The Effect of Diurnal Temperature Differences on Bancroftian Filariasis Distribution

Thomson et al. (1996) used remotely sensed data on diurnal temperature differences (dT) in conjunction with spatial data on case prevalences from 297 villages within the Southern Nile Delta, and showed that this environmental variable may underlie the observed spatial distribution of lymphatic filariasis, at least within their study region. dTs indicate surface and subsurface moisture contained in the soil and plant canopy, and hence may act as a surrogate for the abundance of the mosquito vector, Culex quinquefasciatus. Satellite image data from NOAA-AVHRR were analysed to determine dTs for the southern Nile Delta, while the case prevalence and locational data for each of the

Fig 2.17 Mean diurnal temperature differences of southern Nile delta, 16 August 1990, with study village sites superimposed according to bancroftian filariasis prevalence category

297 villages were inputted into a GIS. Point dT values for each village were obtained by averaging the values for 3x3 pixel areas (10 km2) centred on the corresponding longitude and latitude of each village. The digitized filariasis prevalence data were superimposed on the dT map and assigned to each of four prevalence categories, 0.5%, 5%, 15% and 25%, respectively (Figure 2.17). The association between village dT value and prevalence category was investigated using stepwise polychotomous logistic regression, which indicated a significant relationship between the two variables. Similar applications of dT maps or other remotely sensed data to delineate areas of risk with Bancroftian filariasis in other disease-endemic regions await study.

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