Ticks: physiological aspects with implications for pathogen transmission.

Ticks have attracted a great deal of scientific attention primarily because of their role as vectors of numerous pathogens. The majority of tick researchers worldwide focus primarily on microbiological and clinical issues relating to these pathogens, and on methods (pesticidal and biological) for controlling tick populations. Unfortunately, it is often forgotten that ticks are also interesting in their own right to the general biologist because of their unusual physiological (and other) adaptations. Here I review some of these adaptations relating primarily to osmoregulation. (i) I outline their ability to take up water vapour directly from the atmosphere, an adaptation that enables them to withstand desiccation for extended periods while unfed and, in the case of larvae and nymphs, following engorgement. (ii) I present the remarkable filtration-resorption mechanism of the argasid tick coxal organ, analogous to that of the vertebrate glomerular kidney, that enables them to regulate haemolymph fluid volume and composition following the blood meal. (iii) I then turn attention to the salivary glands of female ixodid ticks, which serve the on-host osmoregulatory function in this family of ticks, (iv) and I discuss the pharmacological control of salivary fluid secretion. (v) Finally, I link the latter to the mechanism of pathogen transmission by the salivary glands, using the tick-borne Thogoto virus as a specific example.