Predicting habitat suitability for Ixodes ricinus and Ixodes persulcatus ticks in Finland

Abstract

Background: Ticks are responsible for transmitting several notable pathogens worldwide. Finland lies in a zone where two human-biting tick species co-occur: Ixodes ricinus and Ixodes persulcatus. Tick densities have increased in boreal regions worldwide during past decades, and tick-borne pathogens have been identified as one of the major threats to public health in the face of climate change.

Methods: We used species distribution modelling techniques to predict the distributions of I. ricinus and I. persulcatus, using aggregated historical data from 2014 to 2020 and new tick occurrence data from 2021. By aiming to fill the gaps in tick occurrence data, we created a new sampling strategy across Finland. We also screened for tick-borne encephalitis virus (TBEV) and Borrelia from the newly collected ticks. Climate, land use and vegetation data, and population densities of the tick hosts were used in various combinations on four data sets to estimate tick species' distributions across mainland Finland with a 1-km resolution.

Results: In the 2021 survey, 89 new locations were sampled of which 25 new presences and 63 absences were found for I. ricinus and one new presence and 88 absences for I. persulcatus. A total of 502 ticks were collected and analysed; no ticks were positive for TBEV, while 56 (47%) of the 120 pools, including adult, nymph, and larva pools, were positive for Borrelia (minimum infection rate 11.2%, respectively). Our prediction results demonstrate that two combined predictor data sets based on ensemble mean models yielded the highest predictive accuracy for both I. ricinus (AUC = 0.91, 0.94) and I. persulcatus (AUC = 0.93, 0.96). The suitable habitats for I. ricinus were determined by higher relative humidity, air temperature, precipitation sum, and middle-infrared reflectance levels and higher densities of white-tailed deer, European hare, and red fox. For I. persulcatus, locations with greater precipitation and air temperature and higher white-tailed deer, roe deer, and mountain hare densities were associated with higher occurrence probabilities. Suitable habitats for I. ricinus ranged from southern Finland up to Central Ostrobothnia and North Karelia, excluding areas in Ostrobothnia and Pirkanmaa. For I. persulcatus, suitable areas were located along the western coast from Ostrobothnia to southern Lapland, in North Karelia, North Savo, Kainuu, and areas in Pirkanmaa and Päijät-Häme.

Conclusions: This is the first study conducted in Finland that estimates potential tick species distributions using environmental and host data. Our results can be utilized in vector control strategies, as supporting material in recommendations issued by public health authorities, and as predictor data for modelling the risk for tick-borne diseases.

Keywords: Borrelia burgdorferi sensu lato; Ensemble prediction; Ixodes persulcatus; Ixodes ricinus; Species distribution modelling; Tick-borne pathogen.

Fig. 1

A map presenting the study area with elevation levels

Fig. 2

Presence–absence data of a I. ricinus and b I. persulcatus after data thinning, presented together with mean air temperature during the activity seasons of each species: May–September for I. ricinus and April–June for I. persulcatus in 2014–2021

Fig. 3

The relative contributions of the explanatory variables in the data set of a the environment and host and b the environment, host, and habitat suitability for the other species (I. ricinus/I. persulcatus) based on the mean ensemble models

Fig. 4

Partial dependency plots for a I. ricinus and b I. persulcatus based on combined host and environmental data produced by the mean ensemble model

Fig. 5

Estimated habitat suitabilities for I. ricinus in mainland Finland by the ensemble mean method over several modelling methods based on a environment only data, b host only data, c combined environmental and host data, and d combined environmental, host and, habitat suitability data for I. persulcatus

Fig. 6

The estimated habitat suitabilities for I. persulcatus in mainland Finland by the ensemble mean method over several modelling methods based on a environment only data, b host only data, c combined environmental and host data, and d combined environmental, host, and habitat suitability data for I. ricinus

Fig. 7

A coefficient of variation in the predictions estimating the uncertainty of the ensemble predictions over several modelling methods for I. ricinus in mainland Finland based on a environment only data, b host only data, c combined environmental and host data, and d combined environmental, host, and habitat suitability data for I. persulcatus

Fig. 8

A coefficient of variation in the predictions assessing the uncertainty of the ensemble predictions over several modelling methods for I. persulcatus in mainland Finland based on a environment only data, b host only data, c combined environmental and host data, and d combined environmental, host, and habitat suitability data for I. ricinus