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CLASSIFICATION of TICKS
General Classification of ticks
Morphological classification of ticks
Biological classification of ticks (Life
Cycle)
Evolution and Phylogeny of
Ticks
General classification of ticks
There are many genera and species of ticks in Australia. Their general classification is as follows:
Phylum Arthropoda (jointed limbs)
Subphylum Chelicerata (anterior fangs/chelicerae)
Class Arachnida (scorpions, spiders, mites, harvestmen, and ticks)
Order Acarina (parasitiformes: gamasid mites and ticks)
Families: Argasidae ("soft ticks") and Ixodidae ("hard ticks") and Nutelliellidae
It has been proposed (Hoogstraal and Kim, 1985) that the ancestors of ticks evolved as obligate parasites of smooth-skinned reptiles during the late Paleozoic era. The argasid life cycle and feeding pattern (ie frequent small feeds) evolved with animals (Chiroptera, Lagomorpha, and Artiodactyla) that occupied and returned to their dens, nests or burrows. Thus they developed features of host and microhabitat specificty, resistance to desiccation, longevity and resistance to starvation. In contrast the more primitive ixodid tick species are, as a group, less host-specific and infest a wide range of hosts including mammals, birds and reptiles.
Morphological classification of ticks
Ticks are ventro-dorsally compressed and usually have no definite division between the head, thorax and abdomen. Sexes are separate. The life cycle is egg, larva, nymph, and adult. Larvae have 3 pairs of legs and the nymphs and adults 4 pairs. The nymph is sexually immature (has no genital aperture). All ticks are blood sucking parasites. There is no distinct head region as we normally define it. The synganglion (the primitive "brain"), the eyes and the salivary glands are all found in the body of the tick. Mouthparts are borne on a moveable part of the body called the basis capituli. Together, the basis capituli, the mouthparts and the palps form the capitulum ("little head"). The cuticle is thick, leathery and flexible with folds which allow for expansion during feeding. Mouthparts comprise 2 palps (sensory function), 2 chelicerae (cutting) and 1 hypostome (anchorage and feeding tube). The hypostome is armed with backward projecting teeth. The chelicerae are armed with movable denticles and the lateral stigmata are without sinuous peritremes. Hard and soft ticks can be compared by examining several biological and morphological criteria:
The Soft Ticks (Argasidae)
The "soft" ticks are characterised by the absence of a dorsal cuticular shield (scutum) and when viewed from above, no projecting mouth parts are visible (they project ventrally). The cuticle is covered with mamillations or spines. Sexual dimorphism is limited to slight differences in the genital opening, the male opening being smaller and more arcuate. The capitulum is subterminal or ventral in adults and not visible from above. Larvae (6 legs) may have terminal capituli. Eggs are laid in several batches of hundreds. There are usually 2 or more nymphal stages. Nymphs and adults feed repeatedly, breed repeatedly, and lay eggs after each blood meal. They usually infest nests, burrows and buildings and attach to sleeping hosts. Adults feed to repletion in minutes to hours. Larvae feed for extended periods as do nymphs of Otobius spp.
Argas persicus is large and has a mamillated cuticle. The dorsal and ventral surfaces of the body meet at a sharp lateral margin. It is found in cracks and crannies.
Otobius spp. Parasitic nymphs have a cuticle covred with spines. The adults do not feed. Nymphs are found in the ears of livestock.
Ornithodoros spp have a mamillated cuticle. The dorsal and ventral surfaces of the body do not meet at a sharp lateral margin. They are found in cracks and crannies.
Antricola spp are parasites of bats, differ from Ornithodorus spp in having a hypostome with a scoop like dorsal surface.
The Hard Ticks (Ixodidae)
The "hard" ticks have a dorsal scutum, and their mouth parts (capitulum) project forward (anteriorly) when viewed from above. The scutum covers the entire dorsal surface in males but only part of the dorsal surface of females. Scutum size remains constant during engorgement of females and thus covers a progressively smaller proportion of the dorsum. Eggs are laid in a single batch of thousands. There is only one nymphal stage. Larvae, nymphs and adults feed only once in each stage [but see life cycle of Ixodes holocyclus, as it thought that engorging adult females can attach to different hosts]. Males often die after mating. Females always die after laying their eggs (ovipositing). They usually live outdoors and attach to passing host animals. They require at least several days to complete engorgement.
