Assessing human risk of exposure to plague bacteria in northwestern Uganda based on remotely sensed predictors

Abstract

Plague, a life-threatening flea-borne zoonosis caused by Yersinia pestis, has most commonly been reported from eastern Africa and Madagascar in recent decades. In these regions and elsewhere, prevention and control efforts are typically targeted at fine spatial scales, yet risk maps for the disease are often presented at coarse spatial resolutions that are of limited value in allocating scarce prevention and control resources. In our study, we sought to identify sub-village level remotely sensed correlates of elevated risk of human exposure to plague bacteria and to project the model across the plague-endemic West Nile region of Uganda and into neighboring regions of the Democratic Republic of Congo. Our model yielded an overall accuracy of 81%, with sensitivities and specificities of 89% and 71%, respectively. Risk was higher above 1,300 meters than below, and the remotely sensed covariates that were included in the model implied that localities that are wetter, with less vegetative growth and more bare soil during the dry month of January (when agricultural plots are typically fallow) pose an increased risk of plague case occurrence. Our results suggest that environmental and landscape features play a large part in classifying an area as ecologically conducive to plague activity. However, it is clear that future studies aimed at identifying behavioral and fine-scale ecological risk factors in the West Nile region are required to fully assess the risk of human exposure to Y. pestis.

Figure 1.

Map of parishes in the West Nile region in northwestern Uganda (inset). Locations of clinics from which confirmed plague cases were reported during 2008–2009 are shown as crosses. Villages of residence of cases and controls are shown as solid or hatched polygons, respectively. Vurra County in Arua District and Okoro County in Nebbi District are shown in dark and light gray, respectively. This figure appears in color at www.ajtmh.org.

Figure 2.

Predicted distribution of areas at elevated risk for exposure to Yersinia pestis within the area of interest (AOI) in the West Nile region of Uganda and in neighboring regions of the Democratic Republic of Congo. Shaded areas represent pixels classified as elevated risk; the color gradient indicates the probability of case occurrence within areas of elevated risk based on dichotimization at a probability value of 0.37. Input variables included in the predictive model include positive associations with elevation above 1,300 meters, brightness, and Landsat Enhanced Thematic Mapper Plus (ETM+) band 3, and a negative association with Landsat ETM+ band 6. Insets show the AOI and parish level incidence for Ugandan parishes within the AOI. This figure appears in color at www.ajtmh.org.