Ticks (Acari:
Ixodidae) and tick-borne diseases, especially Lyme borreliosis caused by Borrelia burgdorferi s.l.
spirochetes and tick-borne encephalitis (TBE) caused by the TBE-virus (TBEV),
are a growing problem in northern Europe and Russia. Surveys conducted in Russia,
Sweden, and Norway have revealed an increase in tick abundance and a northwards
shift in their distribution over the past few decades. In the southern parts of
Finland, Ixodes ricinus is
the primary vector for Borrelia,
TBEV, and several other tick-borne pathogens (TBP). Despite their central role
in the zoonotic transmission of severe diseases, ecological data of I. ricinus populations in
Finland are almost non-existent. Less than 20 research papers focusing on I. ricinus have been
published from Finland between 1961 and 2017, with only a few of them touching
upon any ecological aspects of the ticks themselves. As such, no scientific data
regarding changes in tick and TBP occurrence has been available prior to recent
years. However, citizen science surveys mapping the distribution of I. ricinus in 2014 and 2015,
as well as the numbers of Lyme borreliosis cases that have been increasing
since the mid-90’s, suggest that changes similar to those observed in
neighboring countries are likewise taking place in Finland.

In this
thesis, I have focused on several of the neglected aspects of tick-related
research in Finland, in an effort to determine the current status of tick
populations in southwestern Finland and whether changes similar to those
observed elsewhere in the northern parts of Europe can be observed there. Among
others, study themes included changes in the abundance and distribution of I. ricinus, changes in the
prevalence rates and occurrence of several TBPs, as well as long-term
monitoring of spatial changes in the diversity and occurrence of TBPs.

I found
that I. ricinus numbers
in southwestern Finland have increased over the past few decades in all study
sites with reference data available, with the highest densities being observed
in 2017, the last year of studies for this thesis. Ticks were commonly found
from all coastal areas in southwestern Finland, with particularly high
densities being observed on islands in the Archipelago Sea. Similarly, in 2017 ticks
were detected from several urban and suburban study sites in the city of Turku,
some of which were surveyed but found lacking ticks in 2013. Regarding
pathogens, higher prevalence of B.
burgdorferi s.l  was
observed than in reference studies from 2000, and its geographical range had
also expanded. Disconcertingly, particularly high Borrelia prevalence rates
were observed in tick populations in urbanized areas around Turku. Altogether
seven different pathogen groups were detected from southwestern Finland,
including several species that had not been reported from Finland before. Furthermore,
the probable emergence of two pathogens was observed on Seili Island during the
study, highlighting that ticks are not proliferating alone: tick-borne
pathogens are as well.

To
conclude, the results of this thesis confirm that tick densities are
increasing, tick-borne pathogens are becoming more common, and both ticks and
TBPs are spreading in southwestern Finland. Particularly alarming is the
detection of high numbers of ticks and TBPs in the sphere of influence for
hundreds of thousands of people annually, in urban and suburban areas within
and around Turku. Vigilance is required from both citizens and medical
professionals, in order to prevent and detect harmful tick-borne infections.
Action should be taken to inform citizens of noticeable tick risk also in
urbanized areas. Unfortunately, no decrease in the number of ticks is likely to
occur in the southern parts of Finland in the foreseeable future, so further
increasing tick awareness is the best course of action for minimizing their negative
impact.