article · PLoS neglected tropical diseases
Eliminating onchocerciasis across Africa through mass drug administration of ivermectin requires extensive mapping surveys across implementation units where endemicity remains unknown. To support this effort, boosted regression tree models were used to estimate environmental suitability for transmission at a five-by-five kilometre resolution continent-wide. Receiver operating characteristic analysis established a suitability index threshold of 0.71 based on locations where the disease was previously detected. Applying this threshold showed that large areas in West and Central Africa, alongside focal zones in East Africa, are environmentally suitable. Crucially, 50.2 percent of the implementation units under consideration for field mapping contain at least one location exceeding this threshold. Using spatial predictive data enables national elimination programmes to prioritise high-risk implementation units for surveying, preventing delays in starting treatment where resources, laboratory capacity, and operational schedules are limited.
Onchocerciasis elimination across Africa depends on conducting extensive ground surveys before distributing treatments. Because surveying every unknown area demands immense laboratory and personnel resources, predictive modelling helps public health officials identify which areas are most likely to sustain transmission. Prioritising these locations ensures limited resources are deployed effectively and treatments begin without unnecessary operational delays.
The predictive model provides a decision-support tool for public health bodies and national disease elimination programmes to optimise resource allocation for field surveys. As an applied analytical model tested against continental environmental data, it is ready for operational integration by health authorities and non-governmental organisations planning mass drug administration campaigns, though it does not represent a commercial product for private markets.
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Recent evidence suggests that, in some foci, elimination of onchocerciasis from Africa may be feasible with mass drug administration (MDA) of ivermectin. To achieve continental elimination of transmission, mapping surveys will need to be conducted across all implementation units (IUs) for which endemicity status is currently unknown. Using boosted regression tree models with optimised hyperparameter selection, we estimated environmental suitability for onchocerciasis at the 5 × 5-km resolution across Africa. In order to classify IUs that include locations that are environmentally suitable, we used receiver operating characteristic (ROC) analysis to identify an optimal threshold for suitability concordant with locations where onchocerciasis has been previously detected. This threshold value was then used to classify IUs (more suitable or less suitable) based on the location within the IU with the largest mean prediction. Mean estimates of environmental suitability suggest large areas across West and Central Africa, as well as focal areas of East Africa, are suitable for onchocerciasis transmission, consistent with the presence of current control and elimination of transmission efforts. The ROC analysis identified a mean environmental suitability index of 0·71 as a threshold to classify based on the location with the largest mean prediction within the IU. Of the IUs considered for mapping surveys, 50·2% exceed this threshold for suitability in at least one 5 × 5-km location. The formidable scale of data collection required to map onchocerciasis endemicity across the African continent presents an opportunity to use spatial data to identify areas likely to be suitable for onchocerciasis transmission. National onchocerciasis elimination programmes may wish to consider prioritising these IUs for mapping surveys as human resources, laboratory capacity, and programmatic schedules may constrain survey implementation, and possibly delaying MDA initiation in areas that would ultimately qualify.
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DOI: 10.1371/journal.pntd.0008824
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