Ixodes holocyclus, commonly known as the Australian paralysis tick, is one of about 75 species in the 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 that range, Ixodes holocyclus is the tick most frequently encountered by humans and their pets. Because the same area includes Australia's most densely populated regions, 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. The natural hosts for the paralysis tick include koalas,http://www.tickalert.org.au/ntivefna.htm Tick poisoning in native Australian fauna 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 is Australian paralysis tick or simply paralysis tick. 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 Australian paralysis tick 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 New South Wales, 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; more specifically, conscutum) that 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 holocyclus, 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 holocyclus. 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 Captain William Hovell during his 1824–1825 journey with Hamilton Hume from Lake George to Port Phillip. 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".Hovell WH (1921) Journal kept on the journey from Lake George to Port Phillip, 1824–1825. The Royal Australian Historical Society Journal and Proceedings VII: 307–378.Scott E (1921) Hume and Hovell's Journey to Port Phillip. The Royal Australian Historical Society, 7: 289–380.
James Backhouse, a well-travelled Quaker during the early colonial period, gave the following account:Backhouse, Reverend James (1843) A Narrative of a visit to the Australian Colonies; Hamilton, Adams and Co Paternoster Row, York: John L Linney, Low Ousegate, 1843 (taken from Backhouse and Walker, Illawarra and Shoalhaven, 1836, Illawarra Historical Society (1991) Beale E, Mitchell W and Organ M (eds.), p. 87. "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 described until Louis Georges Neumann did so in 1899. It was further studied by Nuttal and Warburton (1911).Nuttal GHF, Warburton C, Cooper WF, Robinson LE (1908) Ticks, A Monograph of the Ixodoidea, Cambridge University Press, 1908.
By 1921, Dodd had established a definitive link between Ixodes holocyclus and clinical disease in three dogs. His findings were that it took five to six days from time of attachment for clinical signs to develop, with motor paralysis being the major neurological symptom.
The life cycle of the tick was studied chiefly by Ian Clunies Ross. Clunies Ross also demonstrated that a toxin produced by the tick, and not some infective agent carried by the tick, was responsible for the paralysis.Clunies-Ross I. (1935) Tick Paralysis: A fatal disease of dogs and other animals in Eastern Australia, J Commonwealth Scientific and Industrial Research Organisation, 18. The life cycle was further studied by Oxer and Ricardo (1942)Oxer D.T., Ricardo C.L. (1942) Notes on the biology, toxicity and breeding of Ixodes holocyclus (Neumann). Aust Vet J 18: 194. and later summarised by Seddon (1968).Seddon H.R. (1968) Diseases of Domestic Animals in Australia. Part 3: Arthropod Infestations (Ticks and Mites). Serv Publ Dep Hlth Vet Hyg 7: 68
In 1970, Roberts' work Australian TicksRoberts FHS (1970) Australian Ticks. CSIRO, Yeerongpilly, QLD gave the first comprehensive description of Australian ticks, including Ixodes holocyclus.
The first confirmed human death in Australia due to tick envenomation was reported by Cleland in 1912,Cleland J. B. (1912) Australas Med Gaz XXXII: 295 when a large, engorged tick caused flaccid paralysis in a child, progressing to asphyxiation. Headstones at the Cooktown cemetery attribute some human deaths to ticks.National Tick Paralysis Forum 1 (1999) Bulletin #1 Tick Poisoning in Dogs and Cats, Ixodes holocyclus, a unique Australian parasite. Merial Australia Limited.
In the first half of the 20th century, at least 20 human deaths were attributed to the paralysis tick. Eighty percent of the victims reported in New South Wales 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.Australia's Dangerous Creatures (1991) Australian Reader's Digest.
Anatomy, life cycle and behavior
Overview The life cycle of Ixodes holocyclus consists of four stages: egg, larva, nymph, adult. Ixodes holocyclus requires three hosts to complete its life cycle, thus it is a "three-host tick". Ticks hatch as six-legged larvae after an incubation period of 40 to 60 days. Larvae search for a blood meal from a host, feed for four to six days, then drop from the host and moult to become an eight-legged nymph. Nymphs require a second blood meal before they can moult again to become an adult. Female adults then require a further blood meal of up to ten days before dropping off to lay up to 3000 eggs in leaf litter. Male adults will search for females on the host for mating, and to parasitise the females for blood meals. That life cycle takes around a year to complete, with minimum being 135 days, and the maximum 437 days.
To find a host, ticks "quest", climbing onto vegetation and waving forelegs slowly until a host comes within reach. When on the host, they may not attack immediately, but wander for up to two hours until attaching on the back of the host's head or behind an ear. Certain chemicals, such as carbon dioxide (hence the use of "dry ice" baits), as well as heat and movement, serve as stimuli during the questing behaviour.
Both female and male ticks quest for a host, but for different reasons. The female quests for a blood meal, the male to search the host for a female tick in order to mate and feed from her. Males may parasitise female ticks by piercing their cuticles with their mouth parts to feed on the haemolymph, and up to four males have been found feeding on one female tick. Adult male ticks rarely blood-feed on a host. The outside surface, or cuticle, of hard ticks grows to accommodate the large volume of blood ingested, which, in adult ticks, may be anywhere from 200 to 600 times their unfed body weight.Sonenshine, D.E.: Biology of Ticks, 2 volumes: Oxford University Press, New York, Oxford, 1991. When a tick is fully engorged it is said to be replete.
Egg
Adult females lay between 2000 and 6000 eggs in leaf and branch litter, under the scaly or fibrous bark of certain trees and shrubs, or in dense fine foliage near the tips of branches. They use a wax-like substance to make a cohesive mass of eggs and attach them at the selected site. After 40–110 days incubation, a small fraction of the eggs survive and hatch to larvae. Development occurs with suitable warmth and high humidity (e.g. in moist leaf litter).
Larva
Larvae, also known as "seed ticks" and sometimes "grass ticks", emerge from the eggs and move towards lateral branches, or across grassy areas during humid weather, to find and attach to their hosts. Larvae undergo 7–44 days of hardening and then climb vegetation (e.g. to the tips of leaves), from where they attach to a passing host. Larvae feed for 4 to 6 days then drop to the ground. Over a further 19- to 41-day period, larvae moult to become nymphs. The overall period in the larval stage, from hatch to moult, is temperature dependent. It takes about 20 days at 27.5 °C and 40 days at 21 °C, but may extend to 36 weeks. Larvae are just visible to the naked eye. Under laboratory conditions unfed larvae can survive for 162 days.
Ixodes holocyclus larva: a, capitulum (dorsal view); b, scutum; c, hypostome; d, tarsus I; e, tarsus IV; f, coxae
Diagnosis: Capitulum with slender palpi, hypostome rounded apically, dentition 2/2; scutum about as long as wide, with faint lateral carinae; all coxae with small, external spurs. Body: Broadly oval, 0.5 × 0.4 mm (unfed) to 1.15 × 1.0 mm (engorged)