Yaws: a tropical treponemal skin infection

Yaws belongs to the group of treponemal diseases, which also includes syphilis. The World Health Organization describes it as a chronic infection of skin, bone and cartilage caused by Treponema pallidum subspecies pertenue. Humans are currently believed to be the only reservoir of infection. Spread is by direct contact with infectious skin lesions, typically through small cuts and grazes, rather than through sexual contact.

Because the bacterium cannot be told apart from the syphilis organism by routine laboratory tests, doctors rely on the clinical picture and the patient's background to separate the two.

WHO reports that the disease occurs mainly in poor communities in warm, humid and tropical forest areas of Africa, Asia, Latin America and the Pacific, with 15 countries known to be endemic. Around three-quarters or more of cases are in children under 15. DermNet adds that overcrowding and poor sanitation favour spread among children who play together and share skin contact.

For people in the UK, yaws is mainly of interest to those who work in or travel to affected regions for long periods, and to clinicians who might meet an unfamiliar skin ulcer in a migrant or returning traveller.

DermNet describes four phases. A primary papule appears three to four weeks after infection, resembling a wart, and then ulcerates before healing over three to six months, often leaving a scar. In the secondary phase more papules appear on the face, arms and legs and persist for months. A latent phase follows in which the person has no symptoms and is not contagious, and most people remain in this phase. Around 10 per cent progress to tertiary disease with destructive lesions of the bones of the face, jaw and lower leg.

• Primary: single papule that ulcerates and heals with scarring.

• Secondary: multiple skin lesions, sometimes with bone and joint pain.

• Latent: no symptoms; infection detectable only on blood tests.

• Tertiary: rare, destructive disease of bone and skin.

Diagnosis is usually clinical in endemic areas, using the appearance of lesions and the age and location of the patient. Blood tests such as non-treponemal and specific anti-treponemal antibody tests exist, but DermNet notes that they cannot separate yaws from syphilis, so interpretation depends on context. Rapid point-of-care tests are used in field programmes.

WHO states that yaws is cured with a single oral dose of azithromycin, an inexpensive antibiotic, and that more than 95 per cent of cases show complete clinical healing within four weeks. DermNet notes that a single injection of long-acting penicillin is also effective in early disease. Late-stage lesions may leave permanent disfigurement, which is why early treatment matters.

A UK clinician who suspects yaws in a patient from an endemic area would involve a microbiologist or infectious diseases specialist, and would test for syphilis and other infections.

WHO has adopted a strategy of mass drug administration aiming for 90 per cent population coverage, combined with active surveillance for cases. Success requires three consecutive years without new serologically confirmed cases. As of 2023, WHO-secured donations totalled 153 million azithromycin tablets for endemic countries. This makes yaws one of the neglected tropical diseases where elimination is realistically possible.

Short holidays carry negligible risk. Persistent non-healing skin ulcers after prolonged stays in endemic settings, or bone pain with skin lesions, should be reviewed by a GP who can refer to a tropical medicine or sexual health service. Syphilis testing is routinely carried out in such work-up, and a diagnosis does not imply sexual transmission.