Ixodes spp have an anal groove which curves around anterior to the anus. This can be seen with obique illumination of uncleared specimens. Other genera have a groove posterior to the anus if at all. Ixodes spp have no eyes, festoons, or scutal ornamentation. Their palpi are thickest at the junction of palpal segments II and III. The Australian paralysis tick is found in this genus.
Amblyomma spp have mouthparts much longer than the basis capituli; the second palpal segment is at least twice as long as it is wide. They possess eyes and sometimes have an ornate scutum. Feed mainly on kangaroos.
Aponomma spp have palpi elongate, eyes absent, frequently ornate, article 2 not salient basolaterally [cf Haemaphysalis], tarsi of male humped and armed with ventral spurs), in Australia we have 8 species. Feed mainly on reptiles
Haemaphysalis spp have palpi with flared second segments. Like Ixodes, these ticks lack eyes, but they differ in having festoons and a posterior anal groove.
Rhipicephalus spp have a roughly hexagonal basis capituli, festoons and cleft coxae I.
Boophilus spp also have a hexagonal capitulum but they lack festoons. The coxae I are not cleft but have anterior projections. The anal groove is lacking. Posess eyes.
Dermacentor spp have a rectangular basis capituli and 11 festoons. The scutum is ornamented (brightly coloured). The coxae, especially of the males, progress in size from I to IV.
Otocentor spp resemble Dermacentor but have only seven festoons.
Biological classification of ticks
According to the number of hosts they require during their life cycle, the hard ticks generally can be classed in three groups:
1. One host ticks. All three instars engorge on the same animal, the two
ecdyses also taking place on the host- eg Boophilus
decoloratus, B. annulatus.
2. Two host ticks. The larva engorges and moults on the host and
the nymph drops after also having engorged; it moults on the
ground and the imago
seeks seeks a new host- eg Rhipicephalus evertsi, R. bursa.
3. Three host. These require a different host for every instar;
they drop off each time after having engorged and moult on the
ground- eg Ixodes holocyclus, I. ricinus, Rhipicephalus
appendiculatus and most other ticks.
Each species of tick is adapted to certain ranges of temperature and moisture, some occuring only in warm regions with a fair degree of humidity, while others are winter ticks most active in a dry climate. They suck blood and sometimes lymph and are in general not very specific with regard to hosts, although some species, or certain instars of species, show a particular preference for certain host species, or they may be a definite adaptation to certain hosts. When a tick attaches itself to feed, it buries its mouthparts deeply into the tissues of the host and remains attached until it is engorged. Some species (but NOT Ixodes holocyclus) actually glue themselves into place (this is especially important for reptile ticks where there is no body covering on the host which can hide and protect the tick). If it should be detached before engorgement is complete, it will rarely feed again in the same instar.
Bionomics. The Ixodidae lay their eggs in sheltered spots: under stones and clods of soil or crevices of walls and cracks of wood near the ground. The eggs are small, spherical, yellowish brown to dark brown and laid in large massess. The female lays all her eggs in one batch, numbering up to 18,000 in some species, and then dies.
For Ixodidae the newly hatched larvae or "seed ticks" climb onto grass and shrubs and wait there until a suitable host passes, to which they attach themselves with their claws. After having engorged the larva moults and becomes a nymph. The integument of the latter requires a few days to harden, and then the nymph engorges and moults, to become an imago. After hardening of the integument, and often also after copulation, which may take place on the ground, or, more usually on the host, the female engorges, drops off and seeks a sheltered spot to lay. The males remain much longer on the host than the females, in some cases 4 months or even longer, and consequently they accumulate on the host. Although it is not definitely known whether the males of all species feed on the host, many of them certainly do so for a few days and then go in search of females. If no males are present on the host, the females may remain attached for much longer periods than under normal conditions.
