Research Article: Bayesian Geostatistical Model-Based Estimates of Soil-Transmitted Helminth Infection in Nigeria, Including Annual Deworming Requirements

Date Published: April 24, 2015

Publisher: Public Library of Science

Author(s): Akinola S. Oluwole, Uwem F. Ekpo, Dimitrios-Alexios Karagiannis-Voules, Eniola M. Abe, Francisca O. Olamiju, Sunday Isiyaku, Chukwu Okoronkwo, Yisa Saka, Obiageli J. Nebe, Eka I. Braide, Chiedu F. Mafiana, Jürg Utzinger, Penelope Vounatsou, Claudia Munoz-Zanzi.

Abstract: BackgroundThe acceleration of the control of soil-transmitted helminth (STH) infections in Nigeria, emphasizing preventive chemotherapy, has become imperative in light of the global fight against neglected tropical diseases. Predictive risk maps are an important tool to guide and support control activities.MethodologySTH infection prevalence data were obtained from surveys carried out in 2011 using standard protocols. Data were geo-referenced and collated in a nationwide, geographic information system database. Bayesian geostatistical models with remotely sensed environmental covariates and variable selection procedures were utilized to predict the spatial distribution of STH infections in Nigeria.Principal FindingsWe found that hookworm, Ascaris lumbricoides, and Trichuris trichiura infections are endemic in 482 (86.8%), 305 (55.0%), and 55 (9.9%) locations, respectively. Hookworm and A. lumbricoides infection co-exist in 16 states, while the three species are co-endemic in 12 states. Overall, STHs are endemic in 20 of the 36 states of Nigeria, including the Federal Capital Territory of Abuja. The observed prevalence at endemic locations ranged from 1.7% to 51.7% for hookworm, from 1.6% to 77.8% for A. lumbricoides, and from 1.0% to 25.5% for T. trichiura. Model-based predictions ranged from 0.7% to 51.0% for hookworm, from 0.1% to 82.6% for A. lumbricoides, and from 0.0% to 18.5% for T. trichiura. Our models suggest that day land surface temperature and dense vegetation are important predictors of the spatial distribution of STH infection in Nigeria. In 2011, a total of 5.7 million (13.8%) school-aged children were predicted to be infected with STHs in Nigeria. Mass treatment at the local government area level for annual or bi-annual treatment of the school-aged population in Nigeria in 2011, based on World Health Organization prevalence thresholds, were estimated at 10.2 million tablets.Conclusions/SignificanceThe predictive risk maps and estimated deworming needs presented here will be helpful for escalating the control and spatial targeting of interventions against STH infections in Nigeria.

Partial Text: Soil-transmitted helminth (STH) infections belong to the neglected tropical diseases (NTDs). In terms of at-risk population and number of people infected, the STHs are the most frequent NTDs worldwide. The three common STHs are the roundworm (Ascaris lumbricoides), the whipworm (Trichuris trichiura), and the hookworms (Ancylostoma duodenale and Necator americanus) [1–3]. The most recent estimates suggest that 819 million people worldwide are infected with A. lumbricoides, 465 million with T. trichiura, and 439 million with hookworm [4]. STH infections thrive where there are poor hygiene practices, including limited environmental sanitation, unsafe water sources, inadequate toilet facilities, and poor fecal disposal methods, coupled with poverty and low household income [5–7]. School-aged children (5–14 years), in particular, are at high risk of infection and morbidity due to STHs, and hence, are the main target of preventive chemotherapy [8,9].

We provide spatially explicit model-based risk estimates of the three main species of STHs in Nigeria. We used Bayesian geostatistical methods which have become essential tools in infectious disease risk profiling [35]. Our estimates are based on a large ensemble of recent survey data that were obtained using standard protocols. Hence, our estimates are more robust than those obtained from previous mapping exercises that collated historic survey data employing different collection methods and diagnostic approaches [28,36,37]. Our predictive risk maps are important and useful for planning, implementation, and evaluation of STH control programs [21]. Indeed, as a first step, the maps will help prioritize the implementation of intervention programs for the control of STH infections, particularly the spatial targeting of preventive chemotherapy. This is important in light of the current global moves toward control and elimination of NTDs [14,38]. Additionally, the model-based risk map of STH presented here complements a recent model-based risk map of schistosomiasis in Nigeria [19] for concurrent control of STH and schistosomiasis [39,40]. An integrated approach for the control of multiple helminthiases would reduce operational costs in the planning and implementation of control programs, as the primary target risk group for preventive chemotherapy are school-aged children, and hence the education system is the most convenient platform for drug administration [38,41,42]. It should be noted, however, that recent mathematical modeling work revealed that adults should also be targeted by preventive chemotherapy if substantial gains of morbidity control and interruption of transmission are aimed for [43]. A similar result was supported by a sub-continental geostatistical analysis of STH in sub-Saharan Africa [28].



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