Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Arthropoda (arthropods, segmented body, exoskeleton, jointed appendages)
Chelicerata (2 body parts, 8 legs, first mouthparts chelicerae, no antennae, wingless)
Arachnida (abdomen without appendages)
Acari (ticks and mites, ectoparasites)

Family: Ixodidae
Ticks are obligate blood-sucking ectoparasites with two body parts and eight legs. Ixodid (hard) ticks have a characteristic hard cuticle, a terminal capitulum which can be seen in dorsal view and a large shield-shaped plate (scutum). Ticks undergo incomplete metamorphosis whereby eggs hatch larvae which moult to nymphs and then adults. Male and female ticks exhibit marked size and/or colour differences, with males generally being smaller and plainer. Eggs are laid on the ground and emergent larvae (seed ticks) quest for hosts upon which to feed. All ticks have specific life-cycles involving one, two or three hosts depending on whether moulting occurs on or off the host. Some 650 species of hard ticks infest mammals, birds and reptiles. Three species are of particular medical and/or veterinary importance in Australia: the scrub or paralysis tick Ixodes holocyclus, the cattle tick Rhipicephalus (Boophilus) microplus and the brown dog tick Rhipicephalus sanguineus.

Ixodes holocyclus [this species causes tick paralysis in humans and companion animals]

Parasite morphology: Ticks form four developmental stages; eggs, larvae, nymphs and adults. Eggs appear as small brown ovoid bodies (<0.5mm long) clustered together in large masses. The small emergent larvae (<1mm long) have six legs, whereas the larger nymphs (<2mm long) and adults (2-3mm long) have eight legs. Engorging females swell markedly in size and become dark blood-filled sacs (measuring up to 1-2cm in diameter). Ticks have two body parts: a small inconspicuous anterior gnathosoma (containing sensory palps, feeding chelicerae and a barbed hypostome); and a large posterior sac-like idiosoma (to which the legs are attached anteroventral). They have a hard chitinous covering (scutum) covering the whole dorsal surface of adult male ticks but only the anterior idiosoma of larvae, nymphs and adult female ticks. Ixodids are prostriata ticks where the anal groove is located in front of the anus. Adult I. holocyclus ticks are inornate without notches (festoons) or pigmented ‘eyes’ on the scutum.

Host range: The genus Ixodes contains over 200 species of 3-host ticks which are ectoparasitic on small mammals. Ticks are often named after a particular host (e.g. dog tick) but they are generally not host-specific, but rather host-preferential, attempting to feed on many passing animal species. Over 20 ixodid tick species occur in Australia. The paralysis tick, I. holocyclus, is found along the east coast on a range of native animal species, especially bandicoots which appear to be resistant or immune to any toxic effects. The ticks, however, can infest a range of domestic animals (dogs, cats, lambs, foals) and humans, all being more susceptible to toxic sequalae. In America, I. pacificus and I. dammini from rodents, deer and other wildlife act as vectors for Lyme disease (caused by the spirochaete Borrelia burgdorfi).

Site of infection: Larval, nymphal and adult ticks are obligate but transient ectoparasites that attach to the skin of their hosts. Most species have preferred (predilection) sites of attachment on different hosts, often involving cryptic areas among skin folds which are difficult for hosts to groom. Ticks on humans often move to the head behind the ears, or attach to the skin under tight-fitting clothing (such as elasticized waist-bands). Infestations on animals generally involve the head, neck, back and groin.

Pathogenesis: Mouthparts of feeding ticks are embedded in the host forming a tubular food channel through which saliva is injected and blood is ingested. Ticks are relatively long-lived, feeding periodically and taking large blood meals. Tick bites cause irritation, inflammation, hypersensitivity, and even anaemia when present in large numbers. Local reactions to bites vary considerably, although small granulomatous reactions consisting of mixed inflammatory cells with fibrosis are common. Infestation of humans and domestic animals by toxin-producing species, such as I. holocyclus, can result in ascending motor paralysis due to neurotoxic anticoagulants released by engorging females. Clinical signs may appear within 3 days of attachment, first paralysing the legs, then the arms and finally the thorax and throat. Death can result from respiratory failure unless the tick is removed. Tick bites often become infected, especially when ticks are forcibly removed leaving their mouthparts embedded in the skin. Many tick species also transmit viral, bacterial, rickettsial, and protozoan diseases of medical and veterinary importance.

Mode of transmission: Ticks actively seek hosts, not by pursuing them but by sedentary questing; i.e., climbing vegetation and waiting for hosts to brush past. Ticks are prone to desiccation so they quest more actively when hydrated, and return to humid ground level when dehydrated. Once contact is made with a host, the ticks migrate to suitable or preferred sites of attachment. For three-host tick species, larvae, nymphs and adults all feed on different hosts. Blood feeding takes from 3-10 days after which they drop from the host and moult to the next developmental stage or lay eggs. Time spent off the host may be as long as one year for each developmental stage so the entire life-cycle may take up to three or more years. Each female tick can lay several thousand eggs leading to heavy contamination of the environment by larval stages (‘seed’ or ‘pepper’ ticks).

Differential diagnosis: Infestations are detected by visual detection of feeding stages attached to the skin, especially large engorging females. Evidence of recent infestation may be seen at predilection sites as small inflamed nodules. Differential diagnosis is performed by removing ticks and examining them microscopically for species-specific morphotypic characters.

Treatment and control: Individual ticks attached to hosts can be physically removed, preferably by sliding fine forceps under their mouth parts and then exerting gentle backwards pressure until the tick lets go. Excessive force should not be used to avoid squeezing tick contents into the wound as well as to avoid tearing the mouthparts out leaving them behind. Tick removal may be aided by wiping the attached tick with oil or dabbing it with chloroform. A variety of treatment and control strategies have been developed for tick infestations of domestic animals but their efficacy is diminished in many instances by the persistence of ticks on wildlife reservoirs, especially in areas where wildlife and domestic stock constantly intermingle. Many native animal species are genetically resistant to heavy tick infestations. This is being exploited in cross-breeding programs e.g. Bos indicus cattle are tick-resistant whereas Bos taurus cattle are susceptible. More recently, several experimental vaccination programmes have been developed whereby tick gut antigens are used to stimulate protective antibody responses against feeding ticks. Various acaricides have proven effective for treatment when used as dips, sprays pour-ons or slow-release ear-tags. Domestic animals have been treated successfully with topical organophosphates (dichlorvos, cythoate, diazinon, malathion, fenthion, propetamphos, phormet) and pyrethroids (permethrin, deltamethrin), as well as with parenteral macrocyclic lactones or closantel. Companion animals may be treated with topical acaricides, such as fipronil, imidacloprid, selamectin, amitraz and the organophosphates. However, there are growing concerns about the development of resistance to acaricides in some tick populations. Many states and countries have adopted legislature which restricts stock movement into and from endemic areas and facilitates appropriate quarantine. Various management strategies (such as pasture rotation or spelling, cultivation or burning pastures) have also been used to minimize the transmission of infestations and reduce tick burdens on pastures.