Australian Paralysis Tick

Appearance

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, 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. When a tick is fully engorged it is said to be "replete".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, 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. When a tick is fully engorged it is said to be "replete".

Naming

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.

Distribution

"Ixodes holocyclus" is found mainly along coastal eastern Australia, from near Cooktown in far north Queensland to Lakes Entrance in Victoria. In places, it is found more than 100 km inland, particularly in areas of moist escarpments and ranges such as the Bunya Mountains of Queensland and the Lower Blue Mountains of New South Wales.

Habitat

The distribution map gives only an approximation of the tick's distribution as the situation is not static. There are reports from inland Victoria, including north-eastern suburbs of Melbourne. This may reflect general movement of insects and ticks from equatorial to polar latitudes which in turn may be related to climate change.

The need for humid conditions largely determines the botanical niche of this species. Low, leafy vegetation provides higher humidity levels by reducing the desiccating effects of wind and direct sunlight. This environment also suits the tick's major host, the bandicoot, as it seeks out grubs and worms in the leaf litter. Certain vegetation may be conducive to paralysis ticks.

There are mixed reports about whether paralysis ticks climb trees. Some references mention ticks climbing to the tops of trees, while others report that they stay within 50 cm of the ground.

Reproduction

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, 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; 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 to 1.15 × 1.0 mm
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 lengthAttachment of a few larvae to a non-sensitised host provokes little or no response, even though the contact is prolonged over several days. However, towards the end of feeding some itching, redness and localised swelling may develop at the site of attachment.

Repeated infestation with larvae, as occurs in rural and wooded suburban areas where bandicoots are common, rapidly leads to the development of hypersensitivity. Dramatic local redness, numbness, swelling and itching may develop within 2–3 hours of attachment of even one larva if a person has been sensitised by a previous bite. In this case attachment of the larvae leads to formation of intra-epidermal blebs and eventually blisters. These eventually rupture and the larvae detach and are lost. A tick embedded over an eyelid may result in gross facial and neck swelling within a few hours. The person can go on to develop tracheopharyngeal compression within 5–6 hours after the first onset of symptoms.

During damp summers any disturbance of taller plants can produce a shower of tick larvae. When large numbers of larvae attach to a sensitized person, severe allergic dermatitis may result. Such outbreaks are seasonal in southeast Queensland and occur most commonly during January, February and March when larval populations are at their peak. Dermatitis is most commonly encountered in rural workers.

Food

Ticks generally are obligate blood feeders. Active stages of "Ixodes holocyclus" require blood as a source of nutrition. Adults also require the blood for sperm or egg production. The feeding process of ixodid ticks has first a slow phase for several days followed by a fast phase in the last 12–24 hours before detachment. There may be a tenfold increase in fed versus unfed weights by the end of the slow phase, but there is another tenfold increase by the end of the final fast phase. Leaving the full engorgement as late as possible reduces the chances of detection and removal by the host. The hypostome has a groove along its dorsal surface forming a food canal through which blood is drawn from the host and passed on to the mouth and pharynx. During blood feeding by ixodid ticks, the liquid portion of the meal is first concentrated by removal of water and excess ions, which move across the gut epithelium and enter the tick's body cavity. These components are then taken up by the salivary glands, which produce a watery saliva that is injected back into the host.

Blood meal digestion in ticks is similar in all species. The digestive system is histologically divided into foregut, midgut and hindgut. The foregut comprises the sucking pharynx and the oesophagus. The midgut contains a ventriculus with a valve, a variable number of blind diverticula, and a rectal tube. The hindgut is divided into a rectal bulb and the rectum itself.

The mouthpart section of the tick is known as. The capitulum is not a true 'head' in the sense that the structures one normally associates with the head are not contained within it; these structures are located in the 'body' of the tick. "Ixodes holocyclus" has no eyes.

The palps are the paired tactile and positioning limbs, which are not inserted into the skin but splayed out on the surface.
The chelicerae are the paired cutting jaws, which cut a channel into skin for the hypostome. The hypostome is the single feeding tube, which also anchors the tick by its reverse barbs.
The basis capituli forms the basal ring of cuticle to which the palps, chelicerae and hypostome are attached. The basis capituli moves in the dorsoventral plane and articulates with the body proper.

