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The Australian Paralysis Tick

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Associate Professor Kevin Broady from the Department of Medical and Molecular Biosciences at the University of Technology, Sydney puts a native parasite under the microscope.     
Of the 800 species of ticks worldwide the Australian paralysis tick is unusual in terms of the potency of its toxin. Found only on the east coast of Australia, there have been 20 human deaths recorded but most of these were back in the 1920s. While people are not immune to tick toxins, there is a time lag of several days between the bite and tick producing the toxin, allowing for discovery and removal.

Left: The Australian paralysis tick, Ixodes holocyclus attached to a human host. 
Below: Electron micrograph of the mouthparts of Ixodes holocyclus showing barbed hypostome and sensory palps.

However several human deaths do occur every few years as a result of severe allergic reactions to components of the veritable cocktail of chemicals that a tick injects into its host as it feeds. In addition, milder allergic reactions are quite widespread.
Not much is known about what causes the allergic reactions, the culprits are likely to be some of the salivary proteins injected into the host. When biting, a tick injects a wide range of biologically active substances. These include a locally acting anaesthetic, enzymes to dissolve tissue in order to form a feeding chamber so that the tick can drink the blood and lymph that leaks into this chamber. The tick must also release anticoagulants to keep the blood fluid, as well as molecules to control the host’s immune system.
With so many salivary proteins involved, there are many potential triggers of allergic reaction. Since dust mites and ticks are related, their allergens may also be similar.
The ticks have three life stages, larvae, nymphs and adults and each stage is abundant at different times of the year. Although there is some speculation about the toxic nature of the tiny larvae (often called grass ticks) and nymphs, it is the adult tick that is the most dangerous.

Most animals that produce venoms do so because they increase their chances of survival – either in catching prey or protecting them from predation. So it is curious that the totally parasitic Australian paralysis tick continues to produce a very potent toxin that is likely to kill the very host on which it is living.    
Kevin Broady suggests that ticks, which are related to spiders and scorpions, may have evolved from ancestoral forms that were were hunters and so a venom would have survival value. Nowadays, around the world, while there are a few species that remain mildly poisonous, none compares with the toxicity of the Australian species. Indeed, this is the only country where dealing with tick toxicity has spawned an entire industry. Interestingly, our native animals tend not to be adversely affected by the paralysis tick because they build up immunity in the wild through their ongoing exposure to the different life stages each season.

It is only with our domestic animals (including livestock) that even a single tick bite can be fatal. Since these animals have usually been kept naïve to ticks they have not had the opportunity to build any immunity.    
The neurotoxin was first investigated by Clunies-Ross over 70 years ago and the antivenom developed has been on the market since then. It is produced by making dogs increasingly resistant to the neurotoxin (by controlled, gradual exposure to ticks), eventually producing the blood needed for the serum.
Left: Dog with hind limb paralysis due to tick on the neck region.

Since the 1970s there have been many attempts to identify and quantify the active agents in of the venom. The toxin remained elusive, with the common finding that as the extracts were purified, suddenly a stage was reached where all activity suddenly disappeared.    
In the 1990s, at UTS, Michael Thurn, one of Kevin's doctoral students devised a groundbreaking experiment that solved the puzzle. He argued that because it was a neurotoxin, the active agent must bind to nerve tissues.

In this experiment, whole ticks were homogenised and all proteins tagged with a radioactive label. These labelled tick proteins were then mixed with synaptosomes (pinched off nerve ends) and allowed to interact at either 0oC or 37oC. Since it was expected that only a neurotoxin would bind to the nerve tissue, the mixture was first washed by centrifugation to remove any proteins that did not bind to the synaptosomes, then specialised techniques were used to separate the components and the radiolabelled proteins were detected using X-ray film. The experiment showed that while the original tick extract contains many proteins of different sizes that could be radiolabelled (lane 1 in figure), only one of these labelled proteins (about 5kDa in size) would bind to the synaptosomes (lane 3) and so is thought to be the tick neurotoxin. This binding is temperature dependent and was absent at 0oC (lane 2). 
As a result of this research, the toxin was successfully isolated and identified.

Above right: The figure shows an autoradiograph of the critical experiment that revealed that the tick neurotoxin was a small protein that bound to nerves in a temperature-dependent fashion.    
However all attempts to replicate an exact and active form of the molecule by recombinant gene technology and peptide synthesis have failed. This is because the molecule has a number of bonds that can change the orientation of different parts of the molecule and it can be folded in many different ways. Researchers have yet to crack the right folding combination that results in the active toxin that could then lead to developing a vaccine for tick venom.

All photos from Kevin Broady                                                           Text: V.B. November 2008

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