title: Model-Based Geostatistical Mapping of the Prevalence of Onchocerca volvulus in West Africa.
abstract: 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.
  
  
  RESULTS
  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
  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.
authors.affiliation: Department of Infectious Disease Epidemiology, School of Public Health, Faculty of Medicine (St Mary's Campus), Imperial College London, London, United Kingdom.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: O'Hanlon
authors.affiliation: Department of Infectious Disease Epidemiology, School of Public Health, Faculty of Medicine (St Mary's Campus), Imperial College London, London, United Kingdom.|MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom.
authors.forename: Hannah C
authors.initials: HC
authors.lastname: Slater
authors.affiliation: Department of Infectious Disease Epidemiology, School of Public Health, Faculty of Medicine (St Mary's Campus), Imperial College London, London, United Kingdom.|Natural Resources Institute, University of Greenwich at Medway, Chatham, Kent, United Kingdom.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Cheke
authors.affiliation: Lymphatic Filariasis Support Centre, Department of Parasitology, Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Ghana.
authors.forename: Boakye A
authors.initials: BA
authors.lastname: Boatin
authors.affiliation: Department of Public Health, Erasmus MC, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Luc E
authors.initials: LE
authors.lastname: Coffeng
authors.affiliation: UMI 233, Institut de Recherche pour le Développement (IRD) and University of Montpellier 1, Montpellier, France.
authors.forename: Sébastien D S
authors.initials: SD
authors.lastname: Pion
authors.affiliation: UMI 233, Institut de Recherche pour le Développement (IRD) and University of Montpellier 1, Montpellier, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Boussinesq
authors.affiliation: African Programme for Onchocerciasis Control (APOC), World Health Organization (WHO), Ouagadougou, Burkina Faso.
authors.forename: Honorat G M
authors.initials: HG
authors.lastname: Zouré
authors.affiliation: Department of Public Health, Erasmus MC, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Wilma A
authors.initials: WA
authors.lastname: Stolk
authors.affiliation: Department of Infectious Disease Epidemiology, School of Public Health, Faculty of Medicine (St Mary's Campus), Imperial College London, London, United Kingdom.|London Centre for Neglected Tropical Disease Research, Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom.
authors.forename: María-Gloria
authors.initials: MG
authors.lastname: Basáñez
mesh_terms: D000354:Africa, Western / Q000453:epidemiology; D000818:Animals; D001499:Bayes Theorem; D006801:Humans; D015233:Models, Statistical; D017181:Onchocerca volvulus / Q000235:genetics / Q000502:physiology; D009855:Onchocerciasis / Q000453:epidemiology* / Q000469:parasitology / Q000517:prevention & control; D015995:Prevalence