Evolution and Phylogeny
the following is from : Cupp, Eddie W (1991) Biology of Ticks, in The Veterinary Clinics of North America- Small Animal Practice, 21:1 Tick-Transmitted Diseases, Hoskins ed, WB Saunders Co, Philadelphia.
Key aspects of the evolution and phylogeny of the ixodoidea have been developed largely from observations that encompass morphologic and anatomic specialization, host selection, geographic distribution of species, ecologic adaptations, and vector competence. The result is a phylogenetic tree that has remained essentially unchanged for almost a decade. With the exception of recent revisions in taxonomic nomenclature. The evolutionary thesis for this dendrogram suggests that both ixodid and argasid ticks have been in existence since the late Paleozoic to early Mesozoic eras. This estimate for their origin is somewhat hypothetic because tick fossils are rare and, when available, are of relatively recent geologic origin. Nevertheless, Hoogstraal and Kim proposed that antecedent forms evolved as obligate ectoparasitcs of smooth-skinned reptiles during the late Paleozoic era with subsequent tick-host coevolution occurring as the Reptilia radiated, into a variety of habitats, thus occupying differing ecologic niches. Two primary tick lineages emerged that eventually evolved into the two extant major families.
Argasid ticks originally were represented by Argas, Ornithodoros and other genera that became extinct. Other genera of argasids, Nothoaspis, Antricola, and Otobius, probably evolved from an Ornithodoros-like ancestor during the Tertiary period of the Cenozoic era some 65 to 70 million years ago. This evolution within the soft ticks coincided with that time when there was a major radiation of mammals some species of which now serve as hosts for these three tick genera: the Chiroptera. Lagomorpha. and Artiodactyla. Today, although few members of the Argasidae parasitize reptiles, they still retain certain basic biologic patterns developed during their early history as ectoparasites that occupied dens, nests, burrows. and caves. These include resistance to desiccation, host and microhabitat specificity, diapause, longevity, and resistance to starvation.
Ixodid ticks initially were represented by antecedents similar to primitive species in the present day genera of Ixodes and Haemaphysalis, Aponomma, Amblyomma, and Hyalomma probably appeared in the late Mesozoic era the Cretaceous period) at a time when there was a sharp reduction in the availability of many different kinds of reptilian hosts. Margaropus, Boophilus, Rhipicephalus, Dennacentor, and related genera in the subfamily Rhipicephalinae have evolved more recently, probably during the early Cenozoic era (the Tertiary period) its avian and mammalian host species proliferated.
Within the Ixodidae, almost all of the Aponomma species and approximately one third of the Amblyomma species continue to parasitize reptiles, reflecting their early evolutionary history in host selection. The remaining Amblyomma parasitize mammals or birds. Haemaphysalis and Hyalomma, which coevolved with both birds and mammals, generally have retained their preference for these classes of vertebrates. Only one species in each genus depends on reptiles as hosts. Most members of the Rhipicephalinae, which probably evolved in the Cretaceous period, continue to parasitize rodents as immature ticks and artiodactyls as adult ticks. Within the genus Ixodes, a small number of species (<10) feed on reptiles as immatures; approximately 40 species are strictly parasitic on birds. Other species within the genus may parasitize mammals or occasionally birds.
Host Selection and Developmental Patterns
Evolutionary forces such as climate and host availability have shaped both the pattern of the life cycle and the feeding physiology of ticks, beginning with their transition from a predatory life style to that of strict ectoparasitism. It is postulated that ancestral ticks had four developmental stages (as they do today); egg, larva, nymph, and adult." Each feeding stage ingested blood or tissue juices from its reptilian hosts and excreted excess fluid either through coxal glands (Argasidae) or as salivary fluids (Ixodidae). Because this ancestral stock was nidicolous, there were no differentiated eyes on the body surface. As the vertebrate evolutionary process continued, first with the reptilia and later with the appearance and divergence of birds and mammals, the two major tick lines began to separate- argasid and ixodid, ie. soft and hard.
