Ticks climb the mountains: Ixodid tick infestation and infection by tick-borne pathogens in the Western Alps

In mountain areas of northwestern Italy, ticks were rarely collected in the past. In recent years, a marked increase in tick abundance has been observed in several Alpine valleys, together with more frequent reports of Lyme borreliosis. We then carried out a four-year study to assess the distribution and abundance of ticks and transmitted pathogens and determine their altitudinal limit in a natural park area in Piedmont region. Ixodes ricinus and Dermacentor marginatus were collected from both the vegetation and hunted wild ungulates. Tick abundance was significantly associated with altitude, habitat type and signs of animal presence, roe deer’s in particular. Ixodes ricinus prevailed in distribution and abundance and, although their numbers decreased with increasing altitude, we recorded the presence of all active life stages of up to around 1700 m a.s.l., with conifers as the second most infested habitat after deciduous woods. Molecular analyses demonstrated the infection of questing I. ricinus nymphs with B. burgdorferi sensu lato (15.5%), Rickettsia helvetica and R. monacensis (20.7%), Anaplasma phagocytophilum (1.9%), Borrelia miyamotoi (0.5%) and Neoehrlichia mikurensis (0.5%). One third of the questing D. marginatus were infected with R. slovaca. We observed a spatial aggregation of study sites infested by B. burgdorferi s.l. infected ticks below 1400 m. Borrelia-infected nymphs prevailed in open areas, while SFG rickettsiae prevalence was higher in coniferous and deciduous woods. Interestingly, prevalence of SFG rickettsiae in ticks doubled above 1400 m, and R. helvetica was the only pathogen detected above 1800 m a.s.l. Tick infestation on hunted wild ungulates indicated the persistence of tick activity during winter months and, when compared to past studies, confirmed the recent spread of I. ricinus in the area. Our study provides new insights into the population dynamics of ticks in the Alps and confirms a further expansion of ticks to higher altitudes in Europe. We underline the importance of adopting a multidisciplinary approach in order to develop effective strategies for the surveillance of tick-borne diseases, and inform the public about the hazard posed by ticks, especially in recently invaded areas.