Prediction of a Rift Valley fever outbreak

Abstract

El Niño/Southern Oscillation related climate anomalies were analyzed by using a combination of satellite measurements of elevated sea-surface temperatures and subsequent elevated rainfall and satellite-derived normalized difference vegetation index data. A Rift Valley fever (RVF) risk mapping model using these climate data predicted areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. To our knowledge, this is the first prospective prediction of a RVF outbreak.

Fig. 1.

Time series plot of western equatorial Indian Ocean (WIO) (10°N–10°S, 40°–64°E) and equatorial eastern-central Pacific Ocean SST (NINO, 3.4: 5°N–5°S, 170°W–120°W) anomalies. Anomalies are depicted as degree Celsius departures from their respective climatological baseline periods. Convergence of anomalous positive SSTs between the 2 regions is associated with above-normal rainfall over the RVF endemic region of the Horn of Africa.

Fig. 2.

Seasonal global tropical SST anomalies for September to November 2006 expressed in degrees Celsius with respect to the 1982–2006 base mean period. Positive anomalies in the equatorial eastern-central Pacific Ocean are a manifestation of the 2006–2007 warm ENSO event.

Fig. 3.

Seasonal global tropical OLR anomalies (watts per square meter) for September to November 2007 computed with respect to the 1979–2006 base mean period. Negative OLR anomalies are an indicator of convective activity associated with positive SST anomalies in the western equatorial Indian Ocean and the equatorial eastern-central Pacific Ocean regions. Positive OLR anomalies are indicative of severe drought conditions in Southeast Asia.

Fig. 4.

Seasonal rainfall anomalies in millimeters for the Horn of Africa from September to November 2006. The anomalies are computed as deviations from the long-term seasonal mean for the period 1995–2006. RVF endemic areas of the Horn of Africa, especially eastern Kenya and Somalia, received an excess of +400 mm of rainfall during this 3-month period.

Fig. 5.

NDVI anomalies for December 2006. NDVI anomalies are computed as percentage departures from the 1998–2006 mean period. Positive anomalies are associated with above-normal rainfall and are indicative of anomalous vegetation growth, creating ideal eco-climatic conditions for the emergence and survival of large populations of RVF mosquito vectors from dambo habitats.

Fig. 6.

RVF calculated risk map for December 2006 for the Horn of Africa. The areas shown in red represent areas of persistent rainfall and vegetation growth from October through December, where potential RVF infected mosquitoes could emerge and transmit the virus to livestock and human populations.

Fig. 7.

Overall RVF risk areas shown in red for the period September 2006-May 2007 with human case locations depicted by blue and yellow dots. Blue dots indicate areas of RVF human case locations that were mapped to be within the risk areas (red) and within the potential epizootic area (green). Yellow dots represent human case locations outside the risk areas; 64% of all human cases fell within the areas mapped to be at risk to RVF activity during this period.