Predicting the Risk of Lyme Disease: Habitat Suitability for Ixodes scapularis in the North Central United States

Marta Guerra, Edward Walker, Carl Jones, Susan Paskewitz, M. Roberto Cortinas, Ashley Stancil, Louisa Beck, Matthew Bobo, Uriel Kitron

Disclosures

Emerging Infectious Diseases. 2002;8(3) 

In This Article

Abstract and Introduction

The distribution and abundance of Ixodes scapularis were studied in Wisconsin, northern Illinois, and portions of the Upper Peninsula of Michigan by inspecting small mammals for ticks and by collecting questing ticks at 138 locations in state parks and natural areas. Environmental data were gathered at a local level (i.e., micro and meso levels), and a geographic information system (GIS) was used with several digitized coverages of environmental data to create a habitat profile for each site and a grid map for Wisconsin and Illinois. Results showed that the presence and abundance of I. scapularis varied, even when the host population was adequate. Tick presence was positively associated with deciduous, dry to mesic forests and alfisol-type soils of sandy or loam-sand textures overlying sedimentary rock. Tick absence was associated with grasslands, conifer forests, wet to wet/mesic forests, acidic soils of low fertility and a clay soil texture, and Precambrian bedrock. We performed a discriminant analysis to determine environmental differences between positive and negative tick sites and derived a regression equation to examine the probability of I. scapularis presence per grid. Both analyses indicated that soil order and land cover were the dominant contributors to tick presence. We then constructed a risk map indicating suitable habitats within areas where I. scapularis is already established. The risk map also shows areas of high probability the tick will become established if introduced. Thus, this risk analysis has both explanatory power and predictive capability.

Lyme disease, the most common vectorborne disease of humans in the United States, is caused by the spirochete Borrelia burgdorferi and transmitted by the blacklegged tick Ixodes scapularis[1]. The distribution of Lyme disease in the Midwest has been determined largely by mapping the distribution of its vector, I. Scapularis, which was first detected in northwestern Wisconsin in the late 1960s[2]. Its range then expanded southward and eastward[3,4,5,6]. Even though an isolated established population was discovered in northeastern Wisconsin in Marinette County[7], I. scapularis does not appear to have become established in several counties in northeastern Wisconsin. This area is heavily populated with white-tailed deer (Odocoileus virginianus) and white-footed mice (Peromyscus leucopus)[8], which serve as hosts for I. scapularis[1]. Since host densities do not appear to be a limiting factor for the tick population[9], the physical environment, both at the macro and micro levels, may affect the tick's ability to survive in this habitat. Moreover, even if establishment is successful, environmental factors may limit tick population densities.

In northwestern Illinois, well-established I. scapularis populations were found along the Rock River in Ogle County and in Rock Island and Lee counties since the late 1980s[10,11,12,13,14]. Through the early 1990s, Jo Daviess County was the only positive area along the Wisconsin border, and Putnam County was the only positive along the Illinois River. In southern Illinois, no blacklegged ticks were found among white-tailed deer in a survey conducted from 1980 to 1983[15]. Northern Illinois also maintains populations of white-tailed deer and white-footed mice[8], although a large proportion of land is used for agriculture[16].

The phenology of I. scapularis has been studied in Michigan[17], Wisconsin[18], and Illinois[19]. In the Midwest, adults have both a longer activity period as well as higher peak densities in the spring than in the fall.

Studies of habitat preferences of I. scapularis, which have been conducted at various spatial scales[20,21,22], found environmental factors that are associated with vector and host distribution and densities. I. scapularis presence has been correlated with sandy soils[23,24] and wooded vegetation[25,26,27,28]. At the macro level, environmental risk factors for Lyme disease have been determined using satellite, climatological, and ecological data to characterize the habitat of the vector tick using geographic information systems (GIS), both in Europe[29,30,31,32,33] and the United States[22,23,24,34,35,36].

The purpose of this study was to determine the distribution of I. scapularis in the upper Midwest based on data from Wisconsin, northern Illinois, and the Upper Peninsula of Michigan, and to explain the environmental factors that facilitate or inhibit the establishment of I. scapularis. Since host abundance is not a limiting factor for the maintenance of tick populations in this area, survival of I. scapularis may depend on a combination of several environmental risk factors, resulting in a patchy, discontinuous distribution of this vector. We propose a hierarchic interpretation, starting from the bedrock geology through glacial history and climate patterns, to explain the topography, soil, and vegetation patterns that may directly affect tick survival. By characterizing the habitat preferences of I. scapularis using digitized databases (some derived from satellite imagery) and field data integrated into a GIS, the distribution of Lyme disease and other diseases transmitted by the blacklegged tick can be predicted, and the risk of transmission to the human population can be assessed.

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