Ticks are parasiticarachnids of the orderIxodida. They are part of themite superorderParasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks are externalparasites, living byfeeding 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 theCretaceous period. Ticks are widely distributed around the world, especially in warm, humid climates.
Ticks belong to two major families: theIxodidae, or hard ticks, and theArgasidae, 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 (idiosomas) which become engorged with blood when they feed, and eight legs. Theircephalothorax and abdomen are completely fused. In addition to having a hard shield on their dorsal surfaces, known as thescutum, 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.[1]
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-hostlife cycle.[2] Argasid ticks have up to seven nymphal stages (instars), each one requiring blood ingestion, and as such, Argasid ticks undergo a multihost life cycle. Because of their hematophagous (blood-ingesting) diets, ticks act asvectors of many serious diseases that affect humans and other animals.
Relationships of living and extinct tick families, after Chitimia-Dobler et al. (2022); Chitimia-Dobler et al. (2024).[4][5]
Ticks belong to theParasitiformes, a distinctive group ofmites that are separate from the main group of mites, theAcariformes. 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.[6] Within the Parasitiformes, ticks are most closely related to theHolothyrida, a small group of free living scavengers with 32 described species confined to the landmasses that formed the supercontinentGondwana.[7] Thephylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014maximum parsimony study ofamino acid sequences of 12mitochondrial proteins. The Argasidae appearmonophyletic in this study.
Ticks belong to four different families. The majority of tick species belong to the two families: Ixodidae (hard ticks) and Argasidae (soft ticks). The third living family isNuttalliellidae, named for the bacteriologistGeorge Nuttall. It comprises a single extant species,Nuttalliella namaqua,[8][9] and as such is amonotypic taxon.Nuttalliella namaqua is found in southern Africa ranging fromTanzania toNamibia andSouth Africa.[8][10] There is one extinct family,Khimairidae, represented by the fossil speciesKhimaira fossus, currently regarded as the last common ancestral lineage of theArgasidae andIxodidae.[11]
Almost all contemporary taxa fall into one of the two major tick families. TheIxodidae contain 750 species over 18 genera, characterised by ascutum or hard shield. TheArgasidae contain about 220 species over 15 genera.[18] Argasid species have no scutum, and thecapitulum (mouth and feeding parts) is concealed beneath the body.[19][20]
Ticks, likemites, belong to the subclass Acari that lack their primary somatic segmentation of theabdomen (oropisthosoma), rather these parasiticarachnids present a subsequent fusion of the abdomen with thecephalothorax (orprosoma).[21] Thetagmata typical of otherChelicerata have developed into thegnathosoma (head), which is retractable and contains the mouthparts, and idiosoma (body), which contains the legs, digestive tract, and reproductive organs.[22] 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.[21] Features of the gnathosoma include twopalps, twochelicerae, andhypostome. The hypostome acts as stabilizer and helps to anchor the tick's mouthparts to the host.[23] 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 bearssclerites, and the gonopore is located between the fourth pair of legs. In the absence of segmentation, the positioning of the eyes, limbs, andgonopore on the idiosoma provide the only locational guidance.[21]
Larval ticks hatch with six legs, acquiring the other two after a blood meal andmolting into the nymph stage.[24] 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.[25] In addition to being used forlocomotion, thetarsus 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.[26][27][28] Ticks can also use Haller's organs to perceiveinfrared light emanating from a host.[29] When stationary, their legs remain tightly folded against the body.[26][27]
Ticks are extremely resilient animals. They can survive in a near vacuum for as long as half an hour.[30] Their slow metabolism during theirdormant periods enables them to go prolonged durations between meals.[31] 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.[32] To keep from dehydrating, ticks hide in humid spots on the forest floor[33] or absorb water from subsaturated air by secretinghygroscopic fluid produced by thesalivary glands onto the external mouthparts and then reingesting the water-enriched fluid.[34]
Ticks can withstand temperatures just above −18 °C (0 °F) for more than two hours and can survive temperatures between −7 and −2 °C (20 and 29 °F) for at least two weeks. Ticks have even been found in Antarctica, where they feed on penguins.[35]
Most ticks are plain brown or reddish brown. However, the scuta of some species are decorated with white patterns.[36]
Innymphs and adults, thecapitulum is prominent and projects forwards from the body. The eyes are close to the sides of the scutum and the largespiracles are located just behind thecoxae of the fourth pair of legs.[19] The hard protectivescutellum, 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.[37] 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 tomolt; 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.[38]
