Three genera of spirochetes cause human infection:
Treponema, which causes syphilis, yaws, and pinta
Borrelia, which causes Lyme disease and relapsing fever
Leptospira, which causes leptospirosis
The particular spirochete responsible for syphilis is Treponema pallidum.
T pallidum is a fragile spiral bacterium 6-15 micrometers long by 0.25 micrometers in diameter. Its small size makes it invisible on light microscopy; therefore, it must be identified by its distinctive undulating movements on darkfield microscopy. It can survive only briefly outside of the body; thus, transmission almost always requires direct contact with the infectious lesion.
Syphilis is usually classified into 4 stages: primary, secondary, latent, and tertiary. It can be either acquired or congenital. That is, it can be transmitted either by intimate contact with infectious lesions (most common) or via blood transfusion (if blood has been collected during early syphilis), and it can also be transmitted transplacentally from an infected mother to her fetus.
In acquired syphilis, T pallidum rapidly penetrates intact mucous membranes or microscopic dermal abrasions and, within a few hours, enters the lymphatics and blood to produce systemic infection. Incubation time from exposure to development of primary lesions, which occur at the primary site of inoculation, averages 3 weeks but can range from 10-90 days. Studies in rabbits show that spirochetes can be found in the lymphatic system as early as 30 minutes after primary inoculation, suggesting that syphilis is a systemic disease from the outset.
The central nervous system (CNS) is invaded early in the infection; during the secondary stage, examinations condylomata lata syphilis treatment with azithromycin demonstrate that more than 30% of patients have abnormal findings in the cerebrospinal fluid (CSF). During the first 5-10 years after the onset of untreated primary infection, the disease principally involves the meninges and blood vessels, resulting in meningovascular neurosyphilis. Later, the parenchyma of the brain and spinal cord are damaged, resulting in parenchymatous neurosyphilis. Go to Neurosyphilis for complete information on this topic.
Regardless of the stage of disease and location of lesions, histopathologic hallmarks of syphilis include endarteritis (which in some instances may be obliterative in nature) and a plasma cell–rich infiltrate. Endarteritis is caused by the binding of spirochetes to endothelial cells, mediated by host fibronectin molecules bound to the surface of the spirochetes. The resultant endarteritis can heal with scarring in some instances.
The syphilitic infiltrate reflects a delayed-type hypersensitivity response to T pallidum, and in certain individuals with tertiary syphilis, this response by sensitized T lymphocytes and macrophages results in gummatous ulcerations and necrosis. Antigens of T pallidum induce host production of treponemal antibodies and nonspecific reagin antibodies. Immunity to syphilis is incomplete.
For example, host humoral and cellular immune responses may prevent the formation of a primary lesion on subsequent infections with T pallidum, but they are insufficient to clear the organism. This may be because the outer sheath of the spirochete is lacking immunogenic molecules, or it may be because of down-regulation of helper T cells of the TH1 class. [1, 2]
Primary syphilis is characterized by the development of a painless chancre at the site of transmission after an incubation period of 3-6 weeks. The lesion has a punched-out base and rolled edges and is highly infectious.
Histologically, the chancre is characterized by mononuclear leukocytic infiltration, macrophages, and lymphocytes. The inflammatory reaction causes an obliterative endarteritis. In this stage, the spirochete can be isolated from the surface of the ulceration or the overlying exudate of the chancre. Whether treated or not, healing occurs within 3-12 weeks, with considerable residual fibrosis.
Secondary syphilis develops about 4-10 weeks after the appearance of the primary lesion. During this stage, the spirochetes multiply and spread throughout the body. Secondary syphilis lesions are quite variable in their manifestations. Systemic manifestations include malaise, fever, myalgias, arthralgias, lymphadenopathy, and rash.
Widespread mucocutaneous lesions are observed over the entire body and may involve the palms, soles, and oral mucosae. Most often, the lesions are macular, discrete, reddish brown, and 5 mm or smaller in diameter; however, they can be pustular, annular, or scaling. Vesicular rash is typically absent. All such lesions contain treponemes. Of these, wet mucous patches are the most contagious. Histologically, the inflammatory reaction is similar to but less intense than that of the primary chancre.
