Scaling-Down Mass Ivermectin Treatment for Onchocerciasis Elimination: Modeling the Impact of the Geographical Unit for Decision Making

Abstract

Background Due to spatial heterogeneity in onchocerciasis transmission, the duration of ivermectin mass drug administration (MDA) required for eliminating onchocerciasis will vary within endemic areas and the occurrence of transmission ‘hotspots’ is inevitable. The geographical scale at which stop-MDA decisions are made will be a key driver in how rapidly national programmes can scale down active intervention upon achieving the epidemiological targets for elimination. Methods We use two onchocerciasis models (EPIONCHO-IBM and ONCHOSIM) to predict the likelihood of achieving elimination by 2030 in Africa, accounting for variation in pre-intervention endemicity levels and histories of ivermectin treatment. We explore how decision-making at contrasting geographical scales (community vs. larger scale ‘project’) changes projections on populations still requiring MDA or transitioning to post-treatment surveillance. Results The total population considered grows from 118 million people in 2020 to 136 million in 2030. If stop-MDA decisions are made at project level, the number of people requiring treatment declines from 69-118 million in 2020 to 59-118 million in 2030. If stop-MDA decisions are made at community level, the numbers decline from 23-81 million in 2020 to 15-63 million in 2030. The lower estimates in these predictions intervals are based on ONCHOSIM, the upper limits on EPIONCHO-IBM. Discussion/Conclusions The geographical scale at which stop-MDA decisions are made strongly determines how rapidly national onchocerciasis programmes can scale down MDA programmes. Stopping in portions of project areas or transmission zones would free up human and economic resources.

Impact of human movement between hypo- and hyperendemic areas on sustainability of elimination ofOnchocerca volvulustransmission

“…Our study highlights the need to consider imported infection through mobility of human or vector hosts between connected regions when the goal is elimination of transmission in the vast, contiguous, and heterogeneous areas of sub-Saharan Africa. When determining the geographic scale at which stop-MDAi decisions should be made, treating any given location as though it were isolated from the surrounding region is unrealistic and may jeopardize elimination efforts [100].…”

Optimized strategy for real-time qPCR detection of Onchocerca volvulus DNA in pooled Simulium sp. blackfly vectors

“…The global elimination of onchocerciasis represents a massive technical and political challenge involving many governments, agencies, partners, and countries [44][45][46][47][48]. Defining success in the elimination of onchocerciasis transmission is based, in part, on assessing the absence of parasites in the heads of the transmitting Simulium spp.…”

How modelling can help steer the course set by the World Health Organization 2021-2030 roadmap on neglected tropical diseases

“…85% of inferred transmission distances ≤300m) 105 . Similarly for onchocerciasis, modelling shows that the rate at which interventions can be scaled down depend strongly on the spatial units of assessment 13 , 106 . Clustering of T. solium porcine cysticercosis around human taeniasis carriers, particularly evident in South American communities, demonstrates the need for spatially explicit models in certain settings 14 , 107 , such as the recently developed CystiAgent model for Peru 108 , capable of testing spatially structured interventions.…”