Predictive Factors and Risk Mapping for Rift Valley Fever Epidemics in Kenya

Abstract

BackgroundTo-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.MethodsExposure 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.ResultsThe 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).ConclusionRVF risk map serves as an important tool for developing and deploying prevention and control measures against the disease.

Distribution and abundance of key vectors of Rift Valley fever and other arboviruses in two ecologically distinct counties in Kenya

“…This could be attributed to the ecological differences between the regions as well as the diversity of potential vector species. Previous studies have also reported low numbers of arbovirus cases in Isiolo relative to Tana-River [5, 12]. In previous arbovirus surveys conducted in neighboring Garissa county, WNV, Semliki Forest virus and Ndumu virus, traditionally known to be mosquito-borne, were detected in ticks taken off cattle and other wild animals [32] suggesting that although these viruses have traditional reservoirs and vectors which have been documented, they may infect other animal species from which mosquitoes such as Ae .…”

“…Tana-River was more affected with16 human cases than Isiolo that only reported 7 probable cases [5, 12]. …”

“…The mean annual temperature ranges between 30°C and 33°C. Tana-River has been classified as being at high risk for RVF outbreaks [12] and it suffered a significantly high impact with 7 deaths out of 16 reported probable cases during the 2006/2007 outbreak period [5] although these figures may be considered an underestimation as some cases may have been missed due to various reasons including poor access to health facilities and challenges of identifying cases.…”

“…However, 95% of the county falls in the arid to very arid zone. Isiolo suffered RVF outbreak at a smaller scale than Tana-River and in the most recent RVF risk classification for Kenya by counties, Isiolo was classified as being at medium risk for RVF outbreaks [5, 12]. In the scarce data available of RVF cases during outbreaks of 2006/2007, Isiolo documented no deaths out of 7 probable cases [5] although again there may have been significant under- reporting.…”

“…To generate data that would improve assessment of disease risk and regional vulnerability, we investigated the composition and distribution of known vectors of RVFV in two counties, namely Tana-River and Isiolo, known to have different ecologic settings and different levels of disease activity. In 2006/2007, Tana-River suffered a significantly higher impact RVF with 7 deaths out of 16 reported human cases compared to Isiolo with 0 deaths out of 7 probable cases and in addition, Tana-River and Isiolo have been classified as being at high and medium risk of RVF respectively, based on livestock infection data [5, 12]. We also investigated presence of circulating arboviruses in the mosquito population.…”