Tick
Ticks are parasitic arachnids of the order Ixodida. They are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. The timing of the origin of ticks is uncertain, though the oldest known tick fossils are around 100 million years old, and come from the Cretaceous period. Ticks are widely distributed around the world, especially in warm, humid climates.
Ticks belong to two major families: the Ixodidae, or hard ticks, and the Argasidae, or soft ticks. Nuttalliella, a genus of tick from southern Africa, is the only living member of the family Nuttalliellidae, which represents the most primitive living lineage of ticks. Adults have ovoid/pear-shaped bodies which become engorged with blood when they feed, and eight legs. Their cephalothorax and abdomen are completely fused. In addition to having a hard shield on their dorsal surfaces, known as the scutum, hard ticks have a beak-like structure at the front containing the mouthparts, whereas soft ticks have their mouthparts on the underside of their bodies. Ticks locate potential hosts by sensing odor, body heat, moisture, and/or vibrations in the environment.
Ticks have four stages to their life cycle, namely egg, larva, nymph, and adult. Ticks belonging to the Ixodidae family undergo either a one-host, two-host, or three-host life cycle. Argasid ticks have up to seven nymphal stages, each one requiring blood ingestion, and as such, Argasid ticks undergo a multihost life cycle. Because of their hematophagous diets, ticks act as vectors of many serious diseases that affect humans and other animals.
Biology
Taxonomy and phylogeny
Ticks belong to the Parasitiformes, a distinctive group of mites that are separate from the main group of mites, the Acariformes. Whether the two groups are more closely related to each other than to other arachnids is uncertain, and studies often resolve them as not closely related. Within the Parasitiformes, ticks are most closely related to the Holothyrida, a small group of free living scavengers with 32 described species confined to the landmasses that formed the supercontinent Gondwana. The phylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014 maximum parsimony study of amino acid sequences of 12 mitochondrial proteins. The Argasidae appear monophyletic in this study.Ticks belong to four different families. The majority of tick species belong to the two families: Ixodidae and Argasidae. The third living family is Nuttalliellidae, named for the bacteriologist George Nuttall. It comprises a single extant species, Nuttalliella namaqua, and as such is a monotypic taxon. Nuttalliella namaqua is found in southern Africa ranging from Tanzania to Namibia and South Africa. There is one extinct family, Khimairidae, represented by the fossil species Khimaira fossus, currently regarded as the last common ancestral lineage of the Argasidae and Ixodidae.
Discovery and examination of fossilised ticks have driven understanding of basal Ixodida lineages and the evolutionary history of contemporary taxa. Tick paleobiota have been discovered from the end of the Early Cretaceous onwards, most commonly in amber. The discovery of an argasid bird tick in Late Cretaceous New Jersey amber in 2001 was the first mesozoic record of Parasitiformes. Burmese amber has produced the oldest fossil records, helping to resolve the Khimairidae and Nuttalliellidae through the discovery of extinct Khimaira, Deinocroton, Legionaris and Nuttalliella species, as well as identifying ancient species of the living ixodid genera Amblyomma, Ixodes, Haemaphysalis, Bothriocroton and Archaeocroton. Tick paleobiota is also known from late Albian amber, as well as Baltic and Dominican amber. Phylogenetic analysis suggests that the last common ancestor of all living ticks likely lived around 195 M
Almost all contemporary taxa fall into one of the two major tick families. The Ixodidae contain 750 species over 18 genera, characterised by a scutum or hard shield. The Argasidae contain about 220 species over 15 genera. Argasid species have no scutum, and the capitulum is concealed beneath the body.
Anatomy and physiology
Ticks, like mites, belong to the subclass Acari that lack their primary somatic segmentation of the abdomen, rather these parasitic arachnids present a subsequent fusion of the abdomen with the cephalothorax. The tagmata typical of other Chelicerata have developed into the gnathosoma, which is retractable and contains the mouthparts, and idiosoma, which contains the legs, digestive tract, and reproductive organs. The gnathosoma is a feeding structure with mouthparts adapted for piercing skin and sucking blood; it is the front of the head and contains neither the brain nor the eyes. Features of the gnathosoma include two palps, two chelicerae, and hypostome. The hypostome acts as stabilizer and helps to anchor the tick's mouthparts to the host. The chelicerae are specialized appendages used for cutting and piercing into the host's skin while palps are leglike appendages that are sensory in function.The ventral side of the idiosoma bears sclerites, and the gonopore is located between the fourth pair of legs. In the absence of segmentation, the positioning of the eyes, limbs, and gonopore on the idiosoma provide the only locational guidance.
