- University of Ottawa
- University of Guelph
Summary
The blacklegged tick, Ixodes scapularis, is established in several regions of Ontario, Canada, and continues to spread into new geographic areas across the province at a rapid rate. This poses a significant public health risk since I. scapularis transmits the Lyme disease-causing bacterium, Borrelia burgdorferi, and other pathogens of potential public health concern. The objective of this study was to develop species distribution models for I. scapularis and B. burgdorferi to predict and compare the potential distributions of the tick vector and the Lyme disease pathogen as well as the ecological factors most important for species establishment. Ticks were collected via tick dragging at 120 sites across southern, central, and eastern Ontario between 2015 and 2018 and tested for tick-borne pathogens. A maximum entropy (Maxent) approach was used to model the potential distributions of I. scapularis and B. burgdorferi. Two independent datasets derived from tick dragging at 25 new sites in 2019 and ticks submitted by the public to local health units between 2015 and 2017 were used to validate the predictive accuracy of the models. The model for I. scapularis showed high suitability for blacklegged ticks in eastern Ontario and some regions along the shorelines of the Great Lakes, and moderate suitability near Algonquin Provincial Park and the Georgian Bay with good predictive accuracy (tick dragging 2019: AUC = 0.898; ticks from public: AUC = 0.727). The model for B. burgdorferi showed a similar predicted distribution but was more constrained to eastern Ontario, particularly between Ottawa and Kingston, and along Lake Ontario, with similarly good predictive accuracy (tick dragging 2019: AUC = 0.958; ticks from public: AUC = 0.863. The ecological variables most important for predicting the distributions of I. scapularis and B. burgdorferi included elevation, distance to deciduous and coniferous forest, proportions of agricultural land, water, and infrastructure, mean summer/spring temperature, and cumulative annual degree days above 0°C. Our study presents a novel application of species distribution modelling for I. scapularis and B. burgdorferi in Ontario, Canada, and provides an up to date projection of their potential distributions for public health knowledge users.
Methodology
Occurrence data were compiled from field collection of immature and adult I. scapularis ticks made by the University of Guelph and the University of Ottawa from 2015–2018 in Ontario, Canada. We received authorization from Ontario Parks, the City of Ottawa, the National Capital Commission, Queen’s University Biological Station, Upper Canada Migratory Bird Sanctuary as well as relevant regional conservation authorities (e.g. Grey Sauble, Mississippi Valley, Cataraqui). Both field teams employed a standard field dragging protocol in which a one-meter squared white flannel cloth sheet was dragged along surface vegetation and the forest floor for three person-hours in a given site [28, 29]. The drag sheets and surveyors’ clothes were checked for ticks every 3 minutes (University of Guelph) or every 50 meters with step counts adjusted for each individual's walking pace (University of Ottawa) [28, 29]. Latitude/longitude coordinates were recorded for each sample location using a hand-held GPS. All selected sites were visited during the summer months (May to August) to capture the peak season for questing nymphs. In eastern Canada, the density of questing adult ticks often peaks in the spring and fall, while the density of questing nymphs peaks in the summer [5]. However, various sites were revisited multiple times during the summer and fall to address other research questions. For this study, we used all sampling data available to maximize our ability to detect ticks. Locations to be sampled were selected by each University independently as described in earlier work, and included sites both known, suspected, or broadly suitable for I. scapularis as well as control sites, negative sites, or unsuitable I. scapularis sites distributed across three ecoregions (5E, 6E, 7E) of Ontario and within urban, suburban, and rural regions [24, 28–30]. A total of 120 sites were sampled between 2015 and 2018 across southern, central, and eastern Ontario (Fig 1). Ixodes scapularis ticks were found at 52 locations (Fig 1).