Assessing Human Risk of Exposure to Plague Bacteria in Northwestern Uganda Based on Remotely Sensed Predictors

Abstract

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 .

Identification of Risk Factors for Plague in the West Nile Region of Uganda

“…Lower elevation sites are typically warmer and drier and have sandier soils than sites above the escarpment. 11,13,16 Previous studies showed that human plague cases are more common above the escarpment than below. 11,14 Correspondingly, flea species diversity is significantly higher above the escarpment within the plague focus, compared with lower elevation sites outside the focus, and this has been hypothesized to be important for enzootic maintenance of Y. pestis.…”

“…Furthermore, covariates included in these models suggested that localities that are generally wetter, but with discontinuous rainfall, pose an increased risk for plague compared with drier areas. 11,12,14 Although existing spatial models performed well in broadly defining the plague focus, there were many villages within the focus where human plague cases had not been reported by clinics during approximately a decade of surveillance. Such an observation raised the question of whether these disparities in case counts among villages within the risk area were attributable to differences in access to care, care-seeking behavior or knowledge of plague, agricultural or food storage practices, rodent and vector control strategies, or fine-scale ecologic differences (e.g., differences in host and flea community structure).…”

“…10 To aid in better targeting plague prevention resources, recent research efforts in Uganda have sought to define when and where humans are most at risk for plague in the far eastern edge of this plague focus. [11][12][13][14] During August 1999-July 2011, a total of 2,409 suspect plague cases were reported from the West Nile Region of Uganda; most cases occurred during September-December, a time period that corresponds with the primary rainy season. 13 Modeling of inter-annual variation showed that annual plague case counts were negatively associated with dry season rainfall (December-February) and positively associated with rainfall immediately preceding the plague season.…”

“…Ten case villages and five control villages were selected from within areas that were classified by geographic information system-based statistical models as posing an increased risk for plague. 11 In other words, regardless of case or control status, based on remotely sensed landscape level features, all villages enrolled in the study were believed to be ecologically conducive for plague activity. Ascertainment of village plague case histories has been described.…”

“…Ascertainment of village plague case histories has been described. 11,14 In brief, a retrospective review of clinic log books from 31 local health facilities was performed to compile a list of suspect plague cases by village during 1999-2007. Beginning in 2000 in Okoro County, a separate standardized reporting form was used and it captured more detailed information about environmental data often associated with plague cases (e.g., reported rodent die-offs).…”