Larval ticks hatch with six legs, acquiring the other two after a blood meal and molting into the nymph stage. In the nymphal and adult stages, ticks have eight legs, each of which has seven segments and is tipped with a pair of claws. The legs are sometimes ornamented and usually bear sensory or tactile hairs. In addition to being used for locomotion, the tarsus of leg I contains a unique sensory structure, Haller's organ, which can detect odors and chemicals emanating from the host, as well as sensing changes in temperature and air currents. Ticks can also use Haller's organs to perceive infrared light emanating from a host. When stationary, their legs remain tightly folded against the body.
Ticks are extremely resilient animals. They can survive in a near vacuum for as long as half an hour. Their slow metabolism during their dormant periods enables them to go prolonged durations between meals. Even after 18 weeks of starvation, they can endure repeated two-day bouts of dehydration followed by rehydration, but their survivability against dehydration drops rapidly after 36 weeks of starvation. To keep from dehydrating, ticks hide in humid spots on the forest floor or absorb water from subsaturated air by secreting hygroscopic fluid produced by the salivary glands onto the external mouthparts and then reingesting the water-enriched fluid.
Ticks can withstand temperatures just above for more than two hours and can survive temperatures between for at least two weeks. Ticks have even been found in Antarctica, where they feed on penguins.
Most ticks are plain brown or reddish brown. However, the scuta of some species are decorated with white patterns.
Ixodidae
In nymphs and adults, the capitulum is prominent and projects forwards from the body. The eyes are close to the sides of the scutum and the large spiracles are located just behind the coxae of the fourth pair of legs. The hard protective scutellum, a characteristic of this family, covers nearly the whole dorsal surface in males, but is restricted to a small, shield-like structure behind the capitulum in females and nymphs. When an ixodid attaches to a host the bite is typically painless and generally goes unnoticed. They remain in place until they engorge and are ready to molt; this process may take days or weeks. Some species drop off the host to molt in a safe place, whereas others remain on the same host and only drop off once they are ready to lay their eggs.Argasidae
The body of a soft tick is pear-shaped or oval with a rounded anterior portion. The mouthparts cannot be seen from above, as they are on the ventral surface. A centrally positioned dorsal plate with ridges projecting slightly above the surrounding surface, but with no decoration is often present. Soft ticks possess a leathery cuticle as well. A pattern of small, circular depressions expose where muscles are attached to the interior of the integument. The eyes are on the sides of the body, the spiracles open between legs 3 and 4, and males and females only differ in the structure of the genital pore.Nuttalliellidae
Nuttalliellidae can be distinguished from both ixodid and argasid ticks by a combination of a projecting gnathosoma and a soft leathery skin. Other distinguishing characteristics include the position of the , the lack of setae, the strongly corrugated integument, and the form of the fenestrated plates. Nuttalliellidae genera are grouped together with reference to the character of the pseudoscutum and hypostome, but especially the 'ball-and-socket-like' leg joints.Diet and feeding
Ticks are ectoparasites and most species consume blood to satisfy all of their nutritional requirements. They are obligate hematophages, and require blood to survive and move from one stage of life to another. Ticks can fast for long periods of time, but eventually die if unable to find a host. Hematophagy evolved independently at least six times in arthropods living during the late Cretaceous; in ticks it is thought to have evolved 120 million years ago through adaptation to blood-feeding. This behavior evolved independently within the separate tick families as well, with differing host-tick interactions driving the evolutionary change.Some ticks attach to their host rapidly, while others wander around searching for thinner skin, such as that in the ears of mammals. Depending on the species and life stage, preparing to feed can take from ten minutes to two hours. On locating a suitable feeding spot, the tick grasps the host's skin and cuts into the surface. It extracts blood by cutting a hole in the host's epidermis, into which it inserts its hypostome and prevents the blood from clotting by excreting an anticoagulant or platelet aggregation inhibitor.
