Crowdsourced tick observation data from across 60 years reveals major increases and northwards shifts in tick contact areas in Finland

Abstract

There is mounting evidence of increases in tick (Acari: Ixodidae) contacts in Finland during the past few decades, highlighted by increases in the incidence of Lyme borreliosis and tick-borne encephalitis (TBE). While nationwide field studies to map distributions of ticks are not feasible, crowdsourcing provides a comprehensive method with which to assess large-scale changes in tick contact areas. Here, we assess changes in tick contact areas in Finland between 1958 and 2021 using three different nationwide crowdsourced data sets. The data revealed vast increases in tick contact areas, with ticks estimated to be contacted locally approximately 400 km further north in western and approximately 100 km further north in eastern Finland in 2021 than 1958. Tick contact rates appeared to be highest along the coastline and on the shores of large lakes, possibly indicating higher tick abundance therein. In general, tick observations per inhabitant increased from 2015 to 2021. Tick contact areas have expanded in Finland over the past 60 years. It appears that taiga ticks (Ixodes persulcatus) are behind most of the northwards shifts in tick contact areas, with Ixodes ricinus contributing mostly to new contact areas in the south. While ticks are now present in most of Finland, there are still areas where tick abundance is low and/or establishment not possible, mainly in northern Finland.

Adult-Onset Encephalitis over Twelve Years in Easternmost Finland

“…Moreover, in every year from 2005 onwards, the majority of cases had been diagnosed outside the traditional hotspot of Åland archipelago. These developments are not surprising considering that TBE has become more common at least in Europe and Asia and in Finland tick contact areas have expanded over the past 60 years with observations becoming more common also in eastern Finland [12][13][14][15]. The majority of previous studies have globally reported HSV as the most frequent viral pathogen, followed by VZV in several studies.…”

Genomic signatures of hybridization between Ixodes ricinus and Ixodes persulcatus in natural populations

“…When comparing the native I. ricinus with the non‐native I. persulcatus , we would expect lower diversity among non‐native populations because of potential genetic bottlenecks after founder effects (Allendorf & Lundquist, 2003 ). Indeed, the low genetic diversity levels as well as lack of genetically differentiated populations in I. persulcatus could be due its recent introduction into Finland (Sormunen et al., 2023 ). However, we could not investigate the timing and frequency of introductory events and certainly a larger data set would be required to resolve this.…”