Lyme borreliosis bacteria benefit from biodiversity loss

The island of Seili in the Turku Archipelago is best known for its colourful history, beautiful scenery and the Archipelago Research Institute. A lesser known fact is that extensive tick research is also being carried out on the island.

Published 2.7.2020
Blog written by Jani Sormunen 

The University of Turku tick project was initiated on the island of Seili in 2012, at a time when very little was known about ticks in Finland. Researchers interested in ticks and tick-borne pathogens began, at that time, to gather information about phenomena related to ticks by establishing permanent research (tick drag) lines in different habitats across Seili. The studies carried out in Seili form the longest continuous set of research data on ticks in Finland. During these studies, significant fluctuations in the numbers and prevalence of ticks and tick-borne pathogens have been observed on the island.

In my research project, I am investigating how the species diversity, density and movement activity of vertebrates across different habitats affect the occurrence and prevalence of ticks and, above all, tick-borne pathogens on the island.

My research focuses specifically on juvenile ticks, i.e. nymphs, who are also the most likely life stage to transmit pathogens to humans. The life cycle of ticks contains four stages: egg, larva, nymph and adult tick. Ticks do not usually hatch from eggs carrying pathogens, but rather get infected with the pathogens during their blood meals on host animals. Therefore, nymphs, who have enjoyed a blood meal from only a single host animal during the larval stage, will on average carry slightly less pathogens than adult ticks, who have already fed twice, once as larva and once as a nymph.

Using this logic, we can make the generalisation that the pathogens in nymphs originate from the host animals on which they fed during their larval stage. The source of the pathogens in adult ticks is more difficult to assess, since they have already fed on two host animals. Another interesting characteristic of ticks living in Finland is that they rarely change their locations by moving when not aboard host animals. For this reason, we can, with high confidence, state that a nymph that latches on to a research blanket has dropped off a host animal that passed within a radius of a few metres from the spot at which the tick attached to the blanket.

This leads us to the conclusion that the differences in the prevalence of pathogens observed in nymphs collected along the different research lines on the island are deeply connected to differences between the species composition and relative abundances of host animals inhabiting or frequenting the same research lines – and this is precisely the focus of my research project.

Exceptionally precise data

Multiple methods, including camera trapping, are being used to identify the different vertebrate species and their abundances and activity along and proximate to the island’s tick research lines. The aim is to collect as comprehensive a set of research material as possible on host animals, which can then be cross-referenced with information regarding ticks and tick-borne pathogens that has been collected along these same study lines. Data that is as spatially precise as this is exceedingly rare – partly because it is very uncommon to have standardized and fixed research lines in tick studies.

And what can this research tell us? With the help of our data, we can find answers to numerous questions, such as those relating to the minimum spatial scale that is relevant concerning the upkeep and occurrence of ticks and pathogens. Perhaps the most interesting study question concerns the so-called dilution effect. According to this theory, it is likely that the prevalence of pathogens carried by ticks will decrease as host animal diversity increases. This is expected to happen because the more species of host animals there are within an area, the more likely it is that ticks will encounter species that are not well-suited to be carriers for the pathogens.

Conversely, we might say that the less host species there are, the more likely it is that the ticks will carry for example the Borrelia-bacteria that cause Lyme disease. The research being carried out in Seili will provide us with the first assessment regarding the existence of the dilution effect in Northern Europe. At the same time, this data can be used to assess the way in which decreasing biodiversity may affect the threat presented by ticks now and in the future.

Research data to help mitigate tick-borne infection risk

Ticks seem to be one of the plights that is becoming increasingly common in the northern hemisphere as a result of climate change. Unfortunately, predicted future changes in the weather conditions in Finland suggest increasingly beneficial living conditions for ticks. For the moment, at least, there are no effective methods available to reduce the numbers of ticks on the national level without causing considerable damage to our already threatened natural environments and declining biodiversity.

However, it may be possible to reduce the risk of tick-borne infections in, for example, residential yards or city green spaces. By altering the environmental conditions within designated areas or removing certain host animals or protecting them from ticks, we can significantly affect the local risk of infection by ticks.

It is, however, important to first establish the connections between different host animal species, ticks and the pathogens they carry. By identifying the most essential host animals and focusing mitigation measures on them, we may be able to minimise the problems that ticks pose to our national health in a way that will also have the least negativeimpact on the rest of the environment.