| Summary: | Climate change is facilitating the geographic range expansion of populations of the tick vector of Lyme disease <i>Ixodes scapularis</i> in Canada. Here, we characterize and map the spatio-temporal variability of environments suitable for <i>I. scapularis</i> using Earth observation (EO) data. A simple algorithm for <i>I. scapularis</i> occurrence (cumulative degree-days and forest: CSDF) was developed by combining cumulative annual surface degree-days above 0 °C and forest cover. To map the environmental risk of <i>I. scapularis</i> (risk of <i>I. scapularis</i>: RIS) in central and eastern Canada from 2000 to 2015, CSDF was adjusted using data from an <i>I. scapularis</i> population model. CSDF was validated using cumulative annual degree days > 0 °C (CADD) from meteorological stations, and CSDF was strongly associated with CADD (<i>n</i> = 52, R<sup>2</sup> > 0.86, <i>p</i> < 0.001). Data on field surveillance for <i>I. scapularis </i>ticks (2008 to 2018) were used to validate the risk maps. The presence of <i>I. scapularis</i> ticks was significantly associated with CSDF, and at a limit of 2800, sensitivity approached 100%. RIS increased over the study period, with the highest values in 2012 and the lowest in 2000. The RIS was on average higher in Ontario and Quebec compared to other provinces, and it was higher in the southern parts of the provinces. The proportion of the populated areas with a positive RIS increased on average in central and eastern Canada from 2000 to 2015. Predicted <i>I. scapularis</i> occurrence identifies areas with a more probable risk of tick bites, Lyme disease, and other <i>I. scapularis</i>-borne diseases, which can help guide targeted surveillance, prevention, and control interventions.
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