Utilizing Remote Sensing to Explore Hydrological and Climatic Factors of Visceral Leishmaniasis in South Sudan
Abstract:Recent epidemics of visceral leishmaniasis (VL) in Sudan and South Sudan (locally known as Kala Azar) have caused an estimated 100,000 deaths and have renewed the impetus for defining the ecological boundaries of this vector borne disease. In the past 30 years outbreaks have occurred cyclically within this country, but recent shifts in endemicity have necessitated a more robust understanding of the drivers of the disease. Previous work (e.g. Gebre-Michael et al., 2004; Ashford & Thomson, 1991; Hoogstraal & Heyneman, 1969) has suggested that the primary biological vector in this region, the female sand fly Phlebotomus orientalis, exhibits sensitivities to environmental and climatic variables.
Results of this study showed a relationship between precipitation and inundation during months of the transmission season (April-July) and the number of confirmed cases in the following September-January period. Particular months of the transmission season with below-average precipitation were better indicators of lagged reports of VL than others. During VL epidemics (2009, 2010, 2011) the month of June exhibited below average precipitation. The two largest epidemics (2010, 2011) were associated with years of below average precipitation in the month of April.
Inundation during April-July (AMJJ) also exhibited a strong inverse relationship with reported VL cases in the following September- January (SONDJ). This relationship was best explored when comparing the VL case data of a specific medical center to the inundation anomalies. Results are typified by the Lankien Medical Center analysis where below average inundation during April displays an inverse relationship with VL cases in the following SONDJ.
Drought may lead to below average inundation, which could allow for soils to maintain their fissures, thus maintaining the sand fly breeding habitat, resulting in a sustained breeding season for the sandflies (Quate, 1964). Above-average precipitation and inundation might have the inverse effect, eliminating their breeding sites within the soil.
Land surface temperature (LST) Night, LST Day, and relative humidity did not show a particularly strong relationship with VL. Further research is needed, as these variables are known to exist across strong gradients within the northern states of South Sudan (Quate, 1964).