Ticks: poisonous hitchhikers
Stealthy stowaways from a day in the bush, paralysis ticks are superbly adapted to feast on their hosts.
SUE BLOOMFIELD WAS enjoying a summer day mulching in her garden on the Atherton Tableland, in far north Queensland. She loved gardening for its intimacy with earth and the feeling of being connected to nature. And for at least one part of nature, the desire to meld was mutual. Below her busy hands, a creature was poised on the edge of a leaf, and as Sue brushed past, it latched on.
For a few hours the questing creature trundled about, until it settled on Sue's forehead. There, it administered a drop of liquid, probably analgesic, tore the flesh open with two knife-edged appendages, and wedged a barbed, syringe-like feeding tube in the opening.
From giant salivary glands (the biggest of the tick's organs, which increase in mass 25-fold during feeding), it secreted clot-inhibiting enzymes into the wound to create a feeding pool of Sue's seeping blood. She didn't feel a thing; other secretions prevented inflammation - the itching and swelling would otherwise alert her. If all went well, it could be there a week or more, growing from the size of a grain of rice to that of a pea as it ingested enough protein to trigger reproduction and the creation of up to 3000 offspring.
Two Australian paralysis ticks picked off koalas in Port Macquarie, NSW. The small tick had not yet started feeding, while the other had probably been at work for a couple of days. The amount of blood inside the larger tick is probably around 5ml. (Image: Bjørn Christian Tørrissen / Wikimedia)
The blood-sucking parasite burrowing into Sue's forehead was a female Ixodes holocyclus, known as the Australian paralysis tick or shellback, indigenous to the forests and moist woodlands of eastern Australia.
But it's not her bloodlust that's the problem: we can certainly spare the tick the trickle she needs to reproduce. Rather, it's what she dribbles back in exchange that can harm. Tick saliva is a biochemist's delight - vasodilators (which dilate blood vessels), anti-clotting agents and immunosuppressants keep the blood flowing and the host's immune cells at bay. Research on overseas species has found more than 300 proteins in tick saliva, but immunologist Kevin Broady, head of the department of medical and molecular biosciences at the University of Technology, Sydney (UTS), says the compounds in Australian species have been barely studied.
Tick's saliva the real killer
At 2.30am the next morning, Sue was awoken by a slight burning sensation on her forehead, and discovered the tick. "When I touched it, it felt like poison shot to my feet," she says. "They started tingling. Then I started getting blotches all over my body." When her throat started swelling, she woke her husband. By the time they got to Cairns Base Hospital at 4am, her swollen air passages had almost closed.
What came close to killing Sue was not the tick, but anaphylactic shock brought on by her body's extreme allergic response to proteins in the tick's saliva. Medical entomologist Dr Stephen Doggett says allergic reaction is the most widespread and severe consequence of tick bites in Australia. While the majority of reactions are merely irritating - the bites of multiple tick larvae can provoke an intense allergic dermatitis - anaphylactic shock is life threatening. Elsewhere in the world, Lyme disease has a profound impact on human health. But contrary to popular perception, "there's not one good piece of scientific evidence for Lyme disease in Australia," Stephen says.
The most notorious impact of the paralysis tick gives it its name. "A tiny compact protein in its saliva interferes with transmissions from nerve to muscle cells," says Kevin, whose UTS team isolated the neurotoxin. Of more than 800 species of tick worldwide, only Ixodes holocyclus causes significant paralysis requiring medical or veterinary treatment. At least 20 Australians, mostly children, have died (from respiratory failure), although none since 1945.
Evolutionary mystery of why ticks so lethal
Its toxicity is an evolutionary mystery. "It's unusual for an ectoparasite [one external to the body] to kill its host," Kevin says. Ticks are arachnids, related to spiders and scorpions, for which there is an advantage in being able to kill.
Kevin thinks the toxin may be remnant from that evolutionary past. University of Queensland Professor Rick Atwell, who is investigating genetic variability in ticks, wonders whether it might have been advantageous when megafauna roamed Australia (more than 20,000 years ago), by causing localised paralysis and decreasing the parasite's chances of being dislodged, thus allowing it to feed.
For the tick's natural hosts, paralysis isn't usually a problem. Bandicoots (the most common host), koalas and other marsupials develop immunity through regular exposure. But less-preferred hosts, such as dogs and cats, often don't get the chance to acquire immunity. An estimated 10-20,000 are paralysed annually, with hundreds dying. Other animals are also affected, including spectacled flying-foxes, a threatened species native to north Queensland, which have died in their thousands.
Despite the tick's toll, there's little success finding a vaccine. Kevin started work on it a decade ago, but the task has proved difficult. With new techniques now available, he thinks there's potential for a vaccine to protect against ticks during the attachment stage, but funding is now a problem.
Globally, only mosquitoes spread more human and animal diseases than ticks. But the pathogens spread by Australian ticks are less dangerous than those overseas. Stephen estimates there are 100-200 cases of tick typhus - Queensland tick typhus (also known as scrub typhus) and Flinders Island spotted fever - each year caused by Rickettsia australis (an intracellular bacterial parasite).
While being part of a tick's lifecycle is unpleasant, sometimes dangerous, and for many animals the suffering is horrific. But try to spare a drop or two of sympathy for ticks. Their blood-sucking lifestyle isn't easy. Probably fewer than one in 1000 reach breeding age: they often succumb to desiccation. Good hosts are hard to find.
Giving ticks the flick
First and foremost, avoid tick habitat. Wear light-coloured clothing (so you can see them); tuck your shirt inside your pants, and pants inside socks; check your body regularly; and use repellents (20-30 per cent DEET or picaridin).
Different experts recommend different methods for removal of ticks, and there have been no studies to compare them. Health departments advise pulling ticks out with tweezers close to the skin. To minimise an allergic reaction, some specialists advise not to manipulate the tick, but spray it with a synthetic pyrethroid or dab it with Lyclear (a scabies treatment) and allow it to die and drop off. Those who may suffer a severe allergic reaction should only remove a tick under medical supervision.
This article was originally published in the Apr-Jun 2009 edition of Australian Geographic (AG#94).