Ixodes holocyclus, commonly known as the Australian paralysis tick, is one of about 75 species of Australian tick fauna and is considered the most medically important. It can cause paralysis by injecting neurotoxins into its host. It is usually found in a 20-kilometre wide band following the eastern coastline of Australia. Within this range Ixodes holocyclus is the tick most frequently encountered by humans and their pets. As this area also contains the majority of Australia's most densely populated regions, incidents of bites on people, pets and livestock are relatively common.
Paralysis ticks are found in many types of habitat particularly areas of high rainfall such as wet sclerophyll forest and temperate rainforest.[1] The natural hosts for the paralysis tick include koalas,[2] bandicoots, possums and kangaroos.
Common names[]
The use of common names has led to many colloquial expressions for Ixodes holocyclus. The most generally accepted name used within Australia is Australian paralysis tick or simply paralysis tick.[3] The following table gives some of the other names used to describe various stages of Ixodes holocyclus. Many of these common names, such as dog tick or bush tick, are best not used for Ixodes holocyclus because they are also used for some of the other ticks found in Australia.
| Used (and misused) common names for Ixodes holocyclus | Life stage/gender referred to | Comments |
|---|---|---|
| Paralysis tick of Australia | All stages | The preferred common name for Ixodes holocyclus. There are other ticks around the world also causing paralysis. |
| Scrub tick | Adult female, Adult male | In Queensland, scrub tick is also used for Haemaphysalis longicornis. |
| Bush tick | Adult female, Adult male | Throughout Australia, bush tick is also used for Haemaphysalis longicornis. |
| Dog tick | Adult female, Adult male | In NSW, dog tick is more correctly used for Rhipicephalus sanguineus (the Brown Dog Tick). |
| Wattle tick | Adult female, Adult male | Wattle tick was used by pioneers in the Illawarra region of NSW to describe the tick causing paralysis, especially in sheep. |
| Common hardback tick | Adult female, Adult male | Common hardback tick was used in The Northern Herald, Sydney (August 1996). This expression perhaps emphasises that Ixodes holocyclus is indeed a 'hard tick' and that it is also the most common tick encountered by humans and animals in the Sydney region. |
| Bottle tick or blue bottle tick | Adult female | Bottle tick describes that the engorging tick becomes swollen with fluid (the host's blood). The addition of 'blue' probably refers to a bluish hue associated with the mid-sized engorged female. It also sounds like another venomous animal, the marine stinger of the same name, the 'blue bottle' or Portuguese Man o' War. |
| Shell back tick | Adult male | Shell back tick describes the tortoise-shell appearance of the large shield (scutum) which covers the entire dorsum of the adult male. |
| Grass tick | Nymph and Larva | The term grass tick is usually used to refer to the smaller stages of Ixodes holoyclus but the term delivers little useful information because any tick can be found in the grass. |
| Seed tick | Larva | The term seed tick usually is used to refer to the smallest stage of Ixodes holoyclus. |
| Shower tick | Larva | The term shower tick presumably refers to how humans can become seemingly showered by hundreds of larvae at a time – this is because they have hatched from a single cluster of eggs (thousands) which have not yet been distributed by the first of three hosts. |
| Scrub itch tick | Larva | The term scrub itch tick is used in Queensland to describe the larvae of Ixodes holocyclus, which often infest humans and animals in huge numbers, causing a rash. Without careful inspection, the presence of the tiny larval ticks may be missed until they engorge to an appreciable size. |
Early scientific history[]
One of the earliest Australian references to ticks as a problem in human disease is found in the journal kept by Capt William Hilton Howell for his 1824-1825 journey from Lake George to Port Phillip. In this he remarked on "the small insect called the tick, which buries itself in the flesh, and would in the end destroy either man or beast if not removed in time" [4][5]
James Backhouse, a well-travelled Quaker of the early colonial period, gives the following account:[6] "At Colongatta, in Shoal Haven...district, which, like that of Illawarra, is much more favorable for the grazing of horned cattle than for sheep. Among the enemies of the latter in these rich, coast lands, is the Wattle Tick, a hard flat insect of a dark colour, about the tenth of an inch in diameter, and nearly circular, in the body; it insinuates itself beneath the skin, and destroys, not only sheep, but sometimes foals and calves. Paralysis of the hind quarters often precedes death in these cases. Sometimes it occasions painful swellings, when forcibly removed from the human body, after having fixed its anchor-like head and appendages in the skin. To prevent this inconvenience, we several times, made them let go their hold, by smearing them over with oil, or with wet tobacco ashes."
Whilst pioneering settlers knew that ticks posed a threat to their dogs and perhaps to themselves, the paralysis tick was not scientifically identified until 1899 (by Neumann[7]). It was further studied by Nuttal and Warburton (1911).[8]
By 1921 Dodd had established a definitive link between Ixodes holocyclus and clinical disease in three dogs. His findings were that it took 5 to 6 days from time of attachment for clinical signs to develop, with motor paralysis being the major neurological deficit.
The life cycle was studied chiefly by Clunies-Ross (1924).[9] Ian Clunies Ross also demonstrated that a toxin produced by the tick was responsible for the paralysis and not some infective agent carried by the ticks.[10][11] The lifecycle was further studied by Oxer and Ricardo (1942)[12] and later summarised by Seddon (1968).[13]
In 1970 Roberts' work Australian Ticks [14] gave the first comprehensive description of Australian ticks, including Ixodes holocyclus.
The first confirmed human death due to tick venoming in Australia was reported by Cleland in 1912 [15] when a large engorged tick caused flaccid paralysis in a child, progressing to asphyxiation. Headstones at the Cooktown cemetery apparently reveal how some human deaths were attributed to ticks.[16]
In the first half of the 20th century at least 20 human deaths had been attributed to the paralysis tick. Eighty percent of the victims reported in NSW between 1904 and 1945 were children aged under four years. Many cases of 'infantile paralysis' (later known as poliomyelitis) may well have been misdiagnosed and actually been cases of tick paralysis.