Other skin findings of secondary syphilis are condylomata lata and patchy alopecia. Condylomata lata are painless, highly infectious gray-white lesions that develop in warm, moist sites. The alopecia is characterized by patchy hair loss of the scalp and facial hair, including the eyebrows. Patients with this finding have been referred to as having a “moth-eaten” appearance. During secondary infection, the immune reaction is at its peak and antibody titers are high.
Latent syphilis is a stage at which the features of secondary syphilis have resolved, though patients remain seroreactive. Some patients experience recurrence of the infectious skin lesions of secondary syphilis during this period. About one third of untreated latent syphilis patients go on to develop tertiary syphilis, whereas the rest remain asymptomatic.
Currently, tertiary syphilis disease is rare. When it does occur, it mainly affects the cardiovascular system (80-85%) and the CNS (5-10%), developing over months to years and involving slow inflammatory damage to tissues. The 3 general categories of tertiary syphilis are gummatous syphilis (also called late benign), cardiovascular syphilis, and neurosyphilis.
Gummatous syphilis is characterized by granulomatous lesions, called gummas, which are characterized by a center of necrotic tissue with a rubbery texture. Gummas principally form in the liver, bones, and testes but may affect any organ. Histological examination shows palisaded macrophages and fibroblasts, as well as plasma cells surrounding the margins. Gummas may break down and form ulcers, eventually becoming fibrotic. Treponemes are rarely visualized or recovered from these lesions.
Cardiovascular syphilis occurs at least 10 years after primary infection. The most common manifestation is aneurysm formation in the ascending aorta, caused by chronic inflammatory destruction of the vasa vasorum, the penetrating vessels that nourish the walls of large arteries. Aortic valve insufficiency may result.
Neurosyphilis has several forms. If the spirochete invades the CNS, syphilitic meningitis results. Syphilitic meningitis is an early manifestation, usually occurring within 6 months of the primary infection. CSF shows high protein, low glucose, high lymphocyte count, and positive syphilis serology.
Meningovascular syphilis occurs as a result of damage to the blood vessels of the meninges, brain, and spinal cord, leading to infarctions causing a wide spectrum of neurologic impairments.
Parenchymal neurosyphilis includes tabes dorsalis and general paresis. Tabes dorsalis develops as the posterior columns and dorsal roots of the spinal cord are damaged. Posterior column impairment results in impaired vibration and proprioceptive sensation, leading to a wide-based gait.
Disruption of the dorsal roots leads to loss of pain and temperature sensation and areflexia. Damage to the cortical regions of the brain leads to general paresis, formerly called “general paresis of the insane,” which mimics other forms of dementia. Impairment of memory and speech, personality changes, irritability, and psychotic symptoms develop and may advance to progressive dementia.
The Argyll-Robertson pupil, a pupil that does not react to light but does constrict during accommodation, may be seen in tabes dorsalis and general paresis. The precise location of the lesion causing this phenomenon is unknown.
Congenital syphilis, discussed briefly here, is a veritable potpourri of antiquated medical terminology. The treponemes readily cross the placental barrier and infect the fetus, causing a high rate of spontaneous abortion and stillbirth. Within the first 2 years of life, symptoms are similar to severe adult secondary syphilis with widespread condylomata lata and rash. “Snuffles” describes the mucopurulent rhinitis caused by involvement of the nasal mucosae.
Later manifestations of congenital syphilis include bone and teeth deformities, such as “saddle nose” (due to destruction of the nasal septum), “saber shins” (due to inflammation and bowing of the tibia), “Clutton’s joints” (due to inflammation of the knee joints), “Hutchinson’s teeth” (in which the upper incisors are widely spaced and notched), and “mulberry molars” (in which the molars have too many cusps).
Tabes dorsalis and general paresis may develop as in adults, with 8th cranial nerve deafness and optic nerve atrophy as well as a variety of other ophthalmologic involvement leading to blindness being additional features.
From 2012-2014, the number of congenital syphilis cases in the United States increased from 334 to 458. This appears to be associated with an increase in the rate of primary and secondary syphilis among women. 
Go to Pediatric Syphilis for complete information on this topic.
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