Yaws (also frambesia tropica, thymosis, polypapilloma tropicum, pian or parangi, "Bouba," "Frambösie," and "Pian") is a tropical infection of the skin, bones and joints caused by the spirochete bacterium Treponema pallidum pertenue. Other treponemal diseases are bejel (Treponema pallidum endemicum), pinta (Treponema pallidum carateum), and syphilis (Treponema pallidum pallidum).
Yaws is found in humid tropical regions in South America, Africa, Asia and Oceania. Mass treatment campaigns in the 1950s reduced the worldwide prevalence from 50–150 million to fewer than 2.5 million; however during the 1970s there were outbreaks in south-east Asia and there have been continued sporadic cases in South America. It is unclear how many people worldwide are infected at present.
About three quarters of people affected are children under 15 years of age, with the greatest incidence in children 6-10 years old.
The disease is transmitted by skin-to-skin contact with an infective lesion, with the bacterium entering through a pre-existing cut, bite or scratch. Within ninety days (but usually less than a month) of infection a painless but distinctive 'mother yaw' appears, which is a painless nodule which enlarges and becomes warty in appearance. Sometimes nearby 'daughter yaws' also appear simultaneously. This primary stage resolves completely within six months. The secondary stage occurs months to years later, and is characterised by widespread skin lesions of varying appearance, including 'crab yaws' on the palms and soles with desquamation. These secondary lesions frequently ulcerate (and are then highly infectious), but heal after six months or more. About ten percent of people then go on to develop tertiary disease within five to ten years (during which further secondary lesions may come and go), characterised by widespread bone, joint and soft tissue destruction, which may include extensive destruction of the bone and cartilage of the nose (rhinopharyngitis mutilans or 'gangosa').
Most often the diagnosis is made clinically. Dark field microscopy of samples taken from early lesions (particularly ulcerative lesions) may show the responsible organism. Blood tests such as VDRL, Rapid Plasma Reagin (RPR) and TPHA will also be positive, but there are no current blood tests which distinguish among the four treponematoses.
Treatment is normally by a single intramuscular injection of penicillin, or by a course of penicillin, erythromycin or tetracycline tablets. A single oral dose of azithromycin was shown to be as effective as intramuscular penicillin. Primary and secondary stage lesions may heal completely, but the destructive changes of tertiary yaws are largely irreversible.