Public defence: Andrea Cotes Perdomo

Andrea Cotes Perdomo is defending her thesis for the degree philosophiae doctor (PhD) at the University of South-Eastern Norway. 

She has pursued the PhD program in Ecology at the Faculty of Technology, Natural Sciences and Maritime Sciences, Department of Natural Sciences and Environmental Health.

You are invited to follow the trial lecture and the public defence.

• Link to the dissertation will come.

Summary

The occurrence of tick transmitted diseases is increasing, and their distributions are shifting northwards all over the world. In northern Europe, tick-borne encephalitis and Lyme disease are common examples of this. Under climate change, understanding the behaviour and distribution of ticks and the pathogens they transmit is of high importance.

A long-term monitoring of the activity of the castor-bean tick, Ixodes ricinus (the most common species in Norway), and the prevalence of three pathogens was carried out in Agder and Vestfold. The number of ticks one can find is highly variable from year to year and season to season, so it is the prevalence of tick-borne encephalitis virus (TBEV), Borrelia burgdorferi (the bacteria that cause Lyme disease) and Neoehrlichia mikurensis (an opportunistic bacterial pathogen that causes neoehrlichiosis).

Ticks in southern Norway presented a two-peaked activity pattern from 2017 to 2024. In some years the main peak of activity appears in the spring or early-summer, while other years in late-summer or autumn. The reasons for these are unclear, but it was evident that more ticks are active when the weather is warm and humid. In the long term, they seem to be favoured by medium levels of precipitation during the year and snow coverage in the winter. However, the risk of transmission of tick-borne pathogens not only depends on the number of active ticks, but also on the number of infected ones.

Ticks have three active life stages, larvae, nymphs and adults. From them, adults and nymphs are normally the ones transmitting pathogens, especially nymphs that are more numerous and smaller than adults. From 2017 to 2022, we found more nymphs infected with TBEV in the late-summer and autumn and more adults infected during the spring and early-summer. Adults were around six times more likely to be infected than nymphs with the virus. B. burgdorferi and N. mikurensis were only studied during 2020. There was more chance of finding an infected tick with B. burgdorferi in the spring and with N. mikurensis in the autumn.

Surveillance on tick-borne pathogens is expensive and time consuming, relying highly on the standardization of lab protocols, including the purchase of expensive manufactured reagents. For that reason, we compared the performance of cheap and simple DNA extraction methods with commercial ones, finding similar results from both groups. This can make surveillance of ticks and tick-borne bacteria cheaper and easier in a moment when it is critical.

Finally, in Nordland in 2023, we found for the first time individuals of the Taiga tick, Ixodes persulcatus, a tick capable of transmitting more dangerous strains of TBEV and a more efficient vector of B. burgdorferi than I. ricinus. The Taiga tick has been expanding its distribution westwards and northwards for decades. However, in 2024 and 2025, only I. ricinus was found. This could mean that I. persulcatus is not yet stablished, but it is important to keep looking for it and any other species that may arrive.