Etiology

T. pallidum is a member of the order Spirochaetales and the family Treponemataceae (Fig. 18-1). The genus Treponema includes a number of species that reside in human gastrointestinal and genital tracts. T. pallidum, Treponema pertenue, and Treponema carateum are human pathogens responsible for significant worldwide morbidity (Table 18-1). Yaws, pinta, and bejel are diseases caused by bacteria closely related to T. pallidum. Yaws is common in the Caribbean, Latin America, Central Africa, and the Far East. Pinta is found only in Latin America and infection is limited to the skin. Bejel is found in eastern Mediterranean countries, the Balkans, and the cooler areas of North Africa.

Direct examination of the treponemes is most often performed with darkfield microscopy. Pathogenic treponemes appear as fine, spiral (8 to 24 coils) organisms approximately 6 to 15 µm long. They have a trilaminar outer membrane similar to that of gram-negative bacteria.

Pathogenic treponemes are not cultivatable with any consistency in artificial laboratory media. Outside of the host, the pathogenic treponemes are extremely susceptible to a variety of physical and chemical agents. Treponemes may remain viable for up to 5 days in tissue specimens removed from diseased animals and from frozen cryoprotected specimens.

Epidemiology

Sexually transmitted diseases (STDs) remain a major public health challenge in the United States. The surveillance report by the Centers for Disease Control and Prevention (CDC) includes data on the three STDs that physicians are required to report to the agency—chlamydia, gonorrhea, and syphilis. Syphilis is considered to be primarily a venereal disease. It is the most common STD in the United States.

The three treponematoses—yaws, pinta, and bejel—are rarely seen in the United States but are prevalent in other countries. These diseases are associated with poverty, overcrowding, and poor hygiene.

In 2009, for the first time in 5 years, the CDC reported that syphilis cases did not increase overall among women. In addition, cases of congenital syphilis (transmitted from mother to infant) did not increase for the first time in 4 years. In 2008, 63% of the reported primary and secondary (P&S) syphilis cases were among men who have sex with men (MSM). In the surveillance period of 2004 to 2008, rates of P&S syphilis increased the most among 15- to 24-year-old men and women. The incidence of syphilis per capita is higher among blacks and Hispanics than among whites.

Syphilis remains a global problem, with an estimated 12 million people infected each year, despite the existence of effective prevention measures. The last decade has seen a pronounced resurgence of syphilis in countries of the Far East (e.g., China and Africa). Some fundamental social problems (e.g., poverty, inadequate access to health care, lack of education) are associated with disproportionately high levels of syphilis in certain populations.

Pathogenic treponemes are transmitted almost uniformly by direct contact. Treponemal infections of the skin or oral lesions contain many spirochetes that may be transmitted by personal, but not necessarily venereal, contact. These infections are generally acquired during childhood. In each of these diseases, infection elicits antibodies reactive in nontreponemal and treponemal methods.

Syphilis develops in 30% to 50% of the sexual partners of persons with syphilitic lesions. The risk of acquiring syphilis from a single sexual exposure to an infected partner is unknown. A high percentage of partners do seek medical treatment within 90 days of contact.

Syphilis can be acquired by kissing a person with active oral lesions. Very few cases of transfusion-acquired syphilis have been reported in recent years in the United States. During the first half of the twentieth century, however, syphilis was a major bloodborne infectious disease easily transmitted through the prevailing method of direct donor to patient blood transfusion. The danger of syphilis transmission still exists in tropical countries in which the organization of blood banks is deficient and the use of direct blood transfusion prevails in emergency situations. Refrigerated blood storage decreases accidental transmission of the microorganism because T. pallidum has a short survival period in stored blood. Spirochetes do not appear to survive in units of citrated blood at 4° C (39° F) for longer than 72 hours.

Cases have been reported of children who acquired syphilis by sharing a bed with an infected parent. In addition, syphilis may be transmitted transplacentally to the fetus. Spirochetes can be transmitted to the fetus during the last trimester of pregnancy, before the mother manifests postpartum evidence of infection.

Signs and Symptoms

Untreated syphilis is a chronic disease with subacute symptomatic periods separated by asymptomatic intervals, during which the diagnosis can be made serologically. The progression of untreated syphilis is generally divided into stages—primary, secondary, latent (hidden), and tertiary (late) (Table 18-2).