Predicted Northward Expansion of the Geographic Range of the Tick Vector Amblyomma americanum in North America under Future Climate Conditions

Abstract

Background: The geographic range of the tick Amblyomma americanum, a vector of diseases of public health significance such as ehrlichiosis, has expanded from the southeast of the United States northward during the 20th century. Recently, populations of this tick have been reported to be present close to the Canadian border in Michigan and New York states, but established populations are not known in Canada. Previous research suggests that changing temperature patterns with climate change may influence tick life cycles and permit northward range expansion of ticks in the northern hemisphere.

Objectives: We aimed to estimate minimal temperature conditions for survival of A. americanum populations at the northern edge of the tick's range and to investigate the possibility of range expansion of A. americanum into northern U.S. states and southern Canada in the coming decades.

Methods: A simulation model of the tick A. americanum was used, via simulations using climate data from meteorological stations in the United States and Canada, to estimate minimal temperature conditions for survival of A. americanum populations at the northern edge of the tick's range.

Results: The predicted geographic scope of temperature suitability [$\ge \mathrm{3,285}$ annual cumulative degree days (DD) $>0°C$] included most of the central and eastern U.S. states east of longitude 110°W, which is consistent with current surveillance data for the presence of the tick in this region, as well as parts of southern Quebec and Ontario in Canada. Regional climate model output raises the possibility of northward range expansion into all provinces of Canada from Alberta to Newfoundland and Labrador during the coming decades, with the greatest northward range expansion (up to $\mathrm{1,000}\mathrm{km}$ by the year 2100) occurring under the greenhouse gas (GHG) emissions of Representative Concentration Pathway (RCP) 8.5. Predicted northward range expansion was reduced by approximately half under the reduced GHG emissions of RCP4.5.

Discussion: Our results raise the possibility of range expansion of A. americanum into northern U.S. states and southern Canada in the coming decades, and conclude that surveillance for this tick, and the diseases it transmits, would be prudent. https://doi.org/10.1289/EHP5668.

Figure 1.

Geographic locations of the 36 meteorological stations (using abbreviations of the station names) that provided temperature and day-length data for the dynamic population model simulations. $\text{QC},\text{\hspace{0.17em} Quebec}$, $\text{ON},\text{\hspace{0.17em} Ontario}$, $\text{NL},\text{\hspace{0.17em} Newfoundland \hspace{0.17em} and \hspace{0.17em} Labrador}$, $\text{NB},\text{\hspace{0.17em} New \hspace{0.17em} Brunswick}$, $\text{NS},\text{\hspace{0.17em} Nova \hspace{0.17em} Scotia}$.

Figure 2.

A diagram of the model showing each stage or phase of the tick life cycle as boxes linked by life cycle processes (hatching of eggs, host finding, development and molting of ticks) shown by solid arrows. Arrows of different styles indicate mortality that affects each life stage, influences of temperature and day length on life cycle processes, and density-dependent effects on mortality and reproduction rates. Temperature affects questing activity of questing larvae, nymphs, and adults, and acts on development rates from one life stage to the next: the preoviposition period of engorged adult females (POP), the preeclosion period of eggs (PEP), and the development of engorged larvae to nymphs (L to N) and from engorged nymphs to adults (N to A). Effects of day length on questing activity were also included in the model.

Figure 3.

Maximum number of feeding larvae (panel A), nymphs (panel B) and adults (panel C) at equilibrium plotted against the mean annual number of degree days $>0°\mathrm{C}$ ($\text{DD}>0°\mathrm{C}$), for the meteorological stations that provided temperature data for the simulations. The dashed line indicates the lower temperature threshold for A. americanum population survival estimated in the study (3,285 $\text{DD}>0°\mathrm{C}$).

Figure 4.

Regions of Canada and the United States predicted as having temperature conditions suitable for A. americanum population survival ($\ge \mathrm{3,285}$ degree days $>0°\mathrm{C}$) under current climate (1981–2010 normals), computed from observed ANUSPLIN NLDAS data at approximately $10\text{-}\mathrm{km}$ resolution. U.S. counties identified as having “reported” and “established” A. americanum according to Springer et al. (2014) are shown (respectively) by light and heavy cross-hatching. Locations of the 11 meteorological stations in the United States that provided temperature and day-length data for the dynamic population model simulations are also shown.

Figure 5.

Relationship between the maximal number of feeding larvae at equilibrium and the mean annual degree days resulting from modification of mortality rates of feeding (panel A) and nonfeeding (panel B) stages of A. americanum. For each meteorological station (Ottawa, Hamilton, Albany, and Detroit), the results of three simulations are shown: one using starting values for the basal daily mortality rates, and one each using basal daily mortality rates 5% above and below starting values.

Figure 6.

Geographic location of the lower temperature threshold (3,285 degree days $>0°\mathrm{C}$) for A. americanum survival during the 1971–2000, 2011–2040, 2041–2070, and 2071–2,100 periods computed from ensemble mean values of six Regional Climate Models under RCP4.5 (upper panel) and RCP8.5 (lower panel) scenarios.

Figure 7.

Intramodel variability in an ensemble of six Regional Climate Model (RCM) simulations at $50\text{-}\mathrm{km}$ resolution and its effect on the location of the lower temperature threshold (3,285 degree days $>0°\mathrm{C}$) for A. americanum survival in North America in 1971–2000, 2011–2040, 2041–2070, and 2071–2100, under RCP4.5 and RCP8.5 scenarios. Lines indicating the 10th, 50th, and 90th percentiles of the RCM simulated values are shown.