ABSTRACT
Figure 470 Treponema pallidum Thin, delicate spirochete with identical tapering ends in dark-field microscopy of fresh exudate from penile lesion
Figure 471 Treponema pallidum Syphilitic chancre with indurated margin on erythematous non-purulent base
Figure 472 Treponema pallidum Secondary syphilis of penis manifested by dry, flat, non-purulent, plaque-like scaling lesions admixed with primary raised, oozing chancre on scrotum. In 15% of patients with secondary syphilis, the primary chancre may also be present
Figure 473 Treponema pallidum Flat, dry, scaling lesions of secondary syphilis on palms of hands and soles of feet
Treponema pallidum is solely a human pathogen with infection spread from an infected to non-infected individual through sexual contact, by transplacental passage (congenital syphilis), and through blood transfusion. The microorganism gains access to tissues by penetration through intact mucosae or through abraided skin. Local replication takes place and the bacterium enters the lymphatics and is widely disseminated through the bloodstream to many organs and skin (secondary syphilis). At the site of entry a primary lesion (syphilitic chancre) develops after about 3 weeks’ incubation. Skin lesions of secondary syphilis usually involve the entire trunk and extremities, including the palms of the hands and soles of the feet. Lesions are macular or pustular, teeming with treponemes, and are highly infectious. Lesions are not chancre-like because of incipient humoral and cellular immunity. Oral
Figure 474 Treponema pallidum Higher magnification of flat, scaling lesions of secondary syphilis on sole of foot. Lesions, although dry, when unroofed were teeming with spirochetes and, hence, highly infectious
mucosal lesions and gastric lesions also occur. In the absence of antibiotic treatment, secondary disease will resolve and latent syphilis ensues which can remain dormant for years. Relapses during the first 4 years (early latency) of latency can occur. Although spirochetes may be difficult to detect during late latency, non-specific VDRL (Venereal Diseases Research Laboratory) and specific TPHA (Treponema pallidum hemagglutination) antibody tests remain positive. Tertiary syphilis develops after a variable latency, usually exceeding 2 years. It largely reflects manifestations of the immune response to affected tissues and encompasses gummas (granulomatous lesions of skin or bone), cardiovascular (aortitis), and neurosyphilis. Virulence factors of T. pallidum include hyaluronidase, which aids penetration of glycocalyx surrounding host cells, fibronectinassociated adherence to endothelial cells (syphilis is largely a vasculitis), and presence of glycosaminoglycan and sialic acid in its external layer, which inhibit bactericidal activity of the classic and alternative complement pathways. Treponemes can be propagated in laboratory animals. Treponema carateum is the etiologic agent of pinta, a non-sexually transmitted infection involving the skin only. T. carateum is not as contagious as the three subspecies of T. pallidum, but its skin manifestations result in disfigurement and social stigmata. Although the precise mode of T. carateum transmission is unknown, spread of infection is thought to occur by direct contact with open lesions which may be flat (pintids) after progression from an initial papule or plaque. Crops of pintids (symptomatic episodes) may reappear for 40 years, extending pinta infectivity. T. pallidum subspecies pertenue causes yaws, a chronic, relapsing, contagious skin infection, also transmitted by direct skin contact with the exudate from an infectious lesion; in both pinta and yaws, entry of the microorganism is facilitated by breaks in the skin, scabes infestation, etc. Yaws is the most prevalent endemic treponematosis affecting populations in rural areas of the world where high levels of humidity and rainfall prevail. The initial site of entry of the microorganism is on the legs, feet or buttocks, where a small, erythematous papule develops which enlarges into a papilloma. Treponema pallidum subspecies endemicum causes a non-venereal syphilis-like disease called endemic syphilis or bejel, which is prevalent in Africa, Western Asia, and Australia. Transmission of this ‘pre-pubertal’ syphilis is direct by person to person and by sharing contaminated feeding utensils.
