Satellite Observations of a Rift Valley fever Interepizootic Period

Abstract:

Rift Valley fever virus activity in Southern Africa tends to occur during periods of elevated rainfall, cooler than normal conditions and abundant vegetation creating ideal conditions for increase and propagation of populations of Rift Valley fever vectors. These climatic and ecological conditions are modulated by large-scale tropical-wide El Niño–Southern Oscillation (ENSO) phenomena. Against this background “Understanding Rift Valley Fever in Republic of South Africa project” is taking place during the what we classify as an “ intereepizotic” period since 2014. We have collected and are analyzing several NASA satellite data sets: including rainfall, land surface temperature and normalized difference vegetation index data since September 2014 to understand the broad climatic and land surface conditions in the years following a period of sustained and widespread large outbreaks of the region from 2008 to 2011. This interepizootic period is characterized by on average below normal rainfall (~-500mm), high land surface temperatures (~+12^oC), depressed vegetation conditions (60% below normal) and severe drought as manifested particularly during the 2015-2016 growing season. Such conditions reduce the patchwork of appropriate habitats available for the emergence of RVF vectors and diminish the chance for RVF activity. However, 2016-2017growing season saw a marked return to somewhat wetter conditions, without any reported activity, and only during the 2017-2018 growing season, late and above average rainfall, resulted in a focal outbreak in one location, encompassing our study region. The combined use of derived satellite-derived climate observations in conjunction with in situ mosquito vector collections enable us not only better understand the climate and ecological dynamics underlaying disease emergence but also determine the timing of such disease occurrences.