Environmental determinants of the distribution of Phlebotomus orientalis in Sudan

Despite its importance as a vector of visceral leishmaniasis in Sudan, the ecology of Phlebotomus orientalis is still poorly understood. The results of a ground-based survey and a geographical-information-system (GIS) study, carried out to investigate the environmental determinants of the distribution of P. orientalis in the wooded areas of the central savannah belt of Sudan, are described here. The survey, carried out in April-June 1996, consisted of a collection of sandflies over two consecutive nights at each of 44 study sites, using three CDC, miniature, light traps at each site. During the survey, the ecology of each site was described. Phlebotomus orientalis was caught at 17 of the sites. Environmental data on the collection sites (rainfall, minimum and maximum temperatures, soil class, vegetation and land-surface-temperature indices) were extracted from a range of sources of digital data collected by satellites in the National Ocean and Atmospheric Administration's series. These data were then analysed, to ascertain which variables were significantly associated with sites positive for P. orientalis. In line with the results of previous studies, P. orientalis was found to have a significant association with the presence of the tree species Acacia seyal and Balanites aegyptiaca and with the black cotton (vertisolic) soils of eastern Sudan. The positive sites were found to have significantly higher annual mean maximum and minimum daily temperatures than the negative sites and the annual mean maximum normalized difference vegetation index (NDVI) value was also found to be significantly higher in these sites than in sites where no P. orientalis were found.