A | B | C | D | E | F | G | H | CH | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
| Ixodes holocyclus | |
|---|---|
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| Ixodes holocyclus before and after feeding | |
Scientific classification 
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Class: | Arachnida |
| Order: | Ixodida |
| Family: | Ixodidae |
| Genus: | Ixodes |
| Species: | I. holocyclus
|
| Binomial name | |
| Ixodes holocyclus | |
| |
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.[2] The natural hosts for the paralysis tick include koalas,[3] bandicoots, possums and kangaroos.[2]
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.[4] 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.[4]
| 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".[5][6]
James Backhouse, a well-travelled Quaker during the early colonial period, gave the following account:[7] "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.[1]). 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 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.[9] 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.[10][11] The life cycle 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 in Australia due to tick envenomation 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 attribute some human deaths to ticks.[16]
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.[17]
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.[2]
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.[2] 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.[18] 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) Capitulum: About 0.2 mm in length, basis triangular, about 0.16 mm wide, palpi elongate and slender. Hypostome apically rounded, 0.14 mm in length, dentition 2/2 of 10–12 teeth, the teeth of the inner file blunt and small, some minute denticles apically. Scutum: About as long as broad, 0.31 by 0.32 mm and widest a little anterior to mid-length; lateral carinae present but faint; anterolateral margins usually convex and posterolateral margins concave; cervical grooves short but well defined. Anal grooves: Ill-defined anteriorly and do not converge behind
Legs: Coxae with small external spurs; tarsus I 0.14 mm in length, tarsus IV 0.14 mm in length[14]
Nymph
Nymphs are very active and attach to another host on average 5–6 days (but possibly up to 31 days) after moulting. Nymphs feed for 4–7 days, then drop to the ground. After a further 3–11 weeks the nymphs moult to become adult males or females. Again the period is temperature dependent: it takes about 20 days at 24–27 °C but 53–65 days at 10–21 °C. Dry conditions also prolong this period or even kill nymphs. Under laboratory conditions unfed nymphs can survive for 275 days.
Ixodes holocyclus nymph: a, capitulum (dorsal view); b, capitulum (ventral view); c, scutum; d, spiracular plate; e, sternal plate; f, hypostome; g, coxae; h, tarsus I; i, tarsus IV
Diagnosis (male): Capitulum as in female, hypostome dentition mainly 2/2, 3/3 distally; conscutum about as long as wide with lateral carinae; sternal plate present, oval; anal grooves converging posteriorly but remaining narrowly open; legs as in female. Body: Oval with fine parallel striae and some scattered pale hairs; 1.2 × 0.85 mm (unfed) to 3.5 × 2.5 mm (engorged); marginal grooves well developed and complete in unfed specimens. Capitulum: Length 0.40–0.43 mm. Basis dorsally 0.23–0.25 mm in width; posterior margin straight; posterolateral angles not salient; auriculae well defined. Palps as in female; articles 2 and 3 0.30–0.32 mm in length. Hypostome lanceolate and bluntly pointed; dentition 2/2, 3/3 distally. Scutum: About as wide as long, 0.61 × 0.63 mm – 0.71 × 0.70 mm. Lateral carinae well developed. Punctations few, shallow, scattered. Cervical grooves apparent, continuing to mid-scutal region as superficial depressions. Sternal plate: Oval, 0.27–0.30 mm in length and a little more than twice as long as wide. Spiracular plate: Subcircular, greatest diameter about 0.14 mm. Legs: Coxae armed as in female. Tarsus I tapering gradually, other tarsi more abruptly; length of tarsi I and IV about 0.28 mm.[14][19]
Adult female
The newly moulted adult female becomes increasingly active during the first 6 or 7 days and seeks a host. It attaches to the final host after 7 to 9 days (but possibly up to 77 days). After insemination by a male tick, or sometimes before, a female feeds on blood to gain the nutrients for the egg mass to mature. Adult females engorge for a period of 6–30 days, the time again being temperature dependent. (The 30-day engorgement time is derived from laboratory culture colonies.) Under natural conditions, the time taken for an adult female to engorge while on the host varies from 6 to 21 days, the period being longest in cool weather. When replete, the adult female drops off the host to the ground. After 11–20 days the gravid female starts to lay a batch of 2000 to 6000 eggs (20–200 eggs per day over 16–34 days) into moist vegetation such as leaf and branch litter, under the bark of trees and shrubs, or in foliage near the tips of branches. The eggs are attached as a mass with a wax-like substance. The female tick dies one or two days after egg-laying is complete. Under laboratory conditions, female ticks have been kept alive for more than two years.
