Potential corridors and barriers for plague spread in central Asia

Liesbeth I Wilschut,Lise Heier,Anne Laudisoit,Leonid A Burdelov,Steven M De Jong,Stephen Davis,Vladimir M Dubyanskiy,Mike Begon,Hans Heesterbeek,Elisabeth A Addink

doi:10.1186/1476-072x-12-49

Abstract

BackgroundPlague (Yersinia pestis infection) is a vector-borne disease which caused millions of human deaths in the Middle Ages. The hosts of plague are mostly rodents, and the disease is spread by the fleas that feed on them. Currently, the disease still circulates amongst sylvatic rodent populations all over the world, including great gerbil (Rhombomys opimus) populations in Central Asia. Great gerbils are social desert rodents that live in family groups in burrows, which are visible on satellite images. In great gerbil populations an abundance threshold exists, above which plague can spread causing epizootics. The spatial distribution of the host species is thought to influence the plague dynamics, such as the direction of plague spread, however no detailed analysis exists on the possible functional or structural corridors and barriers that are present in this population and landscape. This study aims to fill that gap.MethodsThree 20 by 20 km areas with known great gerbil burrow distributions were used to analyse the spatial distribution of the burrows. Object-based image analysis was used to map the landscape at several scales, and was linked to the burrow maps. A novel object-based method was developed – the mean neighbour absolute burrow density difference (MNABDD) – to identify the optimal scale and evaluate the efficacy of using landscape objects as opposed to square cells. Multiple regression using raster maps was used to identify the landscape-ecological variables that explain burrow density best. Functional corridors and barriers were mapped using burrow density thresholds. Cumulative resistance of the burrow distribution to potential disease spread was evaluated using cost distance analysis. A 46-year plague surveillance dataset was used to evaluate whether plague spread was radially symmetric.ResultsThe burrow distribution was found to be non-random and negatively correlated with Greenness, especially in the floodplain areas. Corridors and barriers showed a mostly NWSE alignment, suggesting easier spreading along this axis. This was confirmed by the analysis of the plague data.ConclusionsPlague spread had a predominantly NWSE direction, which is likely due to the NWSE alignment of corridors and barriers in the burrow distribution and the landscape. This finding may improve predictions of plague in the future and emphasizes the importance of including landscape analysis in wildlife disease studies.

Highlights

Spread of pathogens through a population of hosts can be enhanced or restricted by the landscape
The influence of the landscape or the spatial distribution of the host or vector population on infectious disease dynamics has been shown for several diseases, such as those caused by hantavirus [7,8], Lyme disease [9] and plague [10,11,12]
This study investigates the role of the landscape and the host distribution on the occurrence of plague in the PreBalkhash focus in Eastern Kazakhstan

Summary

Introduction

Spread of pathogens through a population of hosts can be enhanced or restricted by the landscape. The influence of the landscape or the spatial distribution of the host or vector population on infectious disease dynamics has been shown for several diseases, such as those caused by hantavirus [7,8], Lyme disease [9] and plague [10,11,12]. The spatial distribution of the host species is thought to influence the plague dynamics, such as the direction of plague spread, no detailed analysis exists on the possible functional or structural corridors and barriers that are present in this population and landscape. The plague system The study area is located in the Balkhash basin in Eastern Kazakhstan, north of the Ili River (Figure 1), though plague occurs both north and south of the river It is classified as a plague focus, meaning that hosts, vectors and plague bacteria are present. When gerbils disperse, defined as gerbils permanently moving from their natal colony to another colony during the first year of their life, they move larger distances: dispersal distances for males and females are on average 350 m and 150 m, respectively [18]

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