Friends of the Far North Flying Foxes Inc

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The Australian Paralysis Tick
Matthew Shaw
Department of Zoology and Entomology
University of Queensland

(from the Newsletter of the Friends of the Far North Flying Foxes Inc
Edition 5, January 2000)

They seem to be waiting for you. You can't see them yet but they are there in the leaf litter amongst the thickets and rank grasses growing near the edges of moist forests. Pushing through those sticks and swards recalls earlier itchings and looking down you see bandicoot diggings; you're in tick country!

There are 68 tick species recorded from Australia, but if you are on the humid east coastal strip of mainland Australia, almost all tick bites on humans are from just one species, the Australian paralysis tick, lxodes holocyclus. Humans are one of many mammals attacked by lxodes holocyclus and birds that come to ground may also be parasitised.

In late summer and autumn, larvae or "seed" paralysis ticks cluster in the leaf litter or perhaps adjacent to low vegetation. The unlucky host that contacts them is beset by scores of tiny 0.5mm ticks. When feeding is successful, 4-6 days later, bloated larvae detach and fall onto the ground or into the nest of their host. When larva attach to humans however, feeding is usually unsuccessful and the resulting contest with the human's immune system is lost by the ticks. The irritating swellings they cause are often referred to as "scrub-itch". Note that the feeding of larval paralysis ticks, while often called scrub-itch differs from classic scrub-itch - "that maddening affliction" produced by chiggers, larval mites of the family Trombiculidae. Larval ticks are tiny (0.5mm) and pale whereas pest chiggers are minute (ca.0.3mm), bright red and easily dislodged by scratching. A hand lens is usually necessary to see the offender, if it is still present.

For the lucky larvae that manage to clamber aboard a host, this is only the first of three separate feeds needed to complete their life cycle. Most paralysis ticks die waiting for these feeding opportunities, especially at the larval stage. They are extremely sensitive to desiccation and many probably die of thirst rather than hunger. The rainfall and hence humility of an area is actually an exceedingly good predictor of paralysis tick distribution. In Queensland paralysis ticks are not found in areas of less than 1000mm annual rainfall. The intolerance to low humidities by eggs and larvae is probably primarily responsible for this limitation.

After engorgement an immature tick must detach and digest its meal, moult and then ambush another host. Each successful feed brings about a massive increase in body weight. The pinprick-sized larva feed and moult to become the autumn and winter crop of pinhead-sized (1.2mm) nymphs. By spring and early summer the adults (males 3mm and females 5mm) are at peak abundance. Interestingly, males do not feed from hosts at all. They may feed from feeding or engorged female ticks however and females have been found with up to 15 scars from male feeding.

Although paralysis ticks will at times attach to and attempt to feed upon a great many different warm-blooded animals, bandicoots seem to contribute most to paralysis tick survival, especially by supporting juvenile stages. One study in south-east Queensland showed northern brown bandicoots, Isoodon macrourus, harboured on average 500 to 2000 larvae. These figures taken together with measures of feeding success show that on average each of these bandicoots would be forwarding two hundred or more successfully fed larvae to the nymphal stage. Common brushtail possums (Trichosurus vulpecula) on the other hand were found to carry only a few larvae, but more importantly, of the larvae and nymphs that attached less than a quarter were able to engorge successfully. Adult female ticks are common on possums, scrub turkeys, wallabies and other hosts. It would be interesting to know what proportion of the paralysis tick population can be attributed to these non-bandicoot hosts.

Regardless of the life stage or host, a feeding takes at least four and up to seven days. During this time, periods of blood-feeding alternate with periods of salivary gland fluid being secreted back into the host.

The fluid that the host receives is mostly water and salts but also contains many bioactive chemicals including paralysis toxin. Any feeding stage of the tick produces paralysis toxin and the amount of toxin secreted increases as feeding progresses. Feeding adult females are capable of producing truly dangerous, even fatal, volumes of toxin and engorged, late-phase feeding females are the most dangerous of all.

It is surely paradoxical that a parasite should paralyse its benefactor. Does paralysing a host benefit the tick? No one knows but there are reasons to suspect that it might. A diverse assortment of 46 tick species around the world can paralyse and paralysis is achieved by a variety of toxins that act in different ways. One way of explaining this pattern is to postulate that paralysing ability can be a selective advantage to ticks and that this ability has arisen several times during tick evolution. Another possibility is that paralysis toxins have an unknown role in tick feeding and paralysis is a secondary effect.

Spectacled Flying FoxNative animals with regular exposure to paralysis ticks usually have some immunity to paralysis toxin. Nonetheless a long list of mammals and some birds are known to have succumbed to paralysis. In 1986 Spectacled flying foxes (Pteropus conspicillatus) were found dying mysteriously on the Atherton Tableland. In 1990 large numbers were found dying and paralysis ticks were identified as the cause. Since 1990, paralysis has been an annual occurrence and in some years the carriage has been horrific; for example in 1992 more than 1000 deaths were reported from a flying fox camp comprising only 10,000 bats. Affected bats cannot use their hind limbs to roost, hence they are found helpless on the ground, usually stricken by just one semi-engorged tick. Many of the affected bats are mothers carrying babies; tragically female paralysis tick questing coincides with bat reproduction. Paralysed or orphaned bats invariably die unless they are rescued by one of the dedicated bat carers who patrol flying fox camps on the Atherton Tableland.

Research is needed to uncover how a tree-loving mammal contacts ground-loving ticks. Either ticks are climbing higher or bats are foraging lower than realised; moreover, they must be doing it frequently to explain the large numbers of bats being parasitised. Spectacled flying foxes remain one of the least studied of the Australian flying foxes despite the widespread view that this unique rainforest specialist is in decline. This alarming phenomenon has highlighted how little is known about Spectacled flying fox and paralysis tick ecology.

More information about the Australian Paralysis Tick at a website by norbertf.