While most animals have more than enough blood to spare a drop or two for a famished tick, it's the unpleasant diseases that these small arachnids transmit that engender the disgust that they inspire. ‘We are generally interested in understanding tick populations’, says Andrew Rosendale from the University of Cincinnati, USA, who is keen to learn more about the tick's vulnerabilities in a bid to reduce the incidence of tick-borne diseases. As ticks spend most of their time lurking in the humid undergrowth and only emerge to perch on the end of a twig or blade of grass when waiting for a passing victim, Rosendale explains that they are mainly at risk of dehydration when on the hunt for a victim. So, in a bid to understand how these pests protect themselves from desiccation, Rosendale, Megan Dunlevy and Joshua Benoit began investigating which metabolic processes ticks mobilise when water is scarce.
Analysing the 497 tick genes that changed expression pattern when the pests experienced dehydration, Rosendale identified protective genes such as glutathione S-transferase, which protects cells from oxidative damage; heat shock protein 70, which protects damaged proteins; as well as genes that alter cell structure. ‘All three of those gene types would serve to limit damage from dehydration’, says Rosendale. He then teamed up with Lindsey Romick-Rosendale and Miki Watanabe to find out whether the ticks were producing any molecules that might protect them from desiccation, although he admits that identifying the metabolites produced by the dehydrated arachnids was tricky. ‘Lindsey and Miki primarily work with human samples, so processing the small samples [from the ticks] and calibrating the NMR machine to obtain good data was a challenge’, recalls Rosendale. However, their patience was eventually rewarded when the team realised that the ticks were producing GABA and glycerol; although when these molecules were injected into the ticks they did not reduce the animal's water losses in dry conditions. Instead, glycerol and GABA seemed to improve the tick's ability to absorb water from the air when their reserves were low.
‘Although ticks change their physiology to try and defend against the negative consequences of dehydration, they are not particularly resistant to dehydration,’ says Rosendale, adding, ‘Much of their response is likely to limit water loss when possible and to facilitate the reabsorption of water then they can return to a more humid microclimate’. And Rosendale is now keen to find out how the blood-sucking pests fuel the mechanisms that protect them from dehydration to get a better handle on strategies that could help us to avoid their unpleasant attentions.