A soft-bodied tick of the family Argasidae, beside eggs it has just laid
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 leatherycuticle as well. A pattern of small, circular depressions expose where muscles are attached to the interior of theintegument. 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.[39]
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 thestigmata, the lack of setae, the strongly corrugated integument, and the form of the fenestrated plates.[40][41] Nuttalliellidae genera are grouped together with reference to the character of the pseudoscutum and hypostome, but especially the 'ball-and-socket-like' leg joints.[13]
Ticks areectoparasites and most species consumeblood to satisfy all of their nutritional requirements. They are obligatehematophages, 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.[42] Hematophagy evolved independently at least six times in arthropods living during the lateCretaceous; in ticks it is thought to have evolved 120 million years ago through adaptation to blood-feeding.[12][43] This behavior evolved independently within the separate tick families as well, with differing host-tick interactions driving the evolutionary change.[12]
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.[42] It extracts blood by cutting a hole in the host'sepidermis, into which it inserts itshypostome and prevents the blood from clotting by excreting ananticoagulant orplatelet aggregation inhibitor.[44][43]
Ticks find their hosts by detecting an animals' breath and body odors, sensing body heat, moisture, or vibrations.[45] A common misconception about ticks is they jump onto their host; however, they are incapable of jumping, althoughstatic electricity from their hosts has been shown to be capable of pulling the tick over distances several times their own body length.[46] 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 anolfactory stimulus or other environmental indication, they crawl or run across the intervening surface.[45]
Other ticks, mainly the Argasidae, arenidicolous, 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.[45] Many of them feed primarily onbirds, though someOrnithodoros species, for example, feed on smallmammals. 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 detectcarbon dioxide in the breath of their hosts.[47] While all species arehaematophagous at some point in their lifecycle, a few argasid taxa, such asAntricola delacruzi, only take blood-meals as larvae, subsisting in the nymphal and adult stages by consuming guano.[48][49]
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.[50] 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.[39]
Tick saliva contains about 1,500 to 3,000 proteins, depending on the tick species. The proteins with anti-inflammatory properties, calledevasins, 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 causemyocarditis, heart attack, and stroke.[51]The saliva of ticks also containsanticoagulant andantiplatelet proteins (integrin inhibitors), to stop the blood from coagulating while they suck.[52]
Mature oocysts of the seabird soft tickOrnithodoros maritimus and theirCoxiella endosymbionts (labelled in yellow).
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.[53] 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 nutritionalendosymbionts.[53] 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 theCoxiella andFrancisella bacterial genera.[54][55] These intracellular symbiotic microorganisms are specifically associated with ticks and usetransovarial transmission to ensure their persistence.[56][57][58] AlthoughCoxiella andFrancisella endosymbionts are distantly related bacteria, they have converged towards an analogous B vitamin-based nutritional mutualism with ticks.[53] 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.[57][59][60] The genome sequencing ofCoxiella andFrancisella endosymbionts confirmed that they consistently produce three B vitamin types, biotin (vitamin B7), riboflavin (B2) and folate (B9).[57][59][61] 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.[55][57][58] SinceCoxiella andFrancisella 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.[62][63]
Tick species are widely distributed around the world.[64] They tend to flourish more in warm, humid climates, because they require a certain amount of moisture in the air to undergometamorphosis, and low temperatures inhibit their development of eggs to larvae.[65]The occurrence of ticks and tick-borne illnesses in humans is increasing.[66] Tick populations are spreading into new areas, due in part to the warming temperatures ofclimate change.[67][68]
Tick parasitism is widely distributed among host taxa, including marsupial and placental mammals, birds, reptiles (snakes, iguanas, and lizards), and amphibians.[69]Ticks of domestic animals cause considerable harm to livestock through pathogenic transmission, causing anemia through blood loss, and damaging wool and hides.[70] TheTropical Bont tick wreaks havoc on livestock and wildlife in Africa, the Caribbean, and several other countries through the spread of disease, specificallyheartwater disease.[71] Thespinose ear tick has a worldwide distribution, the young feed inside the ears of cattle and various wildlife.[72]