Due to the variation in time taken for the female to engorge, a host may carry a tick for up to three weeks without the tick becoming significantly engorged or causing paralysis. However, in warm weather, the female engorges rapidly and, at the same time, injects her toxin into the host, thus causing paralysis if the host is susceptible. The adult female does not usually inject detectable amounts of toxin until the third or fourth day of attachment to the host, with peak amounts being injected on days five and six.
Diagnosis: A very large tick when fully engorged; scutum about as long as broad and broadest a little posterior to mid length, with strong lateral carinae; capitulum relatively long porose areas deep, cornua usually absent, but when present at most only mild and rounded; auriculae present; hypostome lanceolate, dentition mainly 3/3; no sternal plate; anal grooves meeting at a point behind; all coxae with an external spur decreasing in size posteriorly; trochanters III and IV usually with small, pointed ventral spurs. Body: Unfed specimens, oval, flat, yellowish, 2.6 × 1.1 mm – 3.8 × 2.6 mm; marginal groove well developed and continuous; hairs small, scattered, most numerous in region of marginal fold. Semi-engorged specimens frequently with body widest behind coxa IV and with a waist at level of spiracles. Fully engorged specimens broadly oval, attaining 13.2 × 10.2 mm, living ticks with blue-grey alloscutum, the dorsum light in colour, a dark band in region of marginal groove. Capitulum: Length 1.00–1.035 mm. Basis dorsally 0.60–0.68 mm in width, the lateral submarginal fields swollen and frequently delimited from the depressed, median field by ill-defined carinae; posterior margin sinuous, posterolateral angles swollen, sometimes mildly salient; porose areas large, deep subcircular or oval, the longer axis directed anteriorly, interval frequently depressed, at most about the width of one; basis ventrally with posterior margin rounded and with well-defined, blunt, retrograde auriculae. Palps long and slender, some long hairs ventrally; article I rounded and somewhat salient laterally, inner 'ring' with dorsal tongue-like prolongation and ventrally semicircular and plate-like, the posterior margin of the plate extending beyond the palp; articles 2 and 3 with no apparent suture, 0.75–0.85 mm in length and about four times as long as wide, narrowly rounded distally. Hypostome lanceolate and bluntly pointed; dentition mainly 3/3, the innermost file of small, spaced teeth, basally 2/2. Scutum: As wide as or a little wider than long, widest a little posterior to mid length, 1.6 × 1.7 mm – 2.4 × 2.4 mm, flat medianly, convex external to the long, strong lateral carinae; anterolateral margins practically straight, posterolateral margins mildly concave; posterior anle broadly rounded. Punctations numerous, fine, sometimes a little coarser medianly and laterally, shallow rugae frequently present posteriorly. Cervical grooves well defined but short. Emargination moderate. Scapulae blunt. Genital aperture: On a level with coxa IV, but in engorged specimens sometimes just posterior to this level. Anal grooves: Rounded anteriorly, curving behind anus to meet in a somewhat elongate point. Spiracular plate: Subcircular, greatest dimension 0.40–0.45 mm. Legs: Coxae smooth, I and II sometimes with mild rounded ridges externally, each with a row of long hairs posteriorly and an external spur, longer and more pointed than in male, and decreasing in size posteriorly. Trochanter IV (and sometimes III) frequently with a small, ventral spur. Tarsi tapering a little abruptly; length of tarsus I 0.70–0.80 mm, and of tarsus IV 0.60–0.78 mm.[14]
Adult male
The newly moulted male seeks a host. Male ticks do not engorge from the host, they wander the host searching for unfertilised females with which to mate. The male dies after fertilising the female, although some males can endure for longer if they parasitise the engorging female.
