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The Australian Scrub Tick

Updated on March 9, 2012

The Australian scrub tick, also known as the paralysis tick or dog tick, is a minute parasite that delivers a paralyzing toxin and if not removed can eventually cause death, even in human hosts.

Ticks are notorious for the many serious diseases and ailments they can transmit or cause, but of the 59 species occurring in Australia, the scrub tick, Ixodes holocyclus, is one of the worst offenders. These tiny parasites are not insects but arachnids, members of the spider group.

They are found in eastern Australia between the mountains and the coast from lower Cape York Peninsula down into far eastern Victoria. They are most prevalent within 16 kilometers of the coast but they have been found further inland and are particularly abundant in rainforests.

Birds carrying nymphs of this species have even been recorded in the Canberra area, but whether the tick has become established there has not yet been determined.

Source

No Preferred Victim

Unlike many other ticks which feed only on the blood of certain animals, the scrub tick is not particular and any warmblooded animal will do.

The most common host is the bandicoot but most small and large marsupials, including kangaroos carry scrub ticks, as do spiny anteaters. Many birds are affected, including chickens, ducks, parrots, crows, magpies, curlews and butcherbirds; pets, especially dogs; stock such as horses, cattle, sheep, goats and pigs; mice and rats. There have even been reports of zoo animals, particularly monkeys, carrying scrub ticks. Humans who visit tick-infested areas or come into contact with infested animals are also liable to be chosen as hosts.

Ticks may be found anywhere on a host's body, but tend to prefer softer skin in areas such as the groin and external genitals, the corners of the mouth and eyes, the ears and the head generally. On humans they tend to hide in folds of the skin or above the hairline and have been known to attach themselves to the eardrum. On birds they keep mainly to the head and to bare or sparsely-feathered areas.

Wrong Diagnosis Results in Death

Scrub tick paralysis is caused by a poison in the saliva which causes a progressive muscular paralysis in the host that may eventually affect respiration and heart function, resulting in death. Actually, once a tick is removed the host begins to recover and in severe cases an antitoxin is available.

However, in certain cases where the doctor has not known that the patient was in tick country the symptoms of approaching tick paralysis have been confused with some other disease such as infantile paralysis. There have even been cases reported in which a bandage has been left covering the unrecognized tick until the victim died.

Ticks should never be squeezed as more poison may be thus injected into the host's body. They should be carefully plucked out using either tweezers or the 'v' of slightly open scissors.

Two related species Ixodes cornuatus and Ixodes hirsti are associated with similar paralysis, but further study is required before their significance can be assessed.

Responsible For Many Ailments

Scrub ticks are associated with various other ailments and diseases. Allergic reactions such as itching, rashes, swelling of lymph nodes and respiratory distress have been noted and are in some cases severe.

This species is one of several responsible for the transmission of Q fever, a disease exhibiting pneumonia-like symptoms and caused by the microoganism Coxiella burneti which the ticks can carry. Most noted in humans, natural occurrences of Q fever have been detected in dogs, cattle, sheep, kangaroos and bandicoots.

Although the disease is usually transmitted by a tick's bite, infections in humans are thought to occur mainly through contact with the excretions and secretions of infected animals.

Scrub ticks are also thought to be associated with the spread of Queensland 'tick typhus', a human disease occurring in coastal Queensland.

Complex Life Cycle

Ticks have a complex life cycle consisting of three parasitic stages. Immature forms are no larger than the head of a pin and are oval in shape.

Thousands of these tiny creatures climb grass blades to await a passing host.

As soon as a tick touches the skin of a host, its sharp mouthparts pierce the skin and the tick begins to rapidly engorge itself with blood, becoming partially buried in the skin. In only four days a female tick can expand to as much as 400 times her original size, looking something like a blood blister, her shape now only slightly oval. Size increases in males are not nearly so dramatic.

The tick then drops off to digest its meal, moults and repeats the process. The female feeds during all three stages -larva, nymph, adult - the male feeds from a host only during the first two stages. Females always feed until fully engorged and paralysis is usually associated with the adult female during her final meal. While she consumes her last draught, a male aboard the same host finds her and mates with her. The male has even been observed to refresh itself with a drink from the female body during mating, by piercing her body with its mouthparts.

Fully engorged and fertilized, the adult female drops from the host, lays thousands of eggs in a single batch, then dies. On hatching some seven to nine weeks later, the tiny larvae seek hosts and renew the cycle.

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