Abstract
BackgroundThe World Health Organization (WHO) has set ambitious targets for the elimination of onchocerciasis by 2020–2025 through mass ivermectin treatment. Two different mathematical models have assessed the feasibility of reaching this goal for different settings and treatment scenarios, namely the individual-based microsimulation model ONCHOSIM and the population-based deterministic model EPIONCHO. In this study, we harmonize some crucial assumptions and compare model predictions on common outputs.MethodsUsing a range of initial endemicity levels and treatment scenarios, we compared the models with respect to the following outcomes: 1) model-predicted trends in microfilarial (mf) prevalence and mean mf intensity during 25 years of (annual or biannual) mass ivermectin treatment; 2) treatment duration needed to bring mf prevalence below a provisional operational threshold for treatment interruption (pOTTIS, i.e. 1.4 %), and 3) treatment duration needed to drive the parasite population to local elimination, even in the absence of further interventions. Local elimination was judged by stochastic fade-out in ONCHOSIM and by reaching transmission breakpoints in EPIONCHO.ResultsONCHOSIM and EPIONCHO both predicted that in mesoendemic areas the pOTTIS can be reached with annual treatment, but that this strategy may be insufficient in very highly hyperendemic areas or would require prolonged continuation of treatment. For the lower endemicity levels explored, ONCHOSIM predicted that the time needed to reach the pOTTIS is longer than that needed to drive the parasite population to elimination, whereas for the higher endemicity levels the opposite was true. In EPIONCHO, the pOTTIS was reached consistently sooner than the breakpoint.ConclusionsThe operational thresholds proposed by APOC may have to be adjusted to adequately reflect differences in pre-control endemicities. Further comparative modelling work will be conducted to better understand the main causes of differences in model-predicted trends. This is a pre-requisite for guiding elimination programmes in Africa and refining operational criteria for stopping mass treatment.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-015-1159-9) contains supplementary material, which is available to authorized users.
Highlights
The World Health Organization (WHO) has set ambitious targets for the elimination of onchocerciasis by 2020–2025 through mass ivermectin treatment
Success was reported in several African foci with annual or biannual ivermectin mass treatment [7, 8] and other areas seem to move towards elimination [9], there are reports of ongoing transmission in spite of prolonged ivermectin mass treatment [10, 11]
We present a comparative modelling study to explore the level of agreement between the ONCHOSIM and EPIONCHO models in their projections of estimated programme duration to achieve elimination
Summary
The World Health Organization (WHO) has set ambitious targets for the elimination of onchocerciasis by 2020–2025 through mass ivermectin treatment. The infection can lead to skin disease, visual impairment and eventually blindness It occurs primarily in tropical sub-Saharan Africa (99 % of cases), but some foci exist in Yemen and Latin America. Success was reported in several African foci with annual or biannual ivermectin mass treatment [7, 8] and other areas seem to move towards elimination [9], there are reports of ongoing transmission in spite of prolonged ivermectin mass treatment [10, 11] In view of this evidence, APOC decided to target elimination where feasible [12]. There is broad international commitment towards these goals, expressed through the adoption of World Health Assembly Resolution on Neglected Tropical Diseases (WHA66.12) and the endorsement of the London Declaration on Neglected Tropical Diseases 2012 by pharmaceutical companies, donors, endemic country governments and non-governmental organizations involved in NTD control [15]
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