Seasonality of Ixodes ricinus and Ixodes trianguliceps tick on the bank vole (Myodes glareolus) and on vegetation in Central Finland

Many infectious diseases are zoonoses, that is, they can be transmitted between vertebrate animals and humans. More than a fifth of zoonotic pathogens are transferred from one host to another by vectors, which are often blood-sucking arthropods, such as ticks. In Europe, Lyme borreliosis, spread by Ixodes ricinus tick, is the most prevalent tick-borne zoonosis. I. ricinus spreads also other zoonotic pathogens, such as tick-borne encephalitis virus (TBEV), Anaplasma phagocytophilum bacterium and Babesia microti protozoan. Pathogen transmission between ticks occurs as a susceptible tick feeds on an infected host who has become infected while feeding an infected tick. In some cases ticks must co-feed on a host to enable pathogen circulation, why the synchronous seasonal activity of different tick instars is essential. Wild rodents are reservoir hosts for many tick-borne pathogens, having an important role in the circulation of enzootic tick-borne infections. I. trianguliceps is a tick species that parasitizes only small mammals and lives in their burrows. It can maintain infections within rodent-tick system in the absence of other tick species. I. ricinus, instead, is a generalist species, which searches for a host on vegetation, i.e. quests, and may transfer pathogens e.g. from rodents to humans. In order to understand the circulation of tick-borne pathogens in tick populations and the risks they may cause to humans, it is important to examine the seasonal relationship between ticks and rodents. Very little is known about the population dynamics of ticks in Finland, where climate is strongly seasonal and the abundance of several rodent species varies cyclically. Therefore, my main purpose was to study the seasonal dynamics (between May–September) of I. ricinus and I. trianguliceps on a common host species, the bank vole, and on vegetation in urban and non-urban forests in Central Finland. In addition, I studied other factors that might affect the tick burden of bank voles. I. ricinus was the most common in early summer when larvae and nymphs had the activity peak. I. ricinus was abundant at urban sites, whereas at non-urban sites it was rarely found. I. trianguliceps, instead, infested voles at all sites. I also found that vole characteristics (sex and age) affected its infestation load, which may suggest that certain individuals harbour the majority of ticks and thus, facilitate the transmission of pathogens. I conclude that humans have the greatest risk to become in contact with ticks in early summer when I. ricinus is common on vegetation. Due to the seasonal synchrony of I. ricinus larvae and nymphs, the environmental conditions might be favourable to maintain TBEV in Central Finland, which currently occurs mainly in coastal areas in Finland.