Organism: Treponema pallidum subspecies pallidum (referred to as T. pallidum) is the causative agent of syphilis. The other major pathogens in the genus Treponema and the diseases they cause include: T. pallidum subspecies pertenue, the causative agent of yaws; T. pallidum subspecies endemicum, the cause of bejel; and T .pallidum subspecies carateum, the etiologic agent of pinta.

Clinical Manifestations: The protean manifestations of venereal syphilis prompted Sir William Osler to refer to syphilis as "the great imitator". Infection with T. pallidum occurs through the skin or mucous membranes and disseminates throughout the body during a mean incubation period of three weeks. Infections with T. pallidum are divided into three stages. Primary syphilis encompasses the lesion present at the site of inoculation, the so-called chancre, a slightly tender, ulceration with a smooth base and firm, raised edges. Regional lymphadenopathy is usually present and the chancre heals within 1 to 12 weeks. Secondary syphilis corresponds to the stage of the disease with the most numerous organisms and occurs approximately 1 to 2 months after the appearance of the chancre. This is the stage of syphilis associated with the disseminated macular to maculopapular rash that characteristically involves the palms and soles of feet. Any organ system may be involved during secondary syphilis including the central nervous system. Tertiary or late syphilis is used to define late complications of syphilis including central nervous system and cardiovascular abnormalities. This case demonstrates one complication of tertiary syphilis, syphilitic endarteritis of the aorta or syphilitic aortitis. This has been reported to occur in up to 10% of cases of untreated syphilis. Microscopically, there is a perivascular lymphoplasmacytic infiltrate around vaso vasorum with occasional endarteritis consisting of areas of acute inflammation with focal acute necrosis with neutrophils (microgummas). Rarely, organisms can be identified by Warthin-Starry stain of these microgummas. The pathologic changes are non-specific and may be seen with other causes of aortitis, therefore correlation with serology is essential to making the diagnosis. The inflammatory changes lead to damage and dilation of the aorta and usually occur in the ascending aorta. This leads to insufficiency and regurgitation of blood through the aortic valve.

Diagnosis: The laboratory diagnosis of syphilis depends on the clinical stage of the disease. In primary syphilis, before the appearance of antibody, direct detection of the causative spirochetes by dark field microscopy is the classical means of diagnosis. Newer direct fluorescent antibody tests and PCR for T. pallidum have been developed but are not generally available. Serology usually plays the most important role in the diagnosis of syphilis. These tests are classified into non-treponemal and treponemal based tests. Non-treponemal tests include the Venereal Disease Research Laboratory (VDRL) and rapid plasma regain (RPR) tests. These rely on the observation of agglutination caused by antibodies to tissue lipids, termed cardiolipins, that are seen as a by-product of syphilis infection. The nontreponemal tests are affected by treatment of syphilis and titers may be used for following the response to therapy. The fluorescent treponemal antibody with absorption (FTA-ABS) is a sensitive and specific treponemal test for syphilis. The absorption refers to treatment of the test material by a non-pathogenic spirochete to improve specificity. The test is not affected by anti-treponemal therapy so it cannot be used to follow treatment. The VDRL is the only test available to diagnoses neurosyphilis.