A habitat preferred by ticks is the interface where a lawn meets the forest,[73] or more generally, theecotone, which is unmaintained transitional edge habitat between woodlands and open areas. Therefore, one tick management strategy is to remove leaf litter, brush, and weeds at the edge of the woods.[74] Ticks like shady, moist leaf litter with an overstory of trees or shrubs and, in the spring, they deposit their eggs into such places allowing larvae to emerge in the fall and crawl into low-lying vegetation. The 3 meter boundary closest to the lawn's edge are a tick migration zone, where 82% of tick nymphs in lawns are found.[75]
In general, ticks are found wherever their host species occur. Migrating birds carry ticks with them on through their migrations; a study of migratory birds passing through Egypt discovered more than half the bird species examined were carrying ticks. It was also observed the tick species varied depending on the season of migration, in this study it is spring and autumn migrations, this is thought to occur due to the seasonal periodicities of the different species.[76]
For an ecosystem to support ticks, it must satisfy two requirements; the population density of host species in the area must be great enough and it must be humid enough for ticks to remain hydrated.[22] Due to their role in transmittingLyme disease, Ixodid ticks, particularly the North AmericanI. scapularis, have been studied usinggeographic information systems to develop predictive models for ideal tick habitats. According to these studies, certain features of a given microclimate – such as sandy soil, hardwood trees, rivers, and the presence of deer – were determined to be good predictors of dense tick populations.[47]
Mites and nematodes feed on ticks, which are also a minor nutritional resource for birds. More importantly, ticks act as a disease vector and behave as the primary hosts of many differentpathogens such asspirochaetes. Ticks carry various debilitating diseases therefore, ticks may assist in controlling animal populations and preventing overgrazing.[77]
Ticks can transmit an array of infectious diseases that affect humans and other animals.[78] Ticks that carryzoonotic pathogens often tend to have a wide host range. The infective agents can be present not only in the adult tick, but also in the eggs produced plentifully by the females. Many tick species have extended their ranges as a result of the movements of people, domesticated pets, andlivestock. With increasing participation in outdoor activities such aswilderness hikes, more people and their dogs may find themselves exposed to ticks.[79]
Life-cycle of an ixodid tick (Rhipicephalus appendiculatus, all to same scale); E=eggs, L=larvae, N=nymphs, F=adult female, M=adult male; upper row are unfed ticks, lower row are fully engorged larvae, nymphs, and a female.Two ticks mating. The smaller tick is the adult male. The larger is the adult female, who is engorged after feeding.
All three tick families ticks have four life cycle stages: egg,larva,nymph, and adult.[80]
Ixodidae ticks have three different life cycles. Depending on the species, Ixodids can either possess a one-host life cycle, two-host life cycle, or three-host life cycle.
In one-host ticks the tick remains on the host through the larval, nymphal, and adult stages, only to leave the host to lay eggs. Eggs laid in the environment hatch into larvae, which immediately seek out a host in which to attach and feed. Fed larvae molt into unfed nymphs that remain on the host. After engorging on the host's blood, the nymphs molt into sexually mature adults that remain on the host in order to feed and mate. Once a female is both fed and ready to lay eggs, only then does she leave the host in search of a suitable area to deposit her eggs. Ticks that follow this life cycle are called one-host ticks. The winter tickDermacentor albipictus and the cattle tickRhipicephalus microplus are examples of one-host ticks.[81]
The life cycle of a two-host tick often spans two years.[2] During fall the pregnant female tick will drop off her second host and lay her eggs. The eggs hatch during winter, the following spring the larvae emerge and attach to their first host. Newly hatched larvae attach to a host in order to obtain a blood meal. They remain on the host then develop into nymphs. Once engorged, they drop off the host and find a safe area in the natural environment in which to molt into adults, this typically occurs during the winter. Both male and female adults seek out a host on which to attach, which may be the same body that served as host during their early development but is often a larger mammal. Once attached, they feed and mate.Gravid females drop from the host tooviposit in the environment. Ticks that complete their life cycle in this manner are called two-host ticks, likeHyalomma anatolicum excavatum.[81]
