376 Variable Soil type Solonetz Calcisols Solonchak Planosols All other Soil texture Clay All other Soil drainage Extremely slow Very slow All other Landform Plains All other Ecozone Somali acacia All other Land use Sparse agriculture Dense bush All other Elevation (m) 0–500 501–1,000 1,000+ CL = confidence limit. Case locations percent N = 70 34.3 4.3 7.1 15.7 38.6 N = 70 87.1 12.9 N = 70 21.4 58.6 20 N = 70 92.9 7.1 N = 70 10.0 90.0 N = 70 15.7 57.1 27.14 N = 70 88.6 7.1 4.3 Non-case locations percent N = 46,130 12.4 10.4 2.3 5.6 69.4 N = 45,868 67.0 33.0 N = 45,868 27.6 50.1 22.3 N = 46,130 74.0 25.98 N = 46,130 16.6 83.4 N = 46,003 17.4 41.9 40.7 N = 45,820 36.0 34.5 29.5 HIGHTOWER AND OTHERS Table 1 Geographic and geologic factors Cases (percent) N = 214 74 (34.6) 11 (5.1) 25 (11.7) 57 (26.6) 47 (22.0) N = 214 197 (92.1) 17 (7.9) N = 214 75 (35.0) 117 (54.7) 22 (10.3) N = 214 203 (94.9) 11 (5.1) N = 214 22 (10.3) 192 (89.7) N = 214 31 (14.5) 140 (65.4) 43 (20.1) N = 214 184 (86.0) 24 (11.2) 6 (2.8) Person-time years N = 8,582,024 120,547 76,152 34,433 259,472 8,091,419 N = 8,582,024 6,378,268 2,145,311 N = 833,579 1,875,380 4,896,064 1,762,134 N = 8,582,024 4,472,875 4,109,149 N = 8,582,024 88,240 8,493,784 N = 8,553,042 273,298 881,093 7,398,651 N = 8,559,500 816,663 646,245 7,096,593 Incidence per million person-years 613.9 144.8 726.0 219.7 5.8 30.9 7.9 40.0 23.90 45.4 2.68 249.3 22.6 113.4 158.9 5.8 225.3 37.1 0.85 Relative risk 105.8 24.5 127.1 39.3 Reference 3.8 Reference 3.5 2.1 Reference 16.7 Reference 10.3 Reference 20.0 28.2 Reference 259.0 43.0 Reference Lower 95% CL 73.5 12.1 76.9 26.6 2.4 2.2 1.3 9.6 6.4 12.5 20.1 125.7 18.74 Upper 95% CL 154.3 45.7 205.4 58.3 6.6 5.9 3.5 36.5 15.7 31.8 40.5 658.1 116.0 Rainfall. Locations that had cases of RVF had signifi- cantly less annual rainfall than non-case locations (49.1 versus 57.5 cm/year; RR per cm = 0.947; 95% CI = 0.942, 0.952). On a purely temporal basis, case locations had significantly greater rainfall than non-case locations for the periods of November 1–10, November 11–20, and December 11–20 (all in 2006). Case locations had significantly less rainfall than non-case locations for the periods of November 21–30 and December 1–10 and from December 21 to February 28. Asso- ciations for rainfall for 10-day periods by location are presented in a subsequent section. For the period of Feb- ruary 11–20, there was no rainfall in any case location, and therefore, the RR was undefined. However, a x2 test for any versus no rainfall for case and non-case locations was highly significant (P < 0.0001) for this time period. Rainfall amounts were over 5 cm for case locations for each 10-day period in November but dropped dramatically after January 1, 2007, only once exceeding 0.5 cm for any period. Bivariable results are presented in Table 2. Maps of rainfall in Kenya for selected 10-day periods are shown in Figure 2. NDVI. Case locations had significantly higher NDVIs than non-case locations during the periods of November 11–20, November 21–30, and December 21–31. Non-case locations had significantly higher NDVIs during the periods of Novem- ber 1–10, December 1–10, and December 11–20 and from January 1, 2007 to February 28, 2007 (Table 2). Greenness measures for case locations declined dramatically after Janu- ary 1, 2007. Multivariable model. Soil types were highly significant in the multivariable model (Table 3). Solonchaks, as found near Lake Baringo, had the highest RR (RR = 96.4; 95% CI = 48.1, 195.7) of any other factor in the model. Most rainfall measures remained statistically significant. As a group, the rainfall and soil types were the most significant factors in the model. A number of the NDVI measures became non- significant and were dropped from the model without any impact on the estimates of the remaining variables in the model. Also remaining significant in the multivariable model were plains areas, elevation, densely bushy areas, and the Somalia acacia ecozone, which occurs primarily in the North Eastern province. Incidence. Locations that had baseline values for all vari- ables in the multivariable model, including zero values for elevation, rain, and NDVI, could be expected to have an incidence of 10.8 cases per million person-years (95% CI = 4.3, 25.2). Figure 1 presents model-based incidences per million person-years based on the attributes of the grid cell. Findings by location. Because cases occurred in spatial and temporal clusters in North Eastern province, Kilifi district, and Baringo district, bivariable analyses, where the cases from each cluster were compared with all other non-case location, were performed. Analyses were limited by the small numbers of cases. Baringo case location risk factors were dense bush land use, solonchak soil type, and rain in the first 10 days of February. Rainfall in Baringo and western Kenya was much higher than other parts of the country, including other case locations during this time period (Baringo case locations: 4.9 ± 0.6 cm; other case locations: 0.01 ± 0.05 cm; non-case loca- tions: 1.3 ± 2.1 cm) (Figure 2). Rain during the first 10 days of November was the only significant risk factor for Kilifi loca- tions. Risk factors for North Eastern province case locations included rainfall during November 1–10, November 11–20,