Relationship of climate, geography, and geology to the incidence of Rift Valley fever in Kenya during the 2006-2007 outbreak

Abstract

We estimated Rift Valley fever (RVF) incidence as a function of geological, geographical, and climatological factors during the 2006-2007 RVF epidemic in Kenya. Location information was obtained for 214 of 340 (63%) confirmed and probable RVF cases that occurred during an outbreak from November 1, 2006 to February 28, 2007. Locations with subtypes of solonetz, calcisols, solonchaks, and planosols soil types were highly associated with RVF occurrence during the outbreak period. Increased rainfall and higher greenness measures before the outbreak were associated with increased risk. RVF was more likely to occur on plains, in densely bushed areas, at lower elevations, and in the Somalia acacia ecological zone. Cases occurred in three spatial temporal clusters that differed by the date of associated rainfall, soil type, and land usage.

Figure 1.

Estimated RVF incidence based on the multivariable model (November 1, 2006 to February 28, 2007).

Figure 2.

Rainfall in selected 10-day periods in Kenya. The first epidemic period, involving North Eastern province and Kilifi district, occurred from mid-December of 2006 to mid-January of 2007. The second epidemic period, involving Baringo district, began in mid-January of 2007 and peaked in early February.