Ticks find their hosts by detecting an animals' breath and body odors, sensing body heat, moisture, or vibrations. A common misconception about ticks is they jump onto their host; however, they are incapable of jumping, although static electricity from their hosts has been shown to be capable of pulling the tick over distances several times their own body length. Many tick species, particularly Ixodidae, lie in wait in a position known as "questing". While questing, ticks cling to leaves and grasses by their third and fourth pairs of legs. They hold the first pair of legs outstretched, waiting to grasp and climb on to any passing host. Tick questing heights tend to be correlated with the size of the desired host; nymphs and small species tend to quest close to the ground, where they may encounter small mammalian or bird hosts; adults climb higher into the vegetation, where larger hosts may be encountered. Some species are hunters and lurk near places where hosts may rest. Upon receiving an olfactory stimulus or other environmental indication, they crawl or run across the intervening surface.
Other ticks, mainly the Argasidae, are nidicolous, finding hosts in their nests, burrows, or caves. They use the same stimuli as non-nidicolous species to identify hosts, with body heat and odors often being the main factors. Many of them feed primarily on birds, though some Ornithodoros species, for example, feed on small mammals. Both groups of soft tick feed rapidly, typically biting painfully and drinking their fill within minutes. Unlike the Ixodidae that have no fixed dwelling place except on the host, they live in sand, in crevices near animal dens or nests, or in human dwellings, where they come out nightly to attack roosting birds or emerge when they detect carbon dioxide in the breath of their hosts. While all species are haematophagous at some point in their lifecycle, a few argasid taxa, such as Antricola delacruzi, only take blood-meals as larvae, subsisting in the nymphal and adult stages by consuming guano.
Ixodidae remain in place until they are completely engorged. Their weight may increase by 200 to 600 times compared to their prefeeding weight. To accommodate this expansion, cell division takes place to facilitate enlargement of the cuticle. In the Argasidae, the tick's cuticle stretches to accommodate the fluid ingested, but does not grow new cells, with the weight of the tick increasing five- to tenfold over the unfed state. The tick then drops off the host and typically remains in the nest or burrow until its host returns to provide its next meal.
Tick saliva contains about 1,500 to 3,000 proteins, depending on the tick species. The proteins with anti-inflammatory properties, called evasins, allow ticks to feed for eight to ten days without being perceived by the host animal. Researchers are studying these evasins with the goal of developing drugs to neutralise the chemokines that cause myocarditis, heart attack, and stroke.
The saliva of ticks also contains anticoagulant and antiplatelet proteins, to stop the blood from coagulating while they suck.
Most ticks do not use any other food source than vertebrate blood and therefore ingest high levels of protein, iron and salt, but few carbohydrates, lipids or vitamins. Ticks' genomes have evolved large repertoires of genes related to this nutritional challenge, but they themselves cannot synthesize the essential vitamins that are lacking in blood meal. To overcome these nutritional deficiencies, ticks have evolved obligate interactions with nutritional endosymbionts. The first appearance of ticks and their later diversification were largely conditioned by this nutritional endosymbiosis lasting for millions of years. The most common of these nutritional endosymbionts belong to the Coxiella and Francisella bacterial genera. These intracellular symbiotic microorganisms are specifically associated with ticks and use transovarial transmission to ensure their persistence. Although Coxiella and Francisella endosymbionts are distantly related bacteria, they have converged towards an analogous B vitamin-based nutritional mutualism with ticks. Their experimental elimination typically results in decreased tick survival, molting, fecundity and egg viability, as well as in physical abnormalities, which all are fully restored with an oral supplement of B vitamins. The genome sequencing of Coxiella and Francisella endosymbionts confirmed that they consistently produce three B vitamin types, biotin, riboflavin and folate. As they are required for tick life cycle, these obligate endosymbionts are present in all individuals of the tick species they infect, at least at early stages of development since they may be secondarily lost in males during nymphal development. Since Coxiella and Francisella endosymbionts are closely related to pathogens, there is a substantial risk of misidentification between endosymbionts and pathogens, leading to an overestimation of infection risks associated with ticks.