the infection remains in the body and can last for many years, even decades. This period is divided into early latent (within the first year of infection) and late latent (after one year). If untreated, approximately one-third of individuals will progress to the tertiary stage.
Tertiary syphilis is the most destructive phase, developing years to decades after the initial infection. It can cause severe damage to various organ systems, including the heart (cardiovascular syphilis, such as aortic aneurysms), the brain and nervous system (neurosyphilis, leading to problems like meningitis, paralysis, or dementia), and can form soft, non-cancerous growths called gummas on the skin, bones, or internal organs. These severe manifestations are a direct result of the microbe’s long-term presence and the body’s inflammatory response to it.
Identifying the Microbe and Infection
Detecting Treponema pallidum involves several methods. Direct visualization of the microbe is possible using darkfield microscopy, particularly from fluid collected from a chancre in primary syphilis. This technique allows observation of the bacterium’s distinctive undulating movements, as its small size makes it invisible under standard light microscopy. However, direct visualization is not always feasible or available.
Blood tests are the more common indirect methods. Non-treponemal tests, such as the Rapid Plasma Reagin (RPR) or Venereal Disease Research Laboratory (VDRL) tests, detect antibodies that the body produces in response to tissue damage caused by the infection. These tests are useful for screening and monitoring treatment effectiveness, as their levels typically decrease after successful treatment. Treponemal tests, like the T. pallidum Particle Agglutination (TP-PA) or Fluorescent Treponemal Antibody Absorption (FTA-ABS) tests, detect specific antibodies directed against Treponema pallidum itself. These tests usually remain positive for life, even after successful treatment, indicating past exposure to the microbe.
Eliminating the Microbe
The primary method for eliminating Treponema pallidum involves antibiotic treatment. Penicillin is a highly effective antibiotic against Treponema pallidum across all stages of syphilis. For primary, secondary, and early latent syphilis, a single intramuscular dose of benzathine penicillin G is usually sufficient.
For later stages, such as late latent syphilis, tertiary syphilis, or neurosyphilis, multiple doses of penicillin over several weeks are administered to ensure complete eradication of the microbe, as it replicates slowly. Individuals with a penicillin allergy may be treated with alternative antibiotics, such as doxycycline or tetracycline, although penicillin desensitization may be considered for severe cases like neurosyphilis or for pregnant patients. Prompt and complete treatment is effective in curing the infection and preventing further progression and damage caused by the microbe.
Preventing Microbe Transmission
Preventing the transmission of Treponema pallidum relies on safe sexual practices. Consistent and correct use of condoms during vaginal, anal, and oral sex significantly reduces the risk of transmission. However, condoms may not cover all infectious sores, so direct contact with any lesion can still lead to infection.
Regular testing for sexually transmitted infections, including syphilis, is important for sexually active individuals, especially those with multiple partners. Early detection and treatment prevent further spread of the microbe. Screening pregnant women for syphilis is also a routine public health measure to prevent congenital syphilis, which can have devastating effects on the fetus. Partner notification and treatment are also important to break the chain of transmission, ensuring that anyone who may have been exposed to the microbe also receives testing and appropriate treatment.