To-date, Rift Valley fever (RVF) outbreaks have occurred in 38 of the 69 administrative districts in Kenya. Using surveillance records collected between 1951 and 2007, we determined the risk of exposure and outcome of an RVF outbreak, examined the ecological and
climatic factors associated with the outbreaks, and used these data to develop an RVF risk
map for Kenya.
Exposure to RVF was evaluated as the proportion of the total outbreak years that each district was involved in prior epizootics, whereas risk of outcome was assessed as severity of
observed disease in humans and animals for each district. A probability-impact weighted
score (1 to 9) of the combined exposure and outcome risks was used to classify a district as
high (score 5) or medium (score 2 - <5) risk, a classification that was subsequently subjected to expert group analysis for final risk level determination at the division levels (total =
391 divisions). Divisions that never reported RVF disease (score < 2) were classified as low
risk. Using data from the 2006/07 RVF outbreak, the predictive risk factors for an RVF outbreak were identified. The predictive probabilities from the model were further used to
develop an RVF risk map for Kenya.
The final output was a RVF risk map that classified 101 of 391 divisions (26%) located in 21
districts as high risk, and 100 of 391 divisions (26%) located in 35 districts as medium risk
and 190 divisions (48%) as low risk, including all 97 divisions in Nyanza and Western provinces. The risk of RVF was positively associated with Normalized Difference Vegetation
Index (NDVI), low altitude below 1000m and high precipitation in areas with solonertz, luvisols and vertisols soil types (p <0.05).
RVF risk map serves as an important.