Diagnosis: Body measurements less than 3.0 × 2.5 mm; lateral grooves completely encircling conscutum, no lateral carinae; punctuations fine; basis capituli punctate dorsally, palpi short and very broad; hypostome dentition 2/2, with rounded teeth; anal plate bluntly pointed behind; adanal plate curving inwardly to a point; coxae with well-defined spurs decreasing in size posteriorly; trochanters III and IV frequently with small, ventral spurs. Body: Oval, sometimes broadly so, 1.9 × 1.6 mm – 3.2 × 2.3 mm; marginal body fold narrow but prominent; hairs dorsally sparse medianly, more numerous on marginal body fold. Capitulum: Length 0.51–0.65 mm in width, surface punctate; posterior margin straight; no cornua; posterolateal margins slightly divergent anteriorly; basis ventrally narrowing to the straight posterior margin, surface with a short anterolateral ridge. Palps short and broad; article 1 rounded and a little salient laterally, ventrally with a transverse rounded flange continuous with ridge on basis; articles 2 and 3 with no apparent suture, 0.33–0.40 mm in length, almost twice as long as broad, rounded distally, hairs moderate in number, some long hairs ventrally. Hypostome short and broad, 0.25–0.28 mm in length, narrowing and shallowly rounded distally; dentition 2/2 of large rounded teeth, some small teeth distally and crenulations basally. Scutum: Oval, convex, only a little smaller than body. Lateral grooves deep and completely encircling the scutum, anteriorly somewhat linear and may simulate mild lateral carinae. Punctations fine, usually most numerous submarginally and anteromedianly; pseudoscutum sometimes faintly apparent. Cervical grooves short, shallow. Emargination moderate. Scapulae blunt. Genital aperture: On a level with anterior margin of coxa III, sometimes at level of second intercoxal space. Ventral plates: Pregenital plate wider than long; median plate 1.5–1.2 mm, the width posteriorly about 3/4 of the length; anal plate 0.75–0.50 mm, anterior margin straight or mildly curved, pointed posteriorly; adanal plates curving to points near the point of the anal plate; plates with scattered punctuations and hairs. Spiracular plate: Elongate, oval, narrow posteriorly, the longer axis directed anteriorly, about 0.50–0.53 mm in length. Legs: Length moderate. Coxae practically contiguous, with a row of long hairs near posterior margin; posterointernal angles of coxae I and II may be somewhat sharp but not salient; all coxae with an external spur, strongest and bluntly pointed on coxa I, smallest on coxa IV. Trochanters III and IV with a small, dark ventral spur, only a tuberosity on II. Tarsi ending somewhat abruptly; length of tarsus I 0.65–0.71 mm and of tarsus IV 0.62–0.70 mm.[14]
Distinguishing Ixodes holocyclus from other Australian ticks
The two features which are most easily recognised and which are characteristic of Ixodes holocyclus:
- The first and last pairs of legs are distinctly darker than the two middle pairs of legs
- The anal groove forms a complete, though somewhat pear-shaped, oval around the anus (this feature gives the tick its species name holocyclus, meaning 'complete circle')
Other ticks which commonly need to be differentiated from Ixodes holocyclus include Rhipicephalus sanguineus (brown dog tick), Haemaphysalis longicornis (bush tick) and Rhipicephalus microplus (cattle tick).
A hospital medical entomology department can provides professional identification of ticks and other medically important insects.
Hosts
Common hosts include long-nosed bandicoots (Parameles nasuta), giant brindle bandicoots (Isoodon torosus), echidnas and possums. Potential hosts include many species of mammals, birds and, occasionally, reptiles. Because of continuous infestation, native animals are usually immune to tick toxins.
Most mammals, such as cattle, sheep, goats, horses, pigs, cats, cavies, rats, mice and humans, can be infested by the tick. Fatalities resulting from a single engorged adult female tick are mostly reported in the young animals of the larger species, and all ages and sizes of the pet species (dogs and cats). Larvae and nymphs can also produce toxic reactions in the host. Fifty larvae or five nymphs can kill a 40 g rat. Larger numbers of either can induce paralysis in dogs and cats. They may be easy to find on short-haired animals, but difficult to find on long-haired animals, such as Persian cats. People who live in a tick-prone area, and whose pets are allowed outdoors, are recommended to inspect their pets every day for ticks. Unfortunately, the ticks are often not discovered until they are large enough to be felt. By that time the tick has subjected the animal to a large amount of toxin. One adult female can produce enough toxin during engorgement to kill four rats. Although it is not typical, an engorging adult female has been known to re-attach several times to different dogs.[20]
Natural predators
Natural predators of I. holocyclus include insectivorous birds and wasps.[21]
Seasonality
Humid conditions are essential for survival of the paralysis tick. Dry conditions, relatively high temperatures (32 °C) and low temperatures (7 °C) are lethal for all stages after a few days. An ambient temperature of 27 °C and high relative humidity is thought to be optimal for rapid development.[22] Very dry or very wet conditions are not ideal.
The tick population in a given year is probably governed by the rainfall in the previous year if temperature variation has been moderate. North-easterly weather, with moisture-laden sea breezes on the east coast of the continent, provides ideal conditions. This pattern usually sets in during spring and early summer. Tick envenomation in animals (especially pets) peaks in spring to mid-summer.
Typically, larvae appear late February to April or May, followed by nymphs from March to September or October and then gradually the adult population from August to February, peaking around December. Under favourable conditions a second cycle may result in a second peak in May. However, infestation by adults can occur at any time of the year when conditions are suitable, even in mid-winter. Only in the very hot summer months are they difficult to find.
Small numbers of each instar are present throughout the year, with the numbers in each life cycle segment rising to plateau levels at particular times and then falling away again. Winter occurrence of gravids varied from nil to very small numbers in two of eight years studied.[23]
Relative sizes
Comparison of non-engorged ticks