Mapping suitability for Buruli ulcer at fine spatial scales across Africa: A modelling study.

Hope Simpson,Edwin Ampadu,Jorge Cano,Joseph Timothy,Michael Frimpong,Rachel L Pullan,Richard O Phillips,Paul Saunderson,Earnest Njih Tabah,Issaka Maman

doi:10.1371/journal.pntd.0009157

Abstract

Buruli ulcer (BU) is a disabling and stigmatising neglected tropical disease (NTD). Its distribution and burden are unknown because of underdiagnosis and underreporting. It is caused by Mycobacterium ulcerans, an environmental pathogen whose environmental niche and transmission routes are not fully understood. The main control strategy is active surveillance to promote early treatment and thus limit morbidity, but these activities are mostly restricted to well-known endemic areas. A better understanding of environmental suitability for the bacterium and disease could inform targeted surveillance, and advance understanding of the ecology and burden of BU. We used previously compiled point-level datasets of BU and M. ulcerans occurrence, evidence for BU occurrence within national and sub-national areas, and a suite of relevant environmental covariates in a distribution modelling framework. We fitted relationships between BU and M. ulcerans occurrence and environmental predictors by applying regression and machine learning based algorithms, combined in an ensemble model to characterise the optimal ecological niche for the disease and bacterium across Africa at a resolution of 5km x 5km. Proximity to waterbodies was the strongest predictor of suitability for BU, followed potential evapotranspiration. The strongest predictors of suitability for M. ulcerans were deforestation and potential evapotranspiration. We identified patchy foci of suitability throughout West and Central Africa, including areas with no previous evidence of the disease. Predicted suitability for M. ulcerans was wider but overlapping with that of BU. The estimated population living in areas predicted suitable for the bacterium and disease was 46.1 million.These maps could be used to inform burden estimations and case searches which would generate a more complete understanding of the spatial distribution of BU in Africa, and may guide control programmes to identify cases beyond the well-known endemic areas.

Highlights

Buruli ulcer (BU) is a chronic necrotizing disease of the skin and soft tissue, which causes debilitating symptoms and sequelae, associated with a high burden of morbidity and stigma for patients and economic costs for affected households [1,2,3]
Having previously evaluated and mapped the existing evidence for BU and its causative agent M. ulcerans, we concluded that the disease was likely to occur beyond the range of known endemic areas
We used the compiled evidence and geographical datasets to build statistical models representing the relationship between environmental factors and previously reported cases

Summary

Introduction

Buruli ulcer (BU) is a chronic necrotizing disease of the skin and soft tissue, which causes debilitating symptoms and sequelae, associated with a high burden of morbidity and stigma for patients and economic costs for affected households [1,2,3]. These impacts are felt strongly in impoverished rural communities with poor access to health services [3,4]. The disease is recognised to be underdiagnosed and under-reported, and may occur undetected in other parts of the world [9,10,11,12]

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Conclusion