A recent paper in the Lancet Global Health reviews the global epidemiology of yaws to reveal the geographic scope of this neglected tropical disease. Author Michael Marks, Research Fellow at the London School of Hygiene & Tropical Medicine, writes about requirements for effective yaws control and eventual eradication as per WHO strategy.
Yaws was the first disease ever targeted for eradication by the WHO. Despite this, the disease has been one of the most neglected of all the NTDs. The disease is caused by the bacterium Treponema pallidum subspecies pertenue, and is related to syphilis. Unlike syphilis, the disease is not sexually-transmitted and occurs mostly in children who live in poor, humid, rural settings. The disease results in damage to the skin, bones and joints (Image 1 and 2). Untreated it can lead to permanent disfigurement.
A campaign by WHO and UNICEF in the middle of the twentieth century delivered mass treatment with injectable penicillin to almost 300 million individuals and greatly reduced the prevalence of yaws worldwide. Since then yaws has slipped off the public health agenda and the disease has rebounded in countries in West Africa, South East Asia and the Pacific and is once again a significant public health problem in a number of countries.
A study conducted in 2012 showed that azithromycin was an effective treatment for yaws and a subsequent study in Papua New Guinea has demonstrated that community mass treatment is an extremely effective intervention. There is now a renewed interest in yaws eradication worldwide and in 2012 the WHO launched a new yaws eradication campaign.
Identifying countries where mass treatment needs to be conducted has been identified as a priority for the WHO yaws eradication campaign. Yaws is known to be currently endemic in 13 countries. Transmission is reported to have been interrupted in two countries, Ecuador and India, following mass treatment campaigns conducted in the 1990s, and there are a further 76 countries for which recent data is not available.
For a recent systematic review, we analysed all the available data collected since 1990 on where cases of yaws are found. We included academic publications, national reporting data and data held by the World Health Organization. Between 2010-2013, there were 256,343 cases of yaws reported to WHO of which the majority, 84%, arose in only three countries, Ghana, Papua New Guinea and the Solomon Islands. As many people have latent infection, without clinical signs, the true number of people who are infected is likely to be much higher. A major problem is that yaws is no longer a notifiable disease in many of the countries where it was formally reported. As such it is likely that the figures above are significant under-estimates and that the disease continues to be transmitted in at least some of the 76 countries for which no recent data is available.
One of the clear findings of the systematic review was that even in countries where the disease is known to still be endemic there is a lack of high quality surveillance data. Many countries rely on clinical case reporting only, which is known to be inaccurate. A point-of-care test, originally developed for the diagnosis of syphilis, has recently been validated for use in yaws and making this test available to endemic countries should help to improve the quality of surveillance data.
The situation has been further complicated by the finding that other organisms, in particular Haemophilus ducreyi, can cause ulcers phenotypically similar to those seen in early yaws. Currently differentiating these two infections requires specialist testing at a limited number of reference laboratories.
The priorities for the yaws control programme are:
• Rolling out mass treatment to the 13 countries which are known to be endemic
• Identifying which of the 76 countries without recent data still harbor cases of yaws.
It is likely that a number of other countries in West and Central Africa still have cases of yaws, but these tend to be found in remote, rural populations with limited access to healthcare. As the target of the WHO campaign is eradication, every single case will need to be identified and treated which represents a major undertaking.
Unlike many of the other NTD control programmes, there is not currently a drug donation programme or specific funding allocated to yaws eradication. A study by the WHO estimated the cost of worldwide eradication to be in the region of $350,000,000. A significant investment of funding in active surveillance and mapping, as has been undertaken in other NTDs, will be required to help policy makers and programmes guide interventions for yaws.