Date Published: August 27, 2008
Publisher: Public Library of Science
Author(s): Andrea Rinaldi, Charles H. King
Partial Text: Some diseases are neglected; others are simply forgotten. Yaws belongs to the second group. One of the first diseases to be targeted for eradication on a global scale, yaws almost disappeared, thanks to a massive treatment program started in the 1950s that eliminated it from many endemic areas. Then something went wrong. The combat line somehow broke down in several points, people focused on other health emergencies, and yaws vanished from the political agenda. Not surprisingly, the disease has now resurged in several countries, where mass treatment has had to be reintroduced.
Yaws (the name is believed to originate from “yaya,” which in the indigenous Caribbean language means sore), or framboesia tropica (also known by the local names of buba, pian, paru, and parangi) is a chronic, contagious, nonvenereal infection caused by the spirochete Treponema pallidum subspecies pertenue. A disease of the poor, yaws mainly affects populations living in rural areas of warm and humid subtropical countries, where conditions of overcrowding, poor water supply, and lack of sanitation and hygiene prevail. The original distribution of the infection spanned across Africa, Asia, South America, and Oceania, but past eradication campaigns have strongly reduced the geographic extension and global burden; a few foci resist, however, notably in South-East Asia (Indonesia, Timor-Leste, Papua New Guinea) and Africa (Ghana, Republic of the Congo). Direct skin-to-skin contact is the main route for transmission, together with breaks in the skin caused by injuries, bites, or excoriations . According to older accounts, flies may also play a role as vectors for transmission .
Treponemal infections form a rather compact cluster. The venereal-transmitted syphilis is by far the best known, owing to its long-lasting presence in Western societies, whereas the nonsexual, tropically distributed yaws, bejel (or endemic syphilis), and pinta have received relatively little attention in medical culture. Different T. pallidum subspecies cause syphilis (subsp. pallidum), bejel (subsp. endemicum), and yaws (subsp. pertenue). Pinta, a mild disease characterized by loss of skin pigmentation, is caused by T. carateum. All T. pallidum subspecies are morphologically identical and cross-react to the same serological tests; only subtle genetic differences have been identified that permit them to be distinguished, leaving open the question of how such a limited variation can translate into the observed differences in pathogenesis . The lack of reliable serological or morphological tests to distinguish T. pallidum subspecies obviously limits diagnostic accuracy, which can have serious detrimental effects. For example, treatment for a pregnant woman with burnt-out yaws and one with active syphilis should be different, and assigning the wrong treatment may result in a negative outcome . It is therefore not surprising that an important avenue of current research on treponematoses focuses on the in-depth analysis of the T. pallidum genome, in the attempt to identify and decipher determinants of host specificity, pathogenicity, and virulence among the different subspecies (e.g., ,). Future advances in distinguishing between the subspecies using DNA polymorphisms may lead to serological tests that could be used in the field, facilitating disease control and eradication efforts by permitting the monitoring of its continued impact on populations.
Yaws has been the second disease ever to be targeted for eradication (defined as zero incidence and no evidence of transmission determined through surveys among children under five years in areas that were earlier endemic) on a global scale. (The first one was, unsuccessfully, yellow fever, in 1915.) From 1952 to 1964, the Global Yaws Control Programme was launched by the World Health Organization (WHO) in partnership with UNICEF, treating some 300 million people in 46 countries and reducing the global levels of the disease by 95%. Strategy of the early yaws eradication program was based on injection of either benzathine penicillin or procaine penicillin G, and was changed in the campaign’s earliest phases to include presumptive therapy for household contacts after it was realized that treatment of overt cases alone produced only a transient reduction in incidence. Treatment coverage varied depending on the prevalence of yaws in a particular area. Where prevalence was less than 5%, only active cases and their contacts were treated; while in areas with levels of endemicity greater than 10%, the entire population was treated . Where prevalence ranged between 5% and 10%, all prepubertal children, adults affected by active disease, and obvious contacts received treatment .
Besides the general criteria of biological and technical feasibility, costs and benefits, and societal and political considerations, each disease eradication attempt has its own peculiarities, which should be carefully considered in order to achieve success . Yaws, in this regard, is a one-of-a-kind opportunity, because a lot can be learnt from past practical efforts, with less need for speculation. Networking, for example, is now considered a key element for a new strategy to beat the flesh-eating foe. “There is no international network on yaws eradication like you will find with polio and Guinea worm. We need to build the network and develop the necessary partnership for tackling yaws in a sustainable manner,” remarked Asiedu. “Clearly, greater collaboration, networking and resource mobilisation is required in order to achieve yaws eradication globally or even regionally,” agreed Narain. Besides looking at how India achieved success, Asiedu said, activities from other eradication programs like polio, Guinea worm, and leprosy will also be considered. Indeed, collaboration with existing programs could allow resources to be shared and facilitate elimination and surveillance activities. There is also a clear need for increased advocacy and generation of interest, both in the disease itself and in engaging countries.