Once it has chosen a feeding site, a tick positions itself with its legs so as to elevate its body at a sharp angle. Guided by the palps, the chelicerae cut into the skin with their horizontal cutting action. These rip and tear at the epidermal layers and a small pool of blood is formed. The hypostome is inserted, and this provides the initial attachment strength. In the case of "Ixodes holocyclus", the hypostome is inserted very deep into the dermis. The process by which such ticks feed is termed telmophagy. The resultant pool expands as a result of the anticoagulants released from the salivary glands.

The host reacts against the tick lesion by haemostasis, inflammation and cell-mediated immunity. An array of pharmacologically active substances is injected with the saliva of the tick, including anticoagulants, prostaglandin E2, prostacyclin, apyrase and, in certain tick species, antihistamines. Prostaglandin E2 and prostacyclin inhibit platelet aggregation and dilate blood vessels. Feeding is almost continuous, with pulses of salivation alternating with periods of feeding to ensure continued suppression of host defences.

There is a concentration of saliva and presumably toxin in the granulomatous reaction around the tick mouth parts. It is thought by some experimenters that the residual toxin located in this granuloma is at least partially responsible for the increasing paralysis which occurs after the tick is removed. By comparison, the North American paralysis tick "Dermacentor andersoni" does not produce a granuloma at the site of attachment, and in this case the paralysis rapidly regresses after the tick is removed. Unlike "Dermacentor andersoni", "Ixodes holocyclus" is a deep feeder with a long hypostome.An association between tick bite reactions and mammalian meat allergy in humans has also been reported. The induced allergy is unusual in that the onset of the allergic reaction, which ranges from mild stomach symptoms to life-threatening anaphylaxis, can occur up to 3 to 6 hours after eating mammalian meat and often many months after the tick bite.

All mammals except humans and certain apes have a carbohydrate commonly known as alpha-gal in their tissue fluids. When a tick feeds on the blood of a mammal it takes alpha-gal into the tick's digestive system. When the same tick attaches to the next host it transfers the alpha-gal to the tissues of that next host. The immune system of some humans recognises alpha-gal as foreign and so produces antibodies against it. In this case the antibody produced is IgE, which is the type of antibody responsible for most allergic reactions. Thus the human is primed for a delayed allergic reaction to subsequent ingestion of mammalian meats.

Alpha-gal allergy in a human can now be recognised through a blood test. The reaction to alpha-gal is distinct from any immediate allergic reaction to the tick itself, which is provoked by proteins in the tick's saliva, not the carbohydrate alpha-gal. People who have such allergic reactions, however, may have a greater predisposition to also developing the allergy to mammalian meats over the next several months.

Predators

Natural predators of "I. holocyclus" include insectivorous birds and wasps.

Defense

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

Other ticks which commonly need to be differentiated from "Ixodes holocyclus" include "Rhipicephalus sanguineus", "Haemaphysalis longicornis" and "Rhipicephalus microplus".
A hospital medical entomology department can provides professional identification of ticks and other medically important insects.

Evolution

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".

James Backhouse, a well-travelled Quaker during the early colonial period, gave the following account: "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.

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. The life cycle was further studied by Oxer and Ricardo and later summarised by Seddon.

In 1970, Roberts' work "Australian Ticks" 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, 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.

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" may well have been misdiagnosed and actually been cases of tick paralysis.

Uses

Dogs and cats on the east coast of Australia commonly succumb to the tick paralysis caused by "Ixodes holocyclus". A similar tick species, "Ixodes cornuatus", appears to cause paralysis in Tasmania. The paralyzing toxin is produced in the salivary glands and injected as part of the feeding process.

The adult female tick is usually attached for a minimum of 3 days before the very earliest signs are noticed although a "very" observant person might begin to notice a slight change in behavior after 48 hours of attachment. Typically, however, a person would not notice obvious signs until at least the 4th day of attachment. In colder weather the feeding process is slowed considerably and some animals may not show significant signs of paralysis for as long as two weeks. Furthermore, dogs rarely show significant signs until the adult female has engorged to a width of at least 4 mm.

