Model-Based Geostatistical Mapping of the Prevalence of Onchocerca volvulus in West Africa

Background: The initial endemicity (pre-control prevalence) of onchocerciasis has been shown to be an important determinant of the feasibility of elimination by mass ivermectin distribution. We present the first geostatistical map of microfilarial prevalence in the former Onchocerciasis Control Programme in West Africa (OCP) before commencement of antivectorial and antiparasitic interventions. Methods and Findings: Pre-control microfilarial prevalence data from 737 villages across the 11 constituent countries in the OCP epidemiological database were used as ground-truth data. These 737 data points, plus a set of statistically selected environmental covariates, were used in a Bayesian model-based geostatistical (B-MBG) approach to generate a continuous surface (at pixel resolution of 5 km x 5km) of microfilarial prevalence in West Africa prior to the commencement of the OCP. Uncertainty in model predictions was measured using a suite of validation statistics, performed on bootstrap samples of held-out validation data. The mean Pearson’s correlation between observed and estimated prevalence at validation locations was 0.693; the mean prediction error (average difference between observed and estimated values) was 0.77%, and the mean absolute prediction error (average magnitude of difference between observed and estimated values) was 12.2%. Within OCP boundaries, 17.8 million people were deemed to have been at risk, 7.55 million to have been infected, and mean microfilarial prevalence to have been 45% (range: 2–90%) in 1975. Conclusions and Significance: This is the first map of initial onchocerciasis prevalence in West Africa using B-MBG. Important environmental predictors of infection prevalence were identified and used in a model out-performing those without spatial random effects or environmental covariates. Results may be compared with recent epidemiological mapping efforts to find areas of persisting transmission. These methods may be extended to areas where data are sparse, and may be used to help inform the feasibility of elimination with current and novel tools. Author Summary: Onchocerciasis is a neglected tropical disease caused by Onchocerca volvulus and transmitted by Simulium blackflies. Operational between 1975 and 2002, the Onchocerciasis Control Programme in West Africa (OCP) was one of the first large-scale intervention programmes ever implemented, and was successful in controlling the disease as a problem of public health importance. Currently, national control programmes in the former OCP and the World Health Organization African Programme for Onchocerciasis Control (APOC) are shifting their focus from morbidity control to elimination of the infection where feasible through community-directed treatment with ivermectin (CDTI). The level of endemicity (measured as infection prevalence) prior to control interventions has been shown to be an important determinant of the feasibility of elimination by CDTI. We present the first pre-control Bayesian geostatistical map of infection (microfilarial) prevalence in the OCP with the objectives of: a) identifying important environmental covariates which help to estimate prevalence in areas without observed survey data; b) understanding the distribution of areas of initially high endemicity, where the current CDTI strategy may be insufficient to achieve elimination goals and intensified CDTI or novel/complementary tools may be required; c) re-estimating the initial numbers of persons at-risk and with onchocerciasis in 1975.