Ixodes scapularis (Blacklegged Tick) Population Density and Borrelia burgdorferi Prevalence in Ticks Along Elevation Gradients in Addison County, Vermont

Vector-borne diseases pose a high risk to human health. Lyme disease, the most prevalent vector-borne disease in the United States and western Europe, is transmitted to humans by Ixodes ticks. In eastern North America, Ixodes scapularis, the blacklegged tick or deer tick, is the main vector of Borrelia burgdorferi, the bacterial agent of Lyme disease. While previous studies in North America have investigated how Ixodes scapularis population density varies biogeographically, this study is the first to investigate how Ixodes scapularis population density changes along elevation gradients. Elevation gradients can serve as a proxy for understanding the effects of climate change on tick population density. In this study, questing population densities of adults and nymphs were measured using the drag cloth method in 200m2 plots distributed along elevation gradients, ranging from 70 m to 940m, on six sites near Middlebury, Vermont, in spring and summer 2022. Environmental factors associated with tick population density were also measured to better understand the drivers of tick population density along these elevation gradients. Additionally, extracted DNA from collected ticks was analyzed for the presence of Borrelia burgdorferi via PCR and gel electrophoresis to determine whether infection rate varies with elevation. A generalized linear model with a Poisson distribution was used to evaluate the relationship between elevation and questing tick population density for adults sampled in the spring, while a generalized linear mixed effects model with a Poisson distribution was used to evaluate the relationship between elevation and questing tick population density for nymphs sampled in the summer. A forward stepwise regression was conducted to determine which environmental variables best accounted for the change in tick population density along the studied elevation gradients. A generalized linear model with a binomial distribution was conducted to evaluate how infection rate varied with elevation. Most of these statistical analyses yielded non-significant results; however, questing nymphal population density decreased significantly with increasing elevation. Elevation and deciduous tree basal area were returned as predictor variables, both having negative effects on questing nymph population density, but only the effect of elevation was significant. Since elevation itself does not have a direct impact on ticks, it is likely that an unmeasured variable associated with elevation, possibly temperature related, best predicts questing tick population density along these elevation gradients.