The ecology of Lyme borreliosis risk : interactions between lxodes ricinus, rodents and Borrelia burgdorferi sensu lato

The sheep tick (Ixodes ricinus) is widespread throughout Europe and can transmit Borrelia burgdorferi sensu lato (s.l.), which can cause Lyme borreliosis and B. miyamotoi, the agent of Borrelia miyamotoi disease in humans. Borrelia afzelii is the most common genospecies of Borrelia burgdorferi s.l. in western Europe and it circulates between I. ricinus and rodents. The density of infected nymphs determines disease risk and is affected by complex multi-trophic interactions between factors that are not well understood. The aim of this thesis was, therefore, to gain insight into the multi-trophic interactions that affect the density of B. afzelii-infected and B. miyamotoi-infected I. ricinus nymphs. Interactions between I. ricinus, rodents (wood mice and bank voles) and B. afzelii were reviewed to elucidate knowledge gaps concerning these interactions. The effect of rodent density on the density of infected nymphs and the factors that affect the contribution of a rodent to the density of infected nymphs were investigated in field experiments. The effects of a B. afzelii infection on larval tick burden of bank voles and nymphal body weight were investigated in the laboratory. The effects of CO2 and host odour on the host-seeking behaviour of I. ricinus were investigated in a Y-tube olfactometer. Finally, the ability of field-collected I. ricinus larvae to transmit B. afzelii and B. miyamotoi was analysed. The review showed that rodent density, rodent infection rate, and larval tick burden are the major factors affecting the density of B. afzelii-infected nymphs one year later. The field experiments showed that rodent density can positively affect the density of B. burgdorferi s.l.- infected nymphs one year later. However, the effect of rodent density was small when the density of infected nymphs was limited by the density of questing larvae in the previous year. Rodent density had, however, no effect on the density of B. miyamotoi-infected nymphs. The field studies also showed that tick burdens are 2.4 times higher on wood mice than on bank voles and that body weight of emerged nymphs was 36% higher for ticks that fed on wood mice compared to ticks that fed on bank voles. Bank voles infected with B. afzelii acquired a larger larval tick burden, resulting in a 27% increase in nymphal infection prevalence. Furthermore, infected nymphs had a 10% greater the body weight. Ixodes ricinus nymphs are activated by but not attracted to CO2, whereas they are attracted to and not activated by host odour. Ixodes ricinus larvae transmitted both B. afzelii and B. miyamotoi to rodents and it was calculated that rodents have a chance of about 25-75% of acquiring B. afzelii from larvae relative to the chance of acquiring B. afzelii from nymphs. The main conclusions of this thesis are discussed and suggestions are made for future research.