Climate change and the potential for range expansion of the Lyme disease vector Ixodes scapularis in Canada
Introduction
Ixodes scapularis Say (1821) is a vector of a number of tick-borne zoonoses including Lyme disease, which currently infects in the order of 20,000 people a year in the USA (CDC, 2003). For many terrestrial arthropod species, a northward range expansion is expected in response to projected climate change (Root et al., 2003). Were I. scapularis to follow this trend, the zoonotic diseases it transmits (Lyme borreliosis, human babesiosis and anaplasmosis: Thompson et al., 2001) may present an enhanced public health challenge for Canada, whose southeastern border lies at the northern edge of the tick's range (CDC, 1999). Whether or not changes in the geographic distribution of vector-borne diseases will occur with climate change is much debated. For some diseases there are grounds for such concerns (Peterson and Shaw, 2003), while for others, outcomes of climate change may be minor compared to changes in risk factors such as wealth and lifestyle of target human populations, and alterations to habitats and ecosystems (Hay et al., 2002, Tanser et al., 2003). Analyses using a statistical model have suggested that the geographic range of I. scapularis may extend north with climate change (Brownstein et al., 2005). However, these analyses rest on identification of locations suitable for I. scapularis in the USA and the question remains as to how far and how significantly in terms of public health, could I. scapularis extend north into Canada with climate change.
A coincidence of factors may mean that further spread of I. scapularis into Canada is particularly likely and significant for human health. First, the highest incidence of Lyme disease occurs in the northeastern states of the USA, where the densities of I. scapularis populations are the highest (CDC, 1999) and these states border some of the most densely populated regions of Canada. Second, I. scapularis are carried into Canada each spring by northward migrating birds (Scott et al., 2001). Third, habitats containing high densities of suitable hosts for I. scapularis and rodent reservoirs for tick-borne zoonoses are already widespread in highly populated regions of southeastern Canada (Banfield, 1977, Gallivan et al., 1998). Fourth, a dynamic model of I. scapularis populations suggested that increasing temperature (associated with climate change) could increase the potential for immigrating ticks to establish resident, endemic tick populations (Ogden et al., 2005).
At the northern end of the tick's range, environmental temperature rather than day length-dependent diapause is the main determinant of intersstadial development rates of engorged I. scapularis in the field (Ogden et al., 2004). In model simulations, the colder the environment the longer are intersstadial development times and total generation times, and so the greater is the proportion of ticks that die before reproducing. A theoretical limit of temperature conditions for establishment of self-sustaining I. scapularis populations were obtained, at which tick mortality in each cohort is greater than fecundity. Mean annual degree-days >0 °C (DD>0 °C) was found to be a convenient index of the seasonally variable temperature conditions used in model simulations, which allowed us to quantify and map the current potential geographic range of temperature conditions suitable for I. scapularis establishment in Canada. Temperature effects on development rates and on activity of host seeking ticks, may also determine the seasonal activity patterns of different tick instars (Ogden et al., 2005). The current pattern of seasonal activity of immature I. scapularis in northeastern USA (nymphs questing in spring and larvae in summer) is thought to drive efficient transmission cycles of some tick-borne zoonoses (Yuval and Spielman, 1990). Our findings have raised three important questions regarding potential effects of projected climate change: (i) will the tick's range increase significantly? (ii) will the threshold number of immigrating ticks needed for establishment be reduced? and (iii) will the seasonal timing of activity of different tick instars allow endemic cycles of tick-borne pathogens (including Borrelia burgdorferi sensu stricto, the agent of Lyme borreliosis) to establish with new endemic I. scapularis populations?
Section snippets
Materials and methods
To answer these questions, first we obtained DD>0 °C scenario maps for Canada to examine how the geographic limits for I. scapularis establishment obtained in a previous study (Ogden et al., 2005) may change with climate change scenarios for three future projections: the 2020s, the 2050s and the 2080s. Second, we used the I. scapularis population model (Ogden et al., 2005) to simulate tick populations at four sites in Ontario, Canada under the climate change scenarios. These simulations were
Geographic limits for I. scapularis under climate change scenarios
Both CGCM2 and HadCM3 projected a northwards expansion of the range of the DD>0 °C limit for I. scapularis establishment derived from the tick model, that was evident by the 2020s, marked by the 2050s and wide by the 2080s (Fig. 1). The DD>0 °C maps obtained from both CGCM2 and HadCM3 placed the current potential northern limit for I. scapularis establishment further south than that mapped directly from 1971–2000 observed data from Canadian meteorological stations (Ogden et al., 2005). Two I.
Discussion
There is a general consensus amongst GCMs in the direction and degree of projected temperature and precipitation change for Canada (IPCC, 2001). The CGCM2 and HadCM3 output we used here are not ‘outlier’ projections for climate change in Canada in comparisons with that of other models and the ‘A2’ emission scenario is considered the most realistic if the world remains much as it is today (IPCC, 2001). In our study, there was consensus amongst GCMs and the emissions scenarios that a northern
Acknowledgements
This study was funded by the Climate Change Action Fund of Natural Resources Canada.
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