Venereal syphilis is a chronic systemic infection caused by the spirochete Treponema pallidum and distinguished clinically by three stages: (i) a local primary lesion with regional lymphadenopathy; (ii) a secondary bacteremic stage with disseminated mucocutaneous lesions; and (iii) a tertiary stage of skin, bone, and visceral involvement that develops after a latent period lasting years.

Epidemiology

Treponema pallidum is an obligate human parasite with no animal or environmental reservoir that is nearly always transmitted sexually through contact with infectious mucocutaneous lesions. Transmission can occur also via non-sexual personal contact, accidental direct inoculation, blood transfusion, needle sharing, or transplacentally (Stamm, 1999).

The disease was first identified in Europe when an epidemic of severe infection overran the continent at the end of the fifteenth century, coinciding with the return of Columbus from the West Indies. Because of this, syphilis was believed to be a New World illness communicated to Europe by Columbus's returning sailors. But the infection likely was already present in the Old World, unrecognized and mistaken for leprosy (Hackett, 1963; Hollander, 1981; Sparling, 1999).

In the next five centuries, syphilis spread worldwide, becoming so common that its prevalence reached 5 to 10% in general autopsy series by the first half of the twentieth century (Sparling, 1999). Following the introduction of penicillin and the initiation of public health programmes in the late 1940s, however, the number of new cases fell by over 90% throughout the developed world. In the USA, for example, the incidence peaked in 1947 at 76 primary and secondary cases per 100000 population and declined to a nadir of 2.6 per 100000 in 1998 (Aral & Holmes, 1999; Centers for Disease Control and Prevention, 1999). In the developing world, however, syphilis remains common, and there is now an explosive outbreak in Eastern Europe and parts of the former USSR (Aral & Holmes, 1999).

Etiology

Treponema pallidum is a small (below the limit of resolution of the light microscope), motile, flagellated, spiral bacterium 6–20 μm long and 0.1–0.2 μm wide. It stains poorly with aniline dyes but can be stained by silver impregnation or visualized by either dark field or phase contrast microscopy. It lacks a capsule. Like gram-negative bacteria, it has an outer membrane, inner membrane, and a thin cell wall composed of peptidoglycan.