Yaws forms part of a group of chronic bacterial infections commonly known as the endemic treponematoses. These diseases are caused by spiral bacteria of the genus Treponema, which also includes endemic syphilis (bejel) and pinta. Yaws is the most common of these three infections.
The causative organism, Treponema pallidum subspecies pertenue, is closely related genetically to T. pallidum subspecies pallidum, which causes syphilis, bejel and pinta.
The disease is found primarily in poor communities in warm, humid and tropical forest areas of Africa, Asia, Latin America and the Pacific. Most affected populations live at the “end of the road”, far from health services. Poverty, low socio-economic conditions and poor personal hygiene facilitate the spread of yaws.
Scope of the problem
About 75–80% of people affected by yaws are children under 15 years of age. Peak incidence occurs in children aged 6–10 years, and males and females are equally affected. Transmission is through person-to-person contact of minor injuries. The initial lesion of yaws is teemed with the bacteria. Most lesions occur on the limbs. The incubation period is 9–90 days, with an average of 21 days. Without treatment, infection can lead to chronic disfigurement and disability.
In 2013, 13 countries were known to be endemic with yaws. Since then, through intense surveillance activities, 2 additional countries reported confirmed cases (Liberia and Philippines)(1) and 3 countries reported suspected yaws cases (Colombia, Ecuador and Haiti). Out of the countries and territories known to have been endemic in the 1950s, at least 76 need to be assessed to determine if the disease is still present. This can be done through integrated surveillance with other diseases, especially skin-related neglected tropical diseases. In 2018, 80 472 suspected yaws cases were reported to WHO, out of which 888 cases were confirmed by Dual Path Platform (DPP® Syphilis Screen & Confirm Assay). There is currently an emphasis on strengthening laboratory confirmation of cases and standardizing data collection at both the country and global levels.
Signs and Symptoms
Yaws initially presents as a papilloma teemed with bacteria, which makes clinical diagnosis straightforward. Without treatment, the papilloma will ulcerate. The diagnosis of the ulcerative form is more challenging and requires serological confirmation. Papilloma and ulcers are very infectious and in the absence of treatment can quickly spread to others. Other clinical forms of yaws exist but they are not as infectious.
Secondary yaws occurs weeks to months after the primary infection and typically presents with multiple raised yellow lesions or pain and swelling of long bones and fingers (dactylitis).
Diagnosis
Traditionally, laboratory-based serological tests such as Treponema pallidum particle agglutination (TPPA) and rapid plasma reagin (RPR) are widely used to diagnose treponemal infections (for example, syphilis and yaws). These tests cannot distinguish yaws from syphilis, however, and the interpretation of results from these tests in adults who live in yaws endemic areas therefore needs careful clinical assessment. About 40% of ulcers clinically misidentified as yaws are caused by the unrelated H. ducreyi bacterium.
Field tests are widely available; however, most cannot distinguish between past and current infection and therefore have limited use in monitoring interruption of transmission. DPP can detect both past and present infection.
Polymerase chain reaction (PCR) technology is used to definitively confirm yaws by detecting the DNA in the skin lesions. It can also be used to monitor azithromycin resistance. This will be useful after mass treatment and post-elimination surveillance.
Treatment and care
Either of 2 antibiotics – azithromycin or benzathine penicillin – may be used to treat yaws:
- Azithromycin (single oral dose) at 30 mg/kg (maximum 2 g) is the preferred treatment.
- Benzathine penicillin (single intramuscular dose) at 0.6 million units (children aged under 10 years) and 1.2 million units (people aged over 10 years) can be used for patients with suspected clinical treatment failure after azithromycin, or patients who cannot be treated with azithromycin.
Patients should be reexamined 4 weeks after antibiotic treatment. Complete clinical healing will be observed in over 95% of cases. Any individual with presumed treatment failure requires macrolide resistance testing.
Prevention and control
There is no vaccine for yaws. Health education and improvement in personal hygiene are essential components to reduce transmission. Contacts of patients with yaws should receive empiric treatment.
The eradication approach consists of mass treatment (also called Total community treatment, TCT) in which oral azithromycin (30 mg/kg, maximum 2 g) is administered to the entire population (minimum 90% coverage) in areas known to harbour yaws. Two or three rounds of mass treatment may interrupt transmission but studies are in progress to determine the optimum number of rounds.
Three criteria for eradication of yaws are:
- absence of new serologically confirmed indigenous cases for 3 consecutive years;
- absence of any case proven by PCR; and
- absence of evidence of transmission for 3 continuous years measured with sero-surveys among children aged 1–5 years.
WHO response
WHO's work on yaws eradication involves:
- strategy development to guide countries in planning and implementing yaws eradication activities;
- development of training material to help health workers and community volunteers identify the disease;
- supporting countries via WHO-secured donation of 153 million tablets of azithromycin;
- standardized tools to guide data collection and reporting;
- strengthening collaboration and coordination among partners and stakeholders;
- advocacy and partnerships.
WHO provides azithromycin and technical support to several countries currently implementing mass treatment of yaws, including Benin, Côte Ivoire, Ghana, Papua New Guinea, Togo and Vanuatu.
References:
(1) Yaws in the Philippines: first reported cases since the 1970s (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6990502/)