INTRODUCTION

Saliva-assisted transmission (SAT) is the indirect promotion of arthropod-borne pathogen transmission via the actions of arthropod saliva molecules on the vertebrate host. This phenomenon has been reported for most blood-feeding arthropods that transmit disease causing agents via their saliva, but the greatest number of examples has been recorded in ticks. The skin site where ticks feed is highly modified by the pharmacologically active molecules secreted in tick saliva. For pathogens, it is an ecologically privileged niche they can exploit. Here we review evidence for SAT and consider candidates for SAT factors among the tick pharmacopoeia of antihaemostatic, anti-inflammatory and immunomodulatory molecules. SAT factors appear to differ for different pathogens and tick vector species, and possibly even depend on the vertebrate host species on which a tick feeds. Most probably, SAT is mediated by a suite of molecules that act together to overcome the redundancy in host response mechanisms. The quest to identify the tick molecules that mediate SAT is an exciting one, offering new insights into host inflammatory and immune mechanisms, and novel ways of controlling ticks and tick-borne diseases.

TICK–HOST–PATHOGEN INTERACTIONS

The relationships between tick-borne pathogens, their tick vectors and diverse vertebrate hosts, can be represented by a triangle of parasitic interactions (Fig. 10). The interactions are between (i) pathogen–tick, (ii) pathogen–host and (iii) tick–host. In (i) the pathogen interacts with its vector, infecting and replicating within tick cells or extracellular spaces (including those of the gut, haemocoel and salivary glands).