The Soft Ticks (Argasidae)
The first, the argasid line, remained in or near the sheltered burrows, dens, nests, and resting places of their hosts and developed a pattern of repetitive blood feeding based on the intermittent return of the various host species. Large tick species tended to parasitize large vertebrates, whereas the smaller argasids coevolved with smaller mammals and birds. Thus, with few exceptions, the argasid line remained nidicolous.
Because of their small size, argasid larvae were able to feed unnoticed on their reptile hosts, remaining attached for several days at a time. This pattern still exists. Slow-feeding larvae parasitize birds and bats (for up to 10 days), and faster-feeding forms parasitize land mammals. However, the larger nymphs and adults developed a more rapid feeding behavior to avoid detection. Some species of Argas and Ornithodoros have developed simple eyes to assist in rapidly locating visiting hosts.
All stages in the argasid life cycle generally use the same kind of host as a source of nutrition. However, several developmental variations have occurred as trophic adaptations. For example, the number of nymphal instars may vary between species or within a single species from as few as two to as many as seven. Larvae of certain Ornithodoros species do not feed, nor do the adults of Otobius megnini and Otobius lagophilus because of poorly developed mouthparts. Adults of the genus Antricola also do not seem to feed. Unfed nymphs and males in the genera Ornithodoros arid Argas often feed on the hemolymph and gut contents of engorged nestmates, thus practicing homoparasitism.
The Hard Ticks (Ixodidae)
The ixodid tick evolutionary line exhibited much more flexibility in adjusting to the new array of rapidly evolving bird and mammal species host seeking occurred outside of the nest or den, so that a three-host cycle became fixed among early hard tick species, avoiding the potential dangers associated with complete dependence on the visits of highly mobile birds and mammals . This pattern of host utilization still remains; about 600 of the approximately 650 species exhibit a three-host cycle. The change in host-seeking behavior was accompanied by other adaptations as well. There was a reduction in overall body size in response to host grooming. Larvae, nymphs, and adults, while attached to their hosts, fed slowly and gradually (slow phase) for several days followed by a rapid engorgement phase during the last 12 to 24 hours before detachment. This was necessary because after the slow phase of feeding, the fed:unfed weight ratio of an adult female is about 10:1; however, there is an additional tenfold increase during the 24hour rapid feeding phase. By developing this bimodal feeding pattern, the time in which these enlarged stages were most likely to be detected because of their increased size was reduced considerably. Adult males also fed slowly and took smaller blood meals.
Palpi (pedipalps) became shorter and more compact. Certain species of Hyalomma and some genera within the subfamily Rhipicephalinae, having coevolved during the Tertiary period of the Cenozoic era with large mammals began to exhibit a two-host or one-host cycle in response to changes in host number and season. For example, Hoogstraal postulated that the one- and two-host cycles developed in temperate climates as a result of the movements of wandering mammals over large distances in places where dry seasons were long and hot. Alternatively, the one-host cycle could have developed as a result of feeding during winter months when small mammal hosts are unavailable to immature ticks that do not enter burrows. The former example is best illustrated by Boophilus (and Margaropus) species, one-host ticks that parasitize large bovids, cervids, equids, and giraffes. In some genera, particularly within the Rhipicephalinae, ocelli (eyes) became important in host perception in conjunction with mechanical and olfactory sensoria.
Bibliography
Cupp, Eddie W (1991) Biology of Ticks, in The Veterinary Clinics of North America Small Animal Practice, 21:1 Tick-Transmitted Diseases, Hoskins ed, WB Saunders Co, Philadelphia.
Georgi, J R : Parasitology for Veterinarians, W B Saunders Co, Philadelphia, 1969.
Kelly JD: Canine Parasitology, Veterinary Review No 17; The Post-Graduate Foundation in Veterinary Science, University of Sydney, 1977.
Soulsby, E J L; Helminths, Arthropods and Protozoa of Domesticated Animals; Bailliere Tindall and Cassell, London, 6th ed, 1968.
Whitlock, J H : Diagnosis of Veterinary Parasitisms, Lea and Febiger, Philadelphia, 1960.
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