Spatial Risk Models for Human Plague in the West Nile Region of Uganda

Abstract

The West Nile region of Uganda represents an epidemiologic focus for human plague in east Africa. However, limited capacity for diagnostic laboratory testing means few clinically diagnosed cases are confirmed and the true burden of disease is undetermined. The aims of the study were 1) describe the spatial distribution of clinical plague cases in the region, 2) identify ecologic correlates of incidence, and 3) incorporate these variables into predictive models that define areas of plague risk. The model explained 74% of the incidence variation and revealed that cases were more common above 1,300 m than below. Remotely-sensed variables associated with differences in soil or vegetation were also identified as incidence predictors. The study demonstrated that plague incidence can be modeled at parish-level scale based on environmental variables and identified parishes where cases may be under-reported and enhanced surveillance and preventative measures may be implemented to decrease the burden of plague.

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

“…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.…”

“…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.…”

“…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).…”

“…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).…”

Blood Meal Identification in Off-Host Cat Fleas (Ctenocephalides felis) from a Plague-Endemic Region of Uganda

“…We collected fleas and small mammals in Vurra and Okoro, contiguous counties located along the Democratic Republic of Congo (DRC) border in the West Nile region of Uganda (Figure 1). The Rift Valley escarpment roughly bisects these counties, and most human plague cases are reported from villages located west of the escarpment at elevations above 1,300 m. 32 The western highlands are characterized by lush vegetation, fertile soil, numerous water sources, and highly fragmented land use associated with subsistence farming. 32,33 This sub-humid region experiences heavy, reliable rains from late August through November and a less reliable rainy season between March and June.…”