INTRODUCTION: THE IMPACT OF TICK ECOLOGY ON PATHOGEN TRANSMISSION DYNAMICS

The ecology of ticks, the outcome of their interactions with their natural environment, is fundamental to the spatial and temporal variation in the risk of infection by tick-borne pathogens. Due to the biology of ticks as blood-feeding parasites, their physical environment includes the host itself. This biotic environment reacts to the tick's presence in both the short and the long term in ways that the abiotic environment cannot do, imposing physiological, population and evolutionary pressures on ticks. Ticks, however, are only intermittent parasites, spending the greater part of their life cycle free within their habitat where they are at the mercy of abiotic factors such as habitat structure and climate. They take only one (ixodid ticks) or a few (argasid ticks) very large blood meals per life stage, as larvae, nymphs and adults, then develop to the next stage, which takes weeks, months or even years, depending on the ambient temperature. This inter-stadial period is usually passed off-host, although the relatively few two- and one-host ticks (e.g. Hyalomma anatolicum excavatum and Rhipicephalus (Boophilus) microplus, respectively) remain on the host for one or both of the inter-stadial periods. For simplicity, and because generally less is known about the ecology of argasid ticks, what follows will refer almost exclusively to ixodid ticks.