BAYESIAN SPATIAL RISK PREDICTION OF SCHISTOSOMA MANSONI INFECTION IN WESTERN CÔTE D’IVOIRE USING A REMOTELY-SENSED DIGITAL ELEVATION MODEL CHRISTIAN BECK-WÖRNER, GIOVANNA RASO, PENELOPE VOUNATSOU, ELIÉZER K. N’GORAN, GERGELY RIGO, EBERHARD PARLOW, AND JÜRG UTZINGER* Department of Public Health and Epidemiology, Swiss Tropical Institute, Basel, Switzerland; Molecular Parasitology Laboratory, Queensland Institute of Medical Research, Brisbane, Queensland, Australia; Centre Suisse de Recherches Scientifiques, Abidjan, Côte d’Ivoire; UFR Biosciences, Université d’Abidjan-Cocody, Abidjan, Côte d’Ivoire; Institute of Meteorology, Climatology and Remote Sensing, Department of Environmental Sciences, University of Basel, Basel, Switzerland Abstract. An important epidemiologic feature of schistosomiasis is the focal distribution of the disease. Thus, the identification of high-risk communities is an essential first step for targeting interventions in an efficient and cost- effective manner. We used a remotely-sensed digital elevation model (DEM), derived hydrologic features (i.e., stream order, and catchment area), and fitted Bayesian geostatistical models to assess associations between environmental factors and infection with Schistosoma mansoni among more than 4,000 school children from the region of Man in western Côte d’Ivoire. At the unit of the school, we found significant correlations between the infection prevalence of S. mansoni and stream order of the nearest river, water catchment area, and altitude. In conclusion, the use of a freely available 90 m high-resolution DEM, geographic information system applications, and Bayesian spatial modeling fa- cilitates risk prediction for S. mansoni, and is a powerful approach for risk profiling of other neglected tropical diseases that are pervasive in the developing world. INTRODUCTION Schistosomiasis is a chronic and debilitating disease that occurs in tropical and subtropical environments, where it im- poses a considerable public-health burden. 1,2 The causative agent is a parasitic trematode of the genus Schistosoma, of which five species are known to infect humans. 3,4 An esti- mated 779 million people live in schistosome-endemic areas with more than 200 million individuals currently infected, and approximately 85% of them are concentrated in sub-Saharan Africa. 5 While the global burden of schistosomiasis has been estimated at 1.7–4.5 million disability-adjusted life years lost, 5 recent analyses suggest that the true burden might be consid- erably higher. 2 School-age children are at particular risk of morbidity caused by schistosomiasis because infections during childhood cause growth retardation, anemia, and can lead to cognitive impairment and memory deficits. 1,2 An important epidemiologic feature of schistosomiasis is its focal distribution, which is governed by deficient access to sanitation and clean water, occurrence of suitable freshwater habitats for the intermediate host snail and its abundance within these, and human water contact activities. 6–8 Along with the initiation of the Schistosomiasis Control Initiative (http://www.schisto.org) in 2002, new control efforts have been launched in sub-Saharan Africa. The goal in high- burden areas in Africa is to accomplish morbidity control facilitated by regular administration of praziquantel. 3,7 These efforts are in line with recommendations set forth by the World Health Organization to systematically treat communi- ties with schistosome infection prevalences  50%. 1 As an initial step of a control program, it is thus necessary to identify high-risk communities to target chemotherapy interventions. Remote sensing (RS) of environmental features and geo- graphic information system (GIS) platforms, coupled with Bayesian spatial statistics are powerful tools for risk predic- tion and mapping of schistosomiasis and other helminth dis- eases. 9–11 One of the latest RS products is a digital elevation model (DEM), which is facilitated by the launch of the shuttle radar topography mission (SRTM) in early 2000. 12 The digital topographic data acquired are freely available at three arc- seconds for Africa. Before, such data were mostly generated from optical stereo data, which are often affected by cloud cover and lack of sunlight. 13 The purpose of this study was to derive RS environmental data, to extract variables with a GIS software, and to test their suitability for spatially distributed risk prediction of intestinal schistosomiasis caused by Schistosoma mansoni, focusing on a 79 × 76 km area in Côte d’Ivoire. The accompanying risk map can guide control interventions and spatial targeting against this neglected tropical disease. MATERIALS AND METHODS Study area. The area under investigation is situated in west- ern Côte d’Ivoire in the mountainous region of Man, extend- ing from 7°04' to 7°45'N and from 7°17' to 8°00'W. Altitude ranges from 239 meters to 1,357 meters above sea level and the region extends 79 km from east to west and 76 km from north to south, thus having a surface area of approximately 6,000 km 2 . This area has been known to be endemic for S. mansoni for more than 30 years. 14,15 The climate is tropical wet-dry (Aw; groupings of Köppen- Geiger-Pohl’s climatic types) and has two distinct seasons; a wet season from March to September and a dry season from October to February. The annual precipitation is between 500 mm and 1,750 mm, with most of the rainfall occurring be- tween June and September. The average temperatures are 19–20°C during the winter months and 24–27°C during the summer months. 16 Demographic and parasitologic data. The demographic and parasitologic data presented were obtained during cross- sectional surveys conducted between October 2001 and Feb- ruary 2002. All schools in the two education districts of Man, * Address correspondence to Jürg Utzinger, Department of Public Health and Epidemiology, Swiss Tropical Institute, P.O. Box, CH- 4002 Basel, Switzerland. E-mail: juerg.utzinger@unibas.ch Am. J. Trop. Med. Hyg., 76(5), 2007, pp. 956–963 Copyright © 2007 by The American Society of Tropical Medicine and Hygiene 956