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Mathematical Modeling of Programmatic Requirements for Yaws Eradication

Saturday, 17th of December 2016 Print

Emerging Infectious Diseases,Volume 23, Number 1—January 2017

Abstract below; full text is at https://wwwnc.cdc.gov/eid/article/23/1/16-0487_article

Research

Mathematical Modeling of Programmatic Requirements for Yaws Eradication

Michael Marks , Oriol Mitjà, Christopher Fitzpatrick, Kingsley Asiedu, Anthony W. Solomon, David C.W. Mabey, and Sebastian Funk

Author affiliations: Hospital for Tropical Diseases, London, UK (M. Marks, A.W. Solomon, D.C.W. Mabey); London School of Hygiene and Tropical Medicine, London (M. Marks, A.W. Solomon, D.C.W. Mabey, S. Funk); ISGlobal, Barcelona, Spain (O. Mitjà); World Health Organization, Geneva, Switzerland (C. Fitzpatrick, K. Asiedu, A.W. Solomon)

Abstract

Yaws is targeted for eradication by 2020. The mainstay of the eradication strategy is mass treatment followed by case finding. Modeling has been used to inform programmatic requirements for other neglected tropical diseases and could provide insights into yaws eradication. We developed a model of yaws transmission varying the coverage and number of rounds of treatment. The estimated number of cases arising from an index case (basic reproduction number [R0]) ranged from 1.08 to 3.32. To have 80% probability of achieving eradication, 8 rounds of treatment with 80% coverage were required at low estimates of R0 (1.45). This requirement increased to 95% at high estimates of R0 (2.47). Extending the treatment interval to 12 months increased requirements at all estimates of R0. At high estimates of R0 with 12 monthly rounds of treatment, no combination of variables achieved eradication. Models should be used to guide the scale-up of yaws eradication.

Yaws is a bacterial infection caused by Treponema pallidum subsp. pertenue (1). The disease predominantly affects children living in poor, remote communities and results in lesions of the skin, bone, and cartilage. Previously, yaws was widespread throughout the tropics (2), but in the 20th century a series of control efforts based on mass treatment and case finding led by the World Health Organization (WHO) is estimated to have reduced the burden of cases worldwide by up to 95% (3). Despite these efforts, the disease has resurged in several countries in West and Central Africa, the Pacific, and Southeast Asia.

In 2012, a single dose of azithromycin was shown to be effective treatment for yaws (4). The availability of a well-tolerated oral agent has prompted WHO to develop a new eradication strategy, known as the Morges strategy, based on community mass azithromycin treatment (5). The strategy is supported by World Health Assembly resolution 66.12, which calls for eradication of yaws by 2020 (6). The strategy combines an initial round of total community treatment (TCT) followed by subsequent active case finding and total targeted treatment (TTT) of newly identified patients and their contacts. Pilot studies have shown that community mass treatment with azithromycin is a highly effective strategy for reducing the community prevalence of yaws (7,8).

Data are limited to inform the optimum coverage and number of TCT or TTT rounds that are required to achieve elimination (i.e., interruption of transmission) of yaws at a local level to facilitate country-level elimination and ultimately global eradication. In India, a national yaws elimination campaign conducted during 1996–2004 resulted in substantial reduction in the prevalence of yaws, sustained interruption of transmission, and nationwide elimination (9). This program consisted of case-finding surveys and treatment with parenteral penicillin conducted every 6 months. Although this approach did not include the initial mass treatment round now recommended as part of the Morges strategy, its success indicates that serial rounds of high-coverage treatment might achieve local elimination.

A recent review of important research questions facing the global yaws eradication program has highlighted the need for more accurate data to inform the optimum number and coverage of rounds of TCT and TTT that will be required to achieve yaws eradication (10). Mathematical modeling has been used to inform control efforts for several other neglected tropical diseases (1113) that are also managed by using community mass treatment strategies, and such approaches could be of value for yaws eradication efforts. In particular, this approach might allow a comparison of the differential effects of alternative mass treatment strategies, which would be difficult to assess by empirical randomized controlled trials because of the size and cost of implementing large-scale cluster randomized studies.

Previous mathematical models for yaws (14) have assessed the cost-effectiveness of yaws eradication but have not directly addressed the feasibility of achieving this goal or the number of rounds of treatment that would be required. In this study, we aimed to determine whether the eradication of yaws is feasible based on the Morges strategy and, if it is, the number and coverage of mass treatment rounds needed to achieve the goal.

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