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Epidemiology and Pathology

A clear distinction must be made between venereal and nonvenereal (endemic) syphilis. Although the organisms responsible are almost identical, the natural history of the two diseases differs in many ways. The basic pathological process is similar in many of the treponemal infections, yet there are considerable clinical variations. The major difference between the two types of syphilis is that endemic syphilis seldom involves the aorta and gastrointestinal tract. Congenital infections are rare in endemic syphilis and yaws. It was thought for a long time that neither caused neurological complications, but it now seems probable that tertiary CNS lesions do occur (and even, perhaps, visceral sequelae). This is because there are many unexplained tropical neuropathies in areas where yaws was previously endemic (e.g., the Caribbean and parts of South America). However, there is no question that the common late lesions will be in the skin, joints, and bones.

The treponemes responsible for endemic syphilis and yaws cause arterial damage and may be demonstrated in the adventitial tissue around the small arteries. The spirochetes initiate a marked reaction with plasma cells and lymphocytes, and damage the endothelium of the vessel. In syphilis (of either variety) this may progress to endarteritis obliterans and occlude the vessels, but in yaws the endothelial damage is less severe.

Yaws and syphilis may pass through three stages of infections. The primary lesions occur at the site of infection, although the organism may already be widely distributed. There is edema and cellular infiltration, usually mononuclear, most marked around the dermal blood vessels. Chronic granulomatous ulceration occurs from which the spirochetes may be readily recovered.

In the secondary stage, the findings depend on the location of the lesions. When they occur on moist mucosal surfaces, such as the mouth and anus, ulceration is extensive, but elsewhere they may remain dry. The underlying histological change is an inflammatory exudate. Both the primary and the secondary lesions may heal without scarring or may leave residual scars, some of which become depigmented. Some of the scars are paper-thin and easily secondarily infected: recurrent treponemal infection frequently occurs where there have been skin lesions.

The tertiary lesions are also granulomatous but, because of the ischemia, there is often necrosis. A tissue hypersensitivity has been postulated as an additional factor. There is a marked cellular infiltration which may contain giant cells and proceed to fibrosis. Perivascular cuffing and/or obliterative endarteritis occurs beneath the surface ulceration, and frequently a secondary infection adds to the tissue destruction. Around joints there may be dense fibrotic nodules with little inflammatory reaction.

Bone infection occurs in both the secondary and the tertiary stages and may cause pain. The earlier, secondary stage lesions are usually multiple and occur in numerous bones simultaneously. They may heal completely or result in periosteal thickening and fibrosis. The changes are most frequent around the periosteal arteries and in the subperiosteal tissue; as healing occurs, there is a subperiosteal deposition of dense new bone and sometimes endosteal bone as well. The long bones become greatly thickened and irregular and, because they are architecturally weakened and soft, they may become bowed to give the classical "saber" (or, in Australia, "boomerang") configuration. The pattern of bone infection varies in different parts of the body and with the different treponematoses. However, both syphilis and yaws develop this gummatous pattern with destruction, which may become secondarily infected and result in sinus formation. Pathological fractures can occur because the affected bones, although appearing thicker and denser than normal, are actually weakened and brittle as a result of the chronic osteomyelitis. When the facial bones are involved, the resulting deformities can be bizarre.

The mode of infection varies within the major groups of treponematoses. In endemic syphilis, spread is usually by direct contact and is extragenital. It is probable that the infection is also transmitted by flies (Hippelases species), because the cutaneous sores, on which flies are very common, contain numerous spirochetes in the secondary stage. Infection is most common early in childhood, and can be transmitted by children sharing drinking or eating utensils, as well as by contact. Immunity is acquired early, until well over half the community will have a positive serological reaction.

Yaws is also transmitted by direct contact among children, particularly where nutrition and hygiene are of a low standard. Ninety percent of yaws infections occur under the age of 15 years. Adult immunity also occurs and, for this reason, congenital infections are very rare. They have been recorded where the initial treponemal infection has occurred in nonimmune adults. One way this can happen is reverse infection of mothers who, as is common practice in the tropics, continue to breast-feed their children to the age of 1 or 2 years, old enough for the children to have been in contact with other patients and become infected.

Where yaws is endemic, monkeys have been found to be seropositive (e.g., baboons in West Africa). Such a reservoir is of importance in attempts to eradicate the disease.

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Copyright: Palmer and Reeder