Left untreated, the outcome is often, if not usually, fatal. The toxin or toxins paralyze muscle tissue; in particular:
⤷  Skeletal muscles. This results in the overt paralysis for which the tick is named. Typically the paralysis starts in the hind limbs and subsequently ascends to affect the fore limbs and then the axial muscles.
⤷  Respiratory muscles. Initially this results in rapid, shallow breathing with an inability to cough. In advanced stages it is associated with a slower, exaggerated breathing pattern.
⤷  Laryngeal muscles. This results in an altered 'voice' and an increased risk of aspiration pneumonia. Aspiration pneumonia in this situation results in labored breathing with a distinctively foul breath.
⤷  Oesophageal muscle. This results in drooling and regurgitation. It increases the risks of choking and aspiration pneumonia. Megaoesophagus is commonly recognized in dogs and may be diagnosed on thoracic X-rays.
⤷  Heart muscle. This results in congestive heart failure and pulmonary oedema, seen also as labored breathing.

Spring is the peak season for tick paralysis because this is when the ticks molt and develop into the final adult stage of their life cycle; it is the adult stage that also produces the most toxins during feeding. Sporadic cases of tick paralysis, however, can occur all year, even in mid-winter.

Once attached, the female draws blood and gradually swells. In typical warmer weather conditions the initial engorgement is slow, but after four or more days it becomes very rapid. The rapid engorgement phase is also associated with the greatest injection of the toxic saliva.

The dog or cat may show a great variety of signs that may be very subtle in the early stages. Early signs include lethargy, loss of appetite, apparent groaning when lifted, altered voice, noisy panting, coughing, drooling of saliva, gagging, regurgitation and enlarged pupils. Occasionally a single limb may appear to be weak or lame. A tick attached to the side of the face may cause loss of blink reflex, corneal ulceration and a discharge from the eye. A tick attached near the anus can cause anal incontinence.

As toxicity progresses the combination of signs becomes more characteristic of tick envenomation. There is progressive limb weakness which is seen first in the hind legs. A dog may appear to walk as though 'drunken'. There may be an inability to climb stairs or an inability to turn in tight circles without stumbling. Respiration may become slower and have a grunt at the end of the expiration. Some animals are easily panicked at this stage and should be handled calmly.

Ultimately paralysis becomes so severe that an animal is unable to stand or even lift its head. The breathing becomes slow, exaggerated and gasping. A foul odor on the breath can signal aspiration pneumonia. Although pain is not regarded as being a feature of tick paralysis, animals appear to be in severe distress. Finally, the mucous membranes develop a bluish hue and a state of coma indicates that death is imminent.

After the tick is removed, the signs usually continue to worsen for up to 48 hours, though worsening is usually most pronounced in the first 12–24 hours after removal.

The primary treatment for tick paralysis is the careful administration of anti-tick serum. The effectiveness of anti-tick serum is most dependent on how early it is administered. Early treatment offers the best chance of full recovery. Unlike snake bite, tick envenomation has a relatively slow onset. Despite this slow onset, the fatality rate can be high if antiserum is not given early enough, before the signs are advanced.

Ancillary treatments may include:
⤷  supporting respiration
⤷  minimizing stress and reducing oxygen demand
⤷  maintaining core body temperature
⤷  maintaining hydration and blood pressure
⤷  protecting eyes if eyelids paralyzed
⤷  assisting urination if unable to urinate

Prevention of tick paralysis is mostly achieved by a combination of daily searching and the use of tick-repelling or tick-killing agents. These may be topical sprays, rinses and collars or systemic oral agents. Some owners clip the fur short to help detect ticks.

Daily searching usually gives a person a few days to find an attached tick. However ticks at the early stage of attachment are small and flat and so easily missed. Whilst most ticks on dogs are found around the head, neck and shoulders, they can be anywhere on the dog's surface. They are easily missed on the face, legs and between the toes. Occasionally they are found inside the lips, ear canals, prepuce or vulva, and anus.

Cats mostly have ticks where they cannot reach to groom themselves—often on the back of the neck or between the shoulder blades, under the chin, on the head, or on the upper leg. Ticks can attach elsewhere, even the tail, hard palate or inside the anus.

Long-haired cats that venture outdoors are more at risk. Matted coats and skin with lumpy lesions also make it more difficult to find ticks. Some veterinarians perform a close clip of the entire coat to help find attached ticks.

The incidence of tick toxicity on cats is unknown, and they may be less susceptible to paralysis than dogs.

Development of a vaccine to protect dogs against the tick toxin has been attempted but has not yet been successful.