Etiology and Epidemiology

Yaws is one of the four chronic infectious treponemal diseases that affect humans, and, in contrast to pinta, endemic syphilis, and venereal syphilis, it appears to be especially adapted to hot and humid tropical and subtropical environments. Rural populations were probably more affected than urban groups. The causal organism has been given separate species status, Treponeme pertenue, but the taxonomy of the treponematoses deserves réévaluation. The microorganism was discovered by Aldo Castellani in 1905, and since then its morphology has been to some extent revealed, especially by electron microscopy. Differences between the pathogenic treponemes have not, however, been resolved at this level, and it now seems unlikely that significant morphological differences will appear between T. pertenue and Treponeme pallidum (which causes syphilis).

Only humans appear to be the natural hosts of all the pathogenic human treponemes. None have so far been cultivated in artificial media, and there is still much to learn about their biological characteristics. In terms of the nature of the lesions produced, the most divergent forms are yaws and venereal syphilis. There is variable cross-protection once an individual has one variety of treponeme and comes into contact with another form.

The site of entry for yaws treponemes is not usually the genitalia, but often the legs. Large numbers of treponemes are probably unnecessary to instigate the disease. Infectious yaws lesions are mainly the early-stage papillomas, the infection being spread perhaps to small abrasions by direct or hand contact (via lesions). Transmission by flies is still considered to be unimportant. The yaws organisms are known to remain infectious in serum for up to 2 hours, provided the temperature remains at about 28°C.

Like endemic syphilis, yaws characteristically develops during childhood by nonvenereal contact. Eventually, after chronic progress of the disease, over 8 months or more, individuals commonly undergo spontaneous cure, although some cases continue to a tertiary stage.

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