- Yaws is a chronic infectious disease caused by Treponema pallidum subspecies pertenue.
- The disease affects the skin, bone and cartilage. If left untreated, yaws can lead to deformities of the nose and bones of the leg.
- Yaws has long been considered as a disease that can be eradicated as humans are the only reservoir.
- The discovery in 2012 that a single, oral dose of the antibiotic azithromycin can completely cure yaws has reopened the prospects of yaws eradication.
- 2 countries, Ecuador and India, which were once endemic, reported interruption of transmission in 2003.
- 13 currently endemic countries need support to implement WHO’s new eradication strategy.
- 73 previously endemic countries need to confirm the absence or presence of the disease.
Yaws forms part of a group of chronic bacterial infections caused by treponemes, including endemic syphilis (bejel) and pinta, which are commonly known as endemic treponematoses. Yaws is the most common of these infections.
The causative organism – Treponema pallidum subspecies pertenue, is closely related genetically to the less common nonvenereal endemic treponematoses, bejel and pinta, and T. pallidum subspecies pallidum, the causative agent of syphilis.
The disease is found primarily in poor communities in warm, humid and tropical forest areas of Africa, Asia, Latin America and the Pacific. Most of these populations have limited access to health care.
Yaws is transmitted through direct (person-to-person) non-sexual contact with the fluid from the lesion of an infected person to an uninfected person through minor injuries. Most lesions occur on the limbs. The initial lesion of yaws is teemed with the bacteria.. The incubation period is 9–90 days, with an average of 21 days.
About 75% of people affected by yaws are children under 15 years old, and peak incidence occurs in children aged 6–10 years. Males and females are equally affected.
Overcrowding, poor hygiene and socio-economic conditions facilitate the spread of yaws. Without treatment, infection can lead to chronic disfigurement and disability.
Scope of the problem
A review of historic documents from the 1950s shows that at least 88 countries and territories within the tropical belt 20 degrees north and south of the equator were endemic for yaws. However, only 13 are known to be currently endemic for yaws – these countries need support to implement the WHO Morges eradication strategy2. Recent estimates indicate that about 89 million people live in the areas endemic for yaws in the 13 countries3.
Ecuador and India4 have reported no cases since 2003. A WHO International Verification Team visited India in October 2015 to assess the status of interruption of transmission. The status of at least 73 previously countries needs to be assessed to determine if there is no more transmission of the disease as part of the global eradication process.
In the field, diagnosis is primarily based on clinical and epidemiological findings. There are two basic stages of yaws: early (infectious) and late (non-infectious). A recent discovery that leg ulcers caused by Haemophilus ducreyican clinically mimic yaws calls for the use of rapid diagnostic tests to confirm suspected yaws cases in the field. WHO has published a clinical pictorial guide to help health and community volunteers identify the disease5.
Standard laboratory-based tests
Serological tests such as Treponema pallidum particle agglutination (TPPA) and rapid plasma reagin (RPR) are widely used to diagnose treponemal infections (e.g. syphilis and yaws). These cannot distinguish yaws from syphilis however, and the interpretation of results from these tests on adults who live in yaws endemic areas need careful clinical assessment. Laboratory tests however are labour intensive and require laboratory facilities.
Rapid point-of-care tests
Rapid tests allow the point-of-care2 diagnosis and treatment of patients. There are 2 types of rapid tests:
- Rapid treponemal tests are widely used in diagnosis of syphilis, however these tests cannot distinguish between present active yaws and past infections. Therefore its use alone could lead to overtreatment of patients and over reporting of cases.
- The new Chembio DPP® Syphilis Screen & Confirm Assay is a single-use immunochromatographic rapid screening test for the simultaneous detection of antibodies against non-Treponemal and Treponema pallidum antigens in fingerstick whole blood, venous whole blood, serum, and plasma. This test is able to detect both present and past infections to guide treatment.
Polymerase chain reaction
Genomic analysis using polymerase chain reaction (PCR) can be used to definitively confirm yaws6, and this test will be very useful in the last phase of the eradication programme. The PCR technique can also be used to determine azithromycin resistance from swabs taken from yaws lesions.
Two antibiotics may be used to treat yaws:
- Azithromycin (single oral dose) at 30 mg/kg (maximum 2 gm)
- Benzathine penicillin (single intramuscular dose) at 1.2 million units (adults) and 600 000 units (children).
Without treatment, about 10% of affected people develop disfiguring and disabling complications, such as deformities of the legs and nose, after 5 years. The disease and its complications cause school absenteeism and prevent adults from working.
There is no vaccine for yaws. Prevention is based on the interruption of transmission through early diagnosis and treatment of individual cases and mass or targeted treatment of affected populations or communities. Health education and improvement in personal hygiene are essential components of prevention.
Renewed eradication efforts: progress so far
The “WHO Roadmap for Neglected Tropical Diseases” and World Health Assembly resolution 66.12 have set the year 2020 for the eradication of yaws from the remaining endemic countries. In 2012, WHO developed the new yaws eradication strategy, referred to as the Morges Strategy, based on the use of oral azithromycin, a shift from 60 years use of the injection of benzathine penicillin.
The results of the pilot implementation of the Morges strategy in Lihir island of Papua New Guinea were published in the New England Journal of Medicine in February 20157, showing a dramatic reduction of cases and transmission after one round of mass treatment of the population at risk. Similar findings have been recorded in Ghana and Vanuatu, although these results have not yet been published.
Public health experts have identified a number of operational research topics to guide the implementation of the eradication strategy8. A study is in progress in Ghana and Papua New Guinea to determine the efficacy of a lower dose of 20 mg/kg (the recommended dose used for trachoma) compared to the standard treatment of yaws given at 30 mg/kg. If successful, the results could help harmonize treatment policies of both diseases in countries where they overlap.
Collaboration with other programmes
Collaboration with neglected tropical diseases (NTDs) programmes as well as sexually transmitted infections programmes will be essential to advance the yaws eradication. In particular, trachoma9 because of the common use of azithromycin in areas co-endemic for both diseases10; sexually transmitted infections because of diagnostic facilities and expertise; and other skin-related NTDs such as leprosy and Buruli ulcer for enhanced case detection and surveillance.
Yaws eradication by 2020 is possible provided access to azithromycin can be secured. The mapping of endemic areas is essential prior to any large-scale mass treatment with azithromycin.
2 Summary report of a consultation on the eradication of yaws
3 Global epidemiology of yaws: systematic review
4 Eradicating successfully yaws from India: The strategy & global lessons:
5 Yaws recognition booklet for communities
6 Molecular differentiation of Treponema pallidum subspecies in skin ulceration clinically suspected as yaws in Vanuatu using real-time multiplex PCR and serological methods.
7 Mass Treatment with Single-Dose Azithromycin for Yaws
8 Challenges and key research questions for yaws: eradication
9 Trachoma and Yaws: Common Ground?
10 Impact of Community Mass Treatment with Azithromycin for Trachoma Elimination on the Prevalence of Yaws