GIS ANALYSIS OF 2006–2007 KENYA RVF OUTBREAK DATA 375 Figure 1. Estimated RVF incidence based on the multivariable model (November 1, 2006 to February 28, 2007). locations are shown on the maps in Figure 1. Cases can be seen to be occurring in three clusters: (1) the North Eastern province in the eastern part of the cluster (this cluster is the spatially broadest cluster), (2) the Kilifi district cluster on the southeastern coast, and (3) the Baringo district cluster in the west-central part of the country. Case and non-case grid cells were not significantly different with respect to populations or population densities (relative risk [RR] for population density = 0.9993; 95% confidence interval [CI] = 0.9977, 1.0003; P = 0.30). BIVARIABLE RESULTS Soil type. Four types of soils (solonetz, calcisols, solonchaks, and planosols) had statistically significantly elevated RRs versus all other soil types (Table 1). The soil types, using the three-letter subtypes from the FAO soil taxonomy,16 that were associated with increased RVF incidence were solonetz, calcisols, solonchaks, and planosols. Solonchaks, planosols, solonetzs, and calcisols were present in areas where in 72% of the grid cells, cases occurred versus in 28% of the grid cells, no cases occurred (P < 0.0001, x2 test). Soil texture. Soil types were classified by their texture prop- erties (very clayey, clay, loamy, and sandy). These types were, in turn, recoded into clay soils versus all other soils. RVF incidence was higher in areas with clay or very clayey soils (Table 1). Clay soils were present in areas were 93% of the cases occurred (data not shown) versus 70% for non-case locations (P < 0.0001, x2 test). Soil drainage. Soil types were classified by their drainage properties and classified into three categories for the purposes of this analysis: extremely slow, very slow, and all other (which combines drainage group categories slow, well, rapid, and very rapid). The rapid and very rapid soil drainage groups are uncommon in Kenya. Areas having soils with both extremely slow and very slow drainage had increased risk of having RVF cases (Table 1). Landforms. RVF incidence was higher in landforms catego- rized as plains (Table 1). Plains were 93% of case locations versus 74% of non-case cells (P < 0.0001, x2 test). Ecological zones. RVF incidence was higher in grid cells categorized as Somali acacia (Table 1). These areas, primarily located in the North Eastern province, are semiarid areas where shrubbery grasses and occasionally, trees are growing. Land use. Land usage was classified as agricultural (dense and sparse), barren, forest, grass lands, bushlands (dense and sparse), and other. RVF case grid locations were more likely to be sparse agriculture or dense shrubbery land use locations versus non-case locations (73% versus 58%; P < 0.0001, x2 test). Elevation. The majority of the cases occurred at elevations below 500 m (88.6% for case locations versus 36.0% for non- case locations) (Table 1). No case occurred above 1,100 m. Approximately 30% of Kenya is at an elevation of 1,000 m or higher.