Landscape determinants of density of blacklegged ticks, vectors of Lyme disease, at the northern edge of their distribution in Canada

Abstract

In eastern North America, including Canada, Lyme disease is caused by Borrelia burgdorferi sensu stricto and transmitted to humans by the blacklegged tick, Ixodes scapularis. The last decade has seen a growing incidence of Lyme disease in Canada, following the northward range expansion of I. scapularis tick populations from endemic areas in eastern United States. This may be attributable to movement of the many hosts that they parasitize, including songbirds, deer and small mammals. In this study, we wanted to test the effect of spatial, temporal and ecological variables, on blacklegged tick density and infection rates, near the northern limit of their distribution in Ontario and Quebec, Canada. We found an effect of both proportion of forested areas and distance to roads, on density of I. scapularis ticks and prevalence of infection by B. burgdorferi. We also found an effect of both sampling year and ordinal sampling data on prevalence of infection by B. burgdorferi. In six adjacent sites showing evidence of reproducing I. scapularis populations, we found that forest composition and structure influenced density of I. scapularis ticks. Our results suggest that blacklegged tick density and infection rate in Canada may be influenced by a variety of factors.

Species distribution models for the eastern blacklegged tick, Ixodes scapularis, and the Lyme disease pathogen, Borrelia burgdorferi, in Ontario, Canada

“…Recent studies have also focused on microclimate or microhabitat to identify I. scapularis distributions at a local scale, and have found significant associations between nymphal and adult tick densities with forest type, forest understory, dominant tree type, depth of litter layer, distance from trails, type of trails, and distance to roads, which support our findings for the dominance of forests in I. scapularis habitat suitability [15,21,22,57]. A local ecological niche model for I. scapularis in the city of Ottawa also found that distance to forests and treed land were among the strongest variables predicting the distribution of blacklegged ticks [24].…”

“…Deciduous forests have been shown to be most favourable for I. scapularis establishment in other studies in North America, while coniferous forests were least favourable for ticks [58,59]. However, in our model distance to both types of forest were found to be important for I. scapularis habitat, which may reflect the importance of cedar and maple forests for tick density [22]. This is also likely due to the behaviour of white-tailed deer, the main reproductive host for I. scapularis, which frequent forest edges that are dominated by coniferous trees such as white cedar, eastern hemlock, and white pines [20,60].…”

“…which has been shown to be a strong predictor of tick occurrence globally [21,22,53]. However, landscape structure and connectivity as well as host abundance and diversity may also play a role in the establishment of ticks in forest habitats.…”

“…However, landscape structure and connectivity as well as host abundance and diversity may also play a role in the establishment of ticks in forest habitats. It had been shown that I. scapularis (and its western counterpart I. pacificus) is predominantly found in woodlands with high canopy cover compared to open shrublands, yet, on the other hand forest fragmentation contributes to higher prevalence of B. burgdorferi along forest edges and urban forests [22,54,55]. It is currently unclear how landscape structure affects the colonization ability of I. scapularis and why regions like Algonquin Park, with very dense mixed forests, show a lower suitability for ticks.…”

“…On a broader geographic scale, province-wide surveillance studies have shown that the distribution of I. scapularis populations is affected by climatic factors, which play an important role in the occurrence and abundance of arthropod vectors and in delimiting the potential range of the vector [5-7, 17, 18]. At a local scale, site-level surveillance studies have shown that the distribution of I. scapularis is also affected by ecological factors like understory density, presence of shrubs, dominant tree type, canopy cover, proportion of forested land and forest fragmentation that are integral to the life cycle of the ticks [19][20][21][22]. These factors also contribute to an adequate habitat for mammalian hosts such as white-footed mice (Peromyscus leucopus) and white-tailed deer (Odocoileus virginianus) that are integral to the developmental and reproductive cycle of the ticks, which are obligate ectoparasites [20].…”