Predicting the speed of tick invasion: an empirical model of range expansion for the Lyme disease vector Ixodes scapularis in Canada

Abstract

Summary 1.Over the past two decades, Ixodes scapularis, the primary tick vector of the Lyme disease pathogen Borrelia burgdorferi in North America has expanded its range northward from the USA to colonize new regions in southern Canada. We have previously projected range increases for I. scapularis based on temperature suitability, but to what extent this is matched by actual tick range expansion is unknown. 2. Since 1990, I. scapularis ticks have been collected across Canada offering a unique opportunity to track the range expansion of an arthropod vector. We used these data to model time-to-establishment for tick populations across Canada to identify factors influencing the rate of spread. 3. Our results point to both long-distance dispersal of ticks by migratory birds and local dispersal by resident hosts as important potential mechanisms underlying patterns of tick range expansion. 4. Temperature (accumulated degree days > 0°C) was the most important determinant of environmental suitability for tick population establishment, suggesting that climate warming may facilitate range expansion. 5. Model projections suggest that I. scapularis range will expand c. 46 km year )1 in the coming decade, with climate warming expected to increase the rate of spread. This expansion is likely to result in a substantial increase in human Lyme disease risk, with the proportion of the human population of eastern Canada inhabiting areas with established tick populations increasing from 18% in 2010 to over 80% by 2020. 6. This first empirical model of I. scapularis invasion supports theoretical range projections based on climate suitability and provides a unique data-driven estimate of the speed of northward range expansion for I. scapularis at the continental scale. 7. Synthesis and applications. By tracking I. scapularis invasion in Canada over the past two decades, we show that I. scapularis is rapidly expanding its range and is likely to colonize the most densely populated areas of southern Canada in the coming decade. These projections suggest that prompt action is necessary to prepare the Canadian public for a likely epidemic of Lyme disease, with emphasis on focusing surveillance activities to confirm the locations of emerging Lyme disease risk.

Surveillance de la maladie de Lyme au Canada, de 2009 à 2012

“…La surveillance des tiques (3,14,15) indique que la population d'Ixodes scapularis élargit sa portée géographique au Canada et près d'un quart des cas de la maladie de Lyme au stade disséminé ont été contractés dans des zones non connues comme étant endémiques (et étaient des cas probables P1). Toutefois, la plupart de ces cas sont survenus dans des zones où les populations d'I.…”

Lyme Disease Propelled by Borrelia burgdorferi-Infected Blacklegged Ticks, Wild Birds and Public Awareness ─ Not Climate Change

“…All together, these tick collections constitute an established population of I. scapularis [53]. These epidemiological findings were overlooked on the climate change maps [17,42,45,46,51]. Furthermore, game hunters, who are now seniors, remember ticks on the heads and necks of white-tailed deer during the 1950s and 1960s in northwestern Ontario and southern Manitoba.…”

“…Furthermore, adult I. scapularis were collected at the same location and, collectively, all 3 mobile life stages (adult, nymph, larva) represent a reproducing population. These discoveries were not taken into account on the climate change model maps [17,42,45,46,51]. Likewise, four I. scapularis (2 females, 1 male, 1 nymph) were collected on 12 May and 9 June 1993 from untravelled mammalian hosts (dog, human) residing on the rural property bordering Lake of the Woods north of Rainy River, Ontario; one of the 3 adults was positive for B. burgdorferi [54].…”

“…Leighton et al allege that I. scapularis has expanded its range northward during the past two decades [51], from the United States, to colonize new regions of Canada. However, they admit to not knowing the extent to which the actual tick range expansion transpired.…”

“…Researchers prognosticated that 80% of the human population in eastern and central Canada will be inhabiting areas at risk of Lyme disease by 2020 [51]. However, only 2.5% of Ontario currently has Lyme disease risk areas [50], so these climate change projections and associated climate change maps have subtle scientific shortfalls.…”