Onchocerciasis elimination mapping: a handbook for national elimination programmes
Overview
This ancient disease has a huge economic impact in endemic areas, including substantially reducing productivity and household welfare and increasing health-related expenditures. The World Health Organization (WHO) has set a target for elimination of onchocerciasis by 2030 in its road map for NTDs 2021–2030. Efforts to combat the disease through mass distribution of ivermectin since the early 1990s have registered notable successes, leading to a programmatic shift in focus from control to elimination.
The WHO African Region has made tremendous progress towards eliminating onchocerciasis. In 2022, 159.1 million people received treatment for onchocerciasis, and 14 of 26 countries achieved 100% geographical coverage. Regional coverage of the 244.5 million population who required preventive chemotherapy for onchocerciasis was 65.1%. Preventive chemotherapy was administered in 23 of the 26 African countries where evidence indicates that MDA for onchocerciasis is required. Despite the progress made, treatment with ivermectin has historically targeted only areas where the prevalence of onchocerciasis was estimated to be high, leaving untreated areas of lower transmission.
The existence of ivermectin-naive areas where onchocerciasis transmission is ongoing remains a critical challenge to elimination efforts, and all remaining areas with active transmission must be identified to achieve elimination. Additionally, as countries progress towards submission of a dossier for elimination, they will need to have data or a strong rationale for why an area was declared non-endemic. According to WHO’s guidelines for stopping mass drug administration and verifying elimination of human onchocerciasis, dossiers should include the justification, rationale and criteria for determining the geographical extent of foci and ascertaining that no transmission occurred in areas outside the known foci.
To help countries meet these challenges, a process called onchocerciasis elimination mapping, or OEM, was developed to (i) determine exactly where interventions are required and (ii) provide rigorous evidence supporting the classification of non-endemic areas. Pilot surveys conducted by a number of countries in the African Region have generated evidence to inform the drafting of a simplified method presented in this document. This handbook captures a collation of discussions, documents, advice and evidence submitted by numerous countries and partners. At its heart is the aim to make OEM possible, practical and standardized. The WHO Onchocerciasis Technical Subgroup (OTS) has reviewed the available evidence for OEM and endorsed most aspects of the strategy for programmatic use while provisionally recommending others, namely the Step 3B risk-stratified secondary epidemiological survey described in 3.5
The purpose of this document is, therefore, to guide countries on how best to conduct OEM with the tools available to programmes now. This will help to achieve the aims set out in each national NTD masterplan. The handbook provides step-by-step instructions on how to conduct OEM, including desk review of available data, exclusion mapping, river prospections, identification of first-line villages, population sampling for assessment, choice of diagnostic test and interpretation of results. It is inevitable that as new tools and evidence become available, it will need to be updated. However, the evidence generated by following the guidance herein will be robust, and countries should feel confident in embarking upon their elimination mapping using this document as a guide.
