Abstract
The distribution range of the tick Ixodes ricinus in Europe is expanding in latitude and altitude. Consistently, a rise in human cases of tick-borne diseases has been reported. Environmental and land-use changes, like the increasing abundance of wildlife and the abandonment of marginal territories, as well as the attenuation of winter temperatures and narrowing of the temperature range at high altitudes, led to the infestation of mountain areas previously not infested. In the Italian North-Western Alps, I. ricinus presence has been recorded above 1600m of altitude, and its abundance was related to altitudinal range and forest type. This study aims to re-evaluate previous records about questing I. ricinus nymphs distribution in relation with climatic and environmental factors.
Data from a previous study were employed (Garcia-Vozmediano et al., 2020). In the Susa valley, which stretches eastward for 80km from the French border to the city of Turin in the northwestern Italian Alps, 44 sites were selected. To collect ticks, from 2016 to 2019, the sites were sampled by dragging almost six times per year, from April to November. At sampling, temperature, relative humidity, weather condition and the presence of wildlife’s traces were recorded along with exact geographic location and habitat characteristics. In addition to the five predictors previously measured on-site, we added ten measured from remote sensing. From digital terrain model, altitude, slope, and aspect were obtained. The Normalized Difference Vegetation Index (NDVI), which estimates vegetation coverage, and global horizontal irradiation, the sum of direct and diffuse solar irradiation, were included too. Weather conditions (minimum and maximum temperatures, saturation deficit, wind, and rainfall) were evaluated based on the I. ricinus survival range (Perret et al., 2000) and measured in the variable time spans preceding the sampling.
A spatio-temporal Bayesian LASSO model was implemented to select most relevant variables to explain I. ricinus nymphs’ distribution. Therefore, the model was fitted on a subset of 724 transects that ensured the most complete set of samplings on a subset of 29 sites.
Habitat, altitude, and wildlife were previously reported as key factors affecting ticks’ density, but their relevance decreased when other variables were considered. Relative humidity and saturation deficit measured at sampling showed a non-linear effect on ticks’ distribution but remained valid predictors. In addition, our model showed that nymphs’ density is related to the global solar irradiation and adverse weather conditions during the days preceding the sampling.
The recent infestation of I. ricinus of high Alpine areas requires additional efforts to understand the complex adaptation mechanisms of this species to the environment. This study helps in the comprehension of the role of environmental and climatic factors and demonstrate how remote sensing variables can integrate data measured on-site.
References
Garcia-Vozmediano, A., Krawczyk, A. I., Sprong, H., Rossi, L., Ramassa, E., & Tomassone, L. (2020). Ticks climb the mountains: Ixodid tick infestation and infection by tick-borne pathogens in the Western Alps. Ticks and Tick-Borne Diseases, 11(5), 101489. https://doi.org/10.1016/j.ttbdis.2020.101489
Perret, J. L., Guigoz, E., Rais, O., & Gern, L. (2000). Influence of saturation deficit and temperature on Ixodes ricinus tick questing activity in a Lyme borreliosis-endemic area (Switzerland). Parasitology Research, 86(7), 554–557. https://doi.org/10.1007/s004360000209