Eco-epidemiology of tick- and rodent-borne pathogens in boreal forests

Abstract

Infectious diseases are amongst the ten major causes of human mortality worldwide, 60% of them being animal-borne. Variations of abiotic and biotic conditions are likely to modify the transmission of parasites and pathogens within reservoir species, and, as a consequence, alter the zoonotic risk for human. My thesis aims at elucidating the dynamics and mechanisms of the maintenance of ticks, tick-borne pathogens (TBPs) and the Puumala hantavirus (PUUV) in the reservoir host, the bank vole (Myodes glareolus, BV). In Northern Europe, tick-borne diseases are growing in importance to human because of the latitudinal expansion of the tick Ixodes ricinus. Field monitoring revealed that I. ricinus was the only species found in the vegetation in Central Finland. The abundance of immature I. ricinus in nature was positively associated with the BV abundance. The highest risk periods for tick bites on humans were May– June and September. Ixodes ricinus was positively associated with open water coverage and human density, which might offer suitable moisture conditions and anthropogenic modifications favouring the species. The infection of BV with the zoonotic B. burgdorferi s.l. was associated with the abundance of I. ricinus at the site, indicating that this tick species was required for the transmission and persistence of this pathogen. An experiment revealed, for the first time, that B. afzelii can modify the behaviour and the breeding success of its host, and these effects are both sex- and size-specific and density-dependent. Space-state modelling of longitudinal field data revealed that PUUV infection likelihood was the lowest in BV previously infested with vectors in comparison to Anaplasma phagocytophilum infected BV, or individuals without any previous infections. Altogether, this study shows how seasonality, co-infecting pathogens and host population density influence the risk of tick-borne pathogens and the zoonotic risk in Central Finland.