Prediction, assessment of the Rift Valley fever activity in East and Southern Africa 2006-2008 and possible vector control strategies

Abstract

Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2-4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future.

Figure 1.

Endemic (on left) and epidemic (on right) life cycles of Rift Valley fever involving close association between heavy rainfall conditions, vector Aedes and Culex mosquitoes, domestic animals, and humans. The epidemic cycle is precipitated by excessive heavy rainfall associated with the El Niño/Southern Oscillation (ENSO) climatic phenomena. The three Xs depicted in epidemic cycle represent critical pathways, which can be interrupted by targeted and specific mosquito control activities.

Figure 2.

Cumulative rainfall anomalies for (A) East Africa: September 2006–May 2008. (B) Sudan: May–November 2007. (C) Southern Africa: September 2007–May 2008. (D) Madagascar: September 2007–May 2008. Except for Madagascar, all the regions received an excess of 200 to 400 mm of rainfall during the respective Rift Valley fever (RVF) outbreak periods.

Figure 3.

Summary Risk Valley fever (RVF) risk maps for (A) Eastern Africa: September 2006–May 2007. (B) Sudan: May 2007–December 2007. (C) Southern Africa: September 2007–May 2008. (D) Madagascar: September 2007– May 2008. Areas shown in green represent RVF potential epizootic areas, areas shown in red represent pixels that were mapped by the prediction system to be at risk for RVF activity during the respective time periods, blue dots indicate human cases identified to be in the RVF risk areas, whereas yellow dots represents human cases in areas not mapped to be at risk.

Figure 4.

Comparison between the timing of the first Rift Valley fever (RVF) early warning alert and the first reported human case of RVF based on epidemiological reports for (A) Kenya, (B) Tanzania, and (C) Sudan. In this case there is a 2–4 month time gap between the early warning alert and the first index human case, which would allow for preventive control and mitigation measures to be undertaken.