Risk maps for range expansion of the Lyme disease vector, Ixodes scapularis, in Canada now and with climate change

Nicholas H Ogden,Robbin Lindsay,Charles M Francis,Abdel Maarouf,Michel Bigras-Poulin,Stephanie Brazeau,Louise Trudel,Francois Milord,Pascal Michel,Laurie St-Onge,Christopher J O'Callaghan,Alex Thompson,Dominique F Charron,Ian K Barker,Audrey Heagy

doi:10.1186/1476-072x-7-24

Abstract

BackgroundLyme disease is the commonest vector-borne zoonosis in the temperate world, and an emerging infectious disease in Canada due to expansion of the geographic range of the tick vector Ixodes scapularis. Studies suggest that climate change will accelerate Lyme disease emergence by enhancing climatic suitability for I. scapularis. Risk maps will help to meet the public health challenge of Lyme disease by allowing targeting of surveillance and intervention activities.ResultsA risk map for possible Lyme endemicity was created using a simple risk algorithm for occurrence of I. scapularis populations. The algorithm was calculated for each census sub-division in central and eastern Canada from interpolated output of a temperature-driven simulation model of I. scapularis populations and an index of tick immigration. The latter was calculated from estimates of tick dispersion distances by migratory birds and recent knowledge of the current geographic range of endemic I. scapularis populations. The index of tick immigration closely predicted passive surveillance data on I. scapularis occurrence, and the risk algorithm was a significant predictor of the occurrence of I. scapularis populations in a prospective field study. Risk maps for I. scapularis occurrence in Canada under future projected climate (in the 2020s, 2050s and 2080s) were produced using temperature output from the Canadian Coupled Global Climate Model 2 with greenhouse gas emission scenario enforcing 'A2' of the Intergovernmental Panel on Climate Change.ConclusionWe have prepared risk maps for the occurrence of I. scapularis in eastern and central Canada under current and future projected climate. Validation of the risk maps provides some confidence that they provide a useful first step in predicting the occurrence of I. scapularis populations, and directing public health objectives in minimizing risk from Lyme disease. Further field studies are needed, however, to continue validation and refinement of the risk maps.

Highlights

Lyme disease is the commonest vector-borne zoonosis in the temperate world, and an emerging infectious disease in Canada due to expansion of the geographic range of the tick vector Ixodes scapularis
Throughout the study, the geographic unit was the census sub-division (CSD) whose geographic area, geographic coordinates and population information were obtained as a coverage and spreadsheet from Statistics Canada [24]. This spatial unit is increasingly used in Canadian public health because i) it is a well defined spatial unit available to users, including provincial and municipal public health organisations, ii) its spatial boundaries are relatively stable over time, and iii) accompanying data on human population acts as a denominator for surveillance activities [23]
Selection of risk algorithms and cut-off levels A number of algorithms to estimate risk of establishment of I. scapularis were calculated from a combination of values for i) output of a simulation model of I. scapularis populations indicating the temperature suitability for the tick, ii) an index of the numbers of ticks immigrating on migratory birds, and iii) the percentage coverage of forest habitat

Summary

Introduction

Lyme disease is the commonest vector-borne zoonosis in the temperate world, and an emerging infectious disease in Canada due to expansion of the geographic range of the tick vector Ixodes scapularis. Risk maps will help to meet the public health challenge of Lyme disease by allowing targeting of surveillance and intervention activities. Lyme disease is an emerging infectious disease in Canada, largely due to the expansion of the geographic range of the tick vector Ixodes scapularis in eastern and central Canada [1]. Global warming is anticipated to accelerate expansion of the geographic range of I. scapularis into Canada, provided that suitable habitat and hosts occur [5], and may influence the emergence of tick-borne zoonoses [6]. Predicting the occurrence of vectors by risk maps is, a potentially very useful tool to guide public health policy and target surveillance and intervention activities for vectorborne diseases [13,14]

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