Satellite climatology and the environmental risk of Schistosoma mansoni in Ethiopia and east Africa

Annual and seasonal composite maps prepared from the normalized difference vegetation index (NDVI) and earth surface maximum temperature (Tmax) satellite data from the archives of the Global land 1-km program of the United States Geological Survey (USGS) were studied for. their potential value, using geographic information system (GIS) methods, as surrogates of climate data in the development of environmental risk models for schistosomiasis in Ethiopia. Annual, wet season and dry season models were developed and iteratively analyzed for relationships with Schistosoma mansoni distribution and infection prevalence rates. Model-predicted endemic area overlays that best fit the distribution of sites with over 5% prevalence corresponded to values of NDVI 125–145 and Tmax 20–33°C in the annual composite map, NDVI 125–145 and Tmax 18–29°C for the wet season map, and NDVI 125–140 and Tmax 22–37°C for the dry season map. The model-predicted endemic area was similar to that of a prior model developed using an independent agroecologic zone data set from the United Nations Food and Agriculture Organization (FAO). Results were consistent with field and laboratory data on the preferences and limits of tolerance of the S. mansoni–Biomphalaria pfeifferi system. Results suggest that Global 1-km NDVI and Tmax, when used together, can be used as surrogate climate data for development of GIS risk assessment models for schistosomiasis. The model developed for Ethiopia based on global 1-km satellite data was extrapolated to a broader area of East Africa. When used with FAO agroecologic zone climate data limits of <27°C for average annual mean temperature and annual moisture deficits (annual rain–annual potential evapotranspiration) of <−1300 mm, the model accurately represented the regional distribution of the S. mansoni–B. pfeifferi system in the East Africa extrapolation area.