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

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 Wes...

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Bibliographic Details
Published in:PLoS neglected tropical diseases 2016-01, Vol.10 (1), p.e0004328
Main Authors: O'Hanlon, Simon J, Slater, Hannah C, Cheke, Robert A, Boatin, Boakye A, Coffeng, Luc E, Pion, Sébastien D S, Boussinesq, Michel, Zouré, Honorat G M, Stolk, Wilma A, Basáñez, María-Gloria
Format: Article
Language:English
Subjects:
Africa, Western - epidemiology
Animals
Bayes Theorem
Funding
Geology
Humans
Infections
Mapping
Methods
Models, Statistical
Mortality
Onchocerca volvulus
Onchocerca volvulus - genetics
Onchocerca volvulus - physiology
Onchocerciasis - epidemiology
Onchocerciasis - parasitology
Onchocerciasis - prevention & control
Prevalence
Prevalence studies (Epidemiology)
Simulium
Statistical methods
Studies
Tropical diseases
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Summary: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. 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. 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.
ISSN:1935-2735
1935-2727
1935-2735
DOI:10.1371/journal.pntd.0004328