Geographical and environmental factors driving the increase in the Lyme disease vector Ixodes scapularis.

Camilo E Khatchikian,Melissa Prusinski,Ing-Nang Wang,Dustin Brisson,Melissa Stone,P Bryon Backenson,Michael Z Levy

doi:10.1890/es12-00134.1

Abstract

The population densities of many organisms have changed dramatically in recent history. Increases in the population density of medically relevant organisms are of particular importance to public health as they are often correlated with the emergence of infectious diseases in human populations. Our aim is to delineate increases in density of a common disease vector in North America, the blacklegged tick, and to identify the environmental factors correlated with these population dynamics. Empirical data that capture the growth of a population are often necessary to identify environmental factors associated with these dynamics. We analyzed temporally- and spatially-structured field collected data in a geographical information systems framework to describe the population growth of blacklegged ticks (Ixodes scapularis) and to identify environmental and climatic factors correlated with these dynamics. The density of the ticks increased throughout the study's temporal and spatial ranges. Tick density increases were positively correlated with mild temperatures, low precipitation, low forest cover, and high urbanization. Importantly, models that accounted for these environmental factors accurately forecast future tick densities across the region. Tick density increased annually along the south-to-north gradient. These trends parallel the increases in human incidences of diseases commonly vectored by I. scapularis. For example, I. scapularis densities are correlated with human Lyme disease incidence, albeit in a non-linear manner that disappears at low tick densities, potentially indicating that a threshold tick density is needed to support epidemiologically-relevant levels of the Lyme disease bacterium. Our results demonstrate a connection between the biogeography of this species and public health.

Highlights

Vector-borne diseases are one of the most common classes of emerging and re-emerging infectious diseases and constitute a major public health threat (Taylor et al 2001, Jones et al 2008)
We describe the dynamic pattern of tick population growth and identify environmental variables that account for these patterns
The observed patterns in tick density estimates across space are similar to the observed patterns in reported human Lyme disease cases (Fig. 2B)

Summary

Introduction

Vector-borne diseases are one of the most common classes of emerging and re-emerging infectious diseases and constitute a major public health threat (Taylor et al 2001, Jones et al 2008). These environmental changes can promote increases in population sizes or geographic distributions of the pathogens or their vectors (e.g., Guerra et al 2002, Lounibos 2002, Ogden et al 2006, Ogden et al 2009). The application of empirically validated biogeographic models allows accurate predictions of future population expansions and can constitute a critical component for the development of public health policies (Guerra et al 2002, Gage et al 2008, Diuk-Wasser et al 2010, Kaplan et al 2010, Khatchikian et al 2010b)

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