- Yaws is a chronic disfiguring and debilitating childhood infectious disease caused by Treponema pallidum subspecies pertenue.
- It is one of the first diseases targeted by WHO and UNICEF for eradication in the 1950s.
- The disease affects skin, bone and cartilage. Humans are currently believed to be the only reservoir, and transmission is from person to person.
- Yaws is cured by a single oral dose of an inexpensive antibiotic azithromycin.
- The 13 countries currently known to be endemic for yaws need support to implement WHO’s “Yaws Eradication Strategy” (the Morges strategy).
- There are 73 countries previously endemic for yaws that need to confirm the current status of the disease.
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 infections.
The causative organism, Treponema pallidum subspecies pertenue, is closely related genetically to T. pallidum subspecies pallidum, the causative agent of syphilis and the causative bacteria of 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. The majority of affected populations live at the “end of the road” and therefore have limited access to basic social amenities and health care. Poor socio-economic conditions and personal hygiene (caused by a lack of water and soap for bathing and washing), scanty clothing, and overcrowding facilitate the spread of yaws.
About 75-80% of people affected are children under 15 years of age, and they constitute the main reservoir of infection. Peak incidence occurs in children aged 6–10 years, and males and females are equally affected. Transmission is through direct (person-to-person) non-sexual contact of minor injuries of an uninfected person with the fluid from the yaws lesion of an infected person. 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.
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. Only 13 countries are known to be currently endemic for yaws however – these countries need support to implement the WHO "Yaws Eradication Strategy" (the Morges Strategy). Recent estimates indicate that about 89 million people live in the 13 countries endemic for yaws 1.
Ecuador and India have reported no cases of yaws since 2003 2. A WHO International Verification Team (IVT) visited India in October 2015 to assess the status of interruption of transmission. Based on the report of the IVT and endorsed by the WHO NTD Strategic and Technical Advisory Group (STAG), WHO declared India free of yaws in May 2016. Ecuador has not yet been declared free of yaws by WHO. The status of at least 73 previously endemic countries, including those in Central and South America, needs to be assessed to determine if there is transmission of the disease so that WHO can take steps to verify and certify them as part of the global eradication process.
Clinical Presentation, diagnosis and treatment
Yaws initially presents as a papilloma teemed with bacteria. Without treatment, the papilloma will ulcerate. The papilloma is a typical presentation and clinical diagnosis is straightforward. The diagnosis of the ulcerative form is more challenging.
The recent discovery that Haemophilus ducreyi is an important cause of skin ulcers (mostly on the legs) which clinically mimic the ulcerative form of yaws complicates clinical diagnosis. 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). WHO has developed information materials to help health workers and community volunteers identify the disease.
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 needs careful clinical assessment because of syphillis.
Rapid point-of-care 3 tests that can be used in the field are widely available. However, most of them are treponemal-based and cannot distinguish between past and current infection. Recently dual treponemal and nontreponemal rapid tests have become available, thus simplifying diagnosis in the field. These tests are able to detect both present and past infections to guide treatment of people with active infection.
Polymerase chain reaction (PCR) can be used to definitively confirm yaws by detecting the organisms in the skin lesions 4. It can also be used to monitor azithromycin resistance and this test will be very useful in the last phase of the eradication programme.
Either of 2 antibiotics – azithromycin or benzathine penicillin – may be used to treat yaws:
- Azithromycin (single oral dose) at 30 mg/kg (maximum 2 gm) is the preferred choice in the WHO "Yaws Eradication Strategy" (the Morges Strategy) because of the ease of administration and logistical consideration in large-scale treatment campaigns.
- Benzathine penicillin (single intramuscular dose) at 1.2 million units (adults) and 600 000 units (children). For patients allergic to penicillin and azithromycin, doxycycline 100mg (1 tab) orally, b.d. twice daily for 7 days may be used.
Patients should be examined 4 weeks after antibiotic treatment, and in most cases complete healing is observed. Patients should undergo repeat non-treponemal testing at 6 and 12 months after treatment; in most cases RPR titres become negative within less than 2 years.
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
In 2012, WHO developed the “Yaws Eradication Strategy”, also referred to as “the Morges strategy”, based on the use of oral azithromycin, indicating a shift from 60 years’ use of the injection of benzathine penicillin. In the 2012 “WHO Roadmap for Neglected Tropical Diseases” and resolution WHA66.12 of the 2013 World Health Assembly, yaws is targeted for eradication by the year 2020.
Pilot implementation of the “Yaws Eradication Strategy” in 5 countries (Congo, Ghana, Papua New Guinea, Solomon Islands and Vanuatu) has provided promising results and practical lessons for scale-up 5.
Priorities for operational research have been identified to guide the eradication programme 6. 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 quantities for azithromycin needed for the eradication campaign can be reduced and the same dose may be used in places where yaws and trachoma overlap 7.
Criteria for Eradication
Two criteria for eradication of diseases were established in 1960 by the WHO Expert Committee on Venereal Infections and Treponematoses 9.
These criteria are:
- Absence of new indigenous cases for 3 consecutive years.
- Absence of evidence of transmission for 3 continuous years measured with sero-surveys among children aged between 1–5 years (for example, no young children with RPR seroreactivity).
Since the development of PCR technology in 2010, an additional criterion of negative PCR in suspected lesions has been added. This is also necessary because of lesions like the H. ducreyi lesions which mimic yaws ulcers.
Collaboration with other programmes
Collaboration with other neglected tropical diseases (NTDs) programmes (such as Buruli ulcer, cutaneous leishmaniasis and leprosy) as well as sexually transmitted infections programmes will be essential to advance the yaws eradication.
Prospects for Eradication by 2020
The experience from the implementation of the WHO "Yaws Eradication Strategy" ( the Morges strategy) in the 5 pilot countries clearly demonstrates that if a sustainable supply of azithromycin can be secured, yaws eradication by 2020 is still a possibility. India`s achievement in interrupting transmission with old technology (benzathine pencillin) 8 provides positive lessons to build momentum for reaching the 2020 target using a single dose of oral azithromycin.
1 Global epidemiology of yaws: systematic review.
2 Haemophilus ducreyi as a cause of skin ulcers in children from a yaws-endemic area of Papua New Guinea: a prospective cohort study.
3 Sensitivity and specificity of a rapid point-of-care test for active yaws: a comparative study.
4 Molecular differentiation of Treponema pallidum subspecies in skin ulceration clinically suspected as yaws in Vanuatu using real-time multiplex PCR and serological methods.
5 Mass Treatment with Single-Dose Azithromycin for Yaws.
6 Challenges and key research questions for yaws: eradication.
7 Trachoma and Yaws: Common Ground?
8 Eradicating successfully yaws from India: The strategy & global lessons.