The geographic distribution of onchocerciasis in the 20 participating countries of the African Programme for Onchocerciasis Control: (2) pre-control endemicity levels and estimated number infected.

Honorat Gm Zouré,Afework H Tekle,Peter J Diggle,Mounkaila Noma,Jan Hf Remme,Uche V Amazigo,Emanuele Giorgi

doi:10.1186/1756-3305-7-326

Abstract

BackgroundThe original aim of the African Programme for Onchocerciasis Control (APOC) was to control onchocerciasis as a public health problem in 20 African countries. In order to identify all high risk areas where ivermectin treatment was needed to achieve control, APOC used Rapid Epidemiological Mapping of Onchocerciasis (REMO). REMO involved spatial sampling of villages to be surveyed, and examination of 30 to 50 adults per village for palpable onchocercal nodules. REMO has now been virtually completed and we report the results in two articles. A companion article reports the delineation of high risk areas based on expert analysis. The present article reports the results of a geostatistical analysis of the REMO data to map endemicity levels and estimate the number infected.MethodsA model-based geostatistical analysis of the REMO data was undertaken to generate high-resolution maps of the predicted prevalence of nodules and of the probability that the true nodule prevalence exceeds the high risk threshold of 20%. The number infected was estimated by converting nodule prevalence to microfilaria prevalence, and multiplying the predicted prevalence for each location with local data on population density. The geostatistical analysis included the nodule palpation data for 14,473 surveyed villages.ResultsThe generated map of onchocerciasis endemicity levels, as reflected in the prevalence of nodules, is a significant advance with many new endemic areas identified. The prevalence of nodules was > 20% over an area of 2.5 million km2 with an estimated population of 62 million people. The results were consistent with the delineation of high risk areas of the expert analysis except for borderline areas where the prevalence fluctuated around 20%. It is estimated that 36 million people would have been infected in the APOC countries by 2011 if there had been no ivermectin treatment.ConclusionsThe map of onchocerciasis endemicity levels has proven very valuable for onchocerciasis control in the APOC countries. Following the recent shift to onchocerciasis elimination, the map continues to play an important role in planning treatment, evaluating impact and predicting treatment end dates in relation to local endemicity levels.

Highlights

The original aim of the African Programme for Onchocerciasis Control (APOC) was to control onchocerciasis as a public health problem in 20 African countries
Surveyed and excluded areas The first step in the implementation of Rapid Epidemiological Mapping of Onchocerciasis (REMO) was the exclusion of areas that were considered unsuitable for onchocerciasis transmission, and where, no nodule surveys were carried out
Most of the remaining 6% of unsurveyed area is either not populated or has a very low population density of less than 1 person per km2. It includes a few zones for which it can reasonably be assumed that onchocerciasis is not endemic: the belts between surveyed and unsuitable areas in central Ethiopia and Kenya where the prevalence of nodules was zero in all neighbouring REMO villages; the unsurveyed areas in Mozambique south of latitude 18°S given that only 1 single nodule was detected in 37 villages surveyed below this latitude; and the coastal low lands of Tanzania where onchocerciasis vectors have never been reported [19,25,50]

Summary

Introduction

The original aim of the African Programme for Onchocerciasis Control (APOC) was to control onchocerciasis as a public health problem in 20 African countries. The existing information on the geographic distribution of onchocerciasis in the 20 APOC countries [1,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32] was incomplete and not reliable enough for targeting ivermectin treatment programmes, and there was an urgent need for comprehensive mapping of the geographic distribution of onchocerciasis in all potentially endemic countries in Africa outside the OCP [7,33] This was a vast area of some 14 million km and the survey methods available were difficult to implement at such a large scale. Large scale application of REMO started in 1996, and has since been applied in phase with the expansion of CDTI to cover all potentially endemic areas in APOC countries [33]

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Conclusion