THE ECOLOGY OF TICKS TRANSMITTING LYME-BORRELIOSIS

The main vectors of Borrelia burgdorferi sensu lato, the cause of Lyme borreliosis, are ixodid ticks of the Ixodes persulcatus species compl ex. These ticks, which occur throughout the northern temperate zone, h ave very similar life cycles and ecological requirements. All are thre e-host ticks, with the immature stages mainly parasitizing small to me dium-sized mammals and birds and the adult females parasitizing large mammals such as deer, cattle, sheep and hares. The host-seeking stages show a distinct seasonality, which is regulated by diapause mechanism s and there appear to be major differences in this respect between the Old World and New World species. Most cases of human borreliosis are transmitted in the summer by the nymphal stages, with the exception of the Eurasian species, I. persulcatus, in which the adult females are mainly responsible. The ticks acquire the spirochaetes from a wide var iety of mammals and birds but large mammals do not seem to be infectiv e, so that ticks that feed almost exclusively on large mammals, for ex ample in some agricultural habitats, are rarely infected. The greatest tick infection prevalences occur in deciduous woodland harbouring a d iverse mix of host species and the diversity of the different genospec ies of B. burgdorferi s.l. is also greatest in such habitats. There is evidence that these genospecies have different host predilections but , apart from the fact that I. persulcatus does not seem to be infected by B. burgdorferi sensu stricto, they do not seem to be adapted to di fferent tick strains or species. (C) 1998 Chapman & Hall Ltd.