Most ixodid ticks require three hosts, and their life cycles typically span three years. The female tick drops off its host, often in the fall, and lays thousands of eggs.[2] The larvae hatch in the winter and emerge in the spring. When the larvae emerge, they attach and feed primarily on small mammals and birds. During the summer the larvae become engorged and drop off the first host to molt and become nymphs, this often occurs during the fall. The following spring the nymphs emerge and seek out another host, often a small rodent. The nymphs become engorged and drop off the host in the fall to molt and become adults. The following spring the adult ticks emerge and seek out a larger host, often a large mammal such as cattle or even humans. Females will mate on their third host. Female adults then engorge on blood and prepare to drop off to lay her eggs on the ground, while males feed very little and remain on the host in order to continue mating with other females.[47][81]
Argasid ticks, unlike ixodid ticks, may go through up to seven nymphal stages (instars), requiring a meal of blood each time.[82] Often, egg laying and mating occurs detached from the host in a safe environment.[2] The eggs hatch and the larvae feed on a nearby host for anywhere from a few hours to several days, this depends on the species of tick. After they feed the larvae drop and molt into their first nymphal instars, then the nymph seeks out and feeds on its second host, often this is the same as the first host, within an hour. This process occurs repeatedly and until the last nymphal instar occurs, thus allowing the tick to molt into an adult. Once an adult these ticks feed rapidly and periodically their entire life cycle. In some species an adult female may lay eggs after each feeding. Their life cycles range from months to years. The adult female argasid tick can lay a few hundred to over a thousand eggs over the course of her lifetime. Both male and female adults feed on blood, and they mate off the host. During feeding, any excess fluid is excreted by the coxal glands, a process that is unique to argasid ticks.[47]
Nuttalliellidae is an elusive monotypic family of tick, that is, possesses a single species,Nuttalliella namaqua. There is little to nothing known about the life cycle and feeding habits ofN. namaqua but it is speculated this species of tick has multiple different hosts.[83]
Some species, notably theAustralian paralysis tick, are also intrinsicallyvenomous and can causetick paralysis. Eggs can become infected with pathogens inside a female tick'sovaries, in which case the larval ticks are infectious immediately at hatching, before feeding on their first host.[82]Tropical bont ticks transmit theheartwater, which can be particularly devastating in cattle.[72] The ticks carried by migratory birds act as reservoirs and vectors of foreign infectious diseases. In the Egyptian migratory bird study, over 20 strains of pathogenic viruses were detected within the tick sample from autumn.[76]
Not all ticks in an infective area are infected with transmittable pathogens, and both attachment of the tick and a long feeding session are necessary for diseases to be transmitted.[79] Consequently, tick bites often do not lead to infection, especially if the ticks are removed within 36 hours.[89] Adult ticks can be removed with fine-tipped tweezers or proprietary tick removal tools, before then disinfecting the wound.[90][91] In Australia and New Zealand, where tick-borne infections are less common than tick reactions, theAustralasian Society of Clinical Immunology and Allergy recommends seeking medical assistance or killing ticksin-situ by freezing and then leaving them to fall out to prevent allergic/anaphylactic reactions.[92][93] ProfessorSheryl van Nunen, whose research in 2007 identified tick-induced mammalian meat allergy, famously said "tweezers are tick squeezers",[94][95] referring to the tick toxins squeezed into people attempting to remove ticks with tweezers. Ticks can be disposed of by flushing them down the toilet, placing them in a container of soapy water or alcohol, or sticking them to tape that can then be folded over and thrown away.[24][90]
Bifenthrin andpermethrin, bothpyrethroids, are sometimes used as tick-control measures, although they have the disadvantage of beingcarcinogenic and able to attack the nervous systems of other species besides ticks. Those who walk through tick-infested areas can make it harder for ticks to latch onto them by tucking their trousers into boots made of smooth rubber, which ticks have trouble climbing.[96][97]
Research since 2008 has documented red-meat allergies (mammalian meat allergy andAlpha-gal allergy) in the US due tolone star tick bites. The range of the problem has been expanding with the range of the tick.[67] Other species of ticks are known for being responsible for meat allergies in other countries, including Sweden, Germany, and Australia.[98]
Researcher collecting ticks using the "tick dragging" method
With the possible exception of widespreadDDT use in theSoviet Union, attempts to limit the population or distribution of disease-causing ticks have been quite unsuccessful.[100]The parasitoidencyrtid waspIxodiphagus hookeri has been investigated for its potential to control tick populations. It lays its eggs in ticks;[101][a] the hatching wasps kill their hosts.[102]
Predators and competitors of tick hosts can indirectly reduce the density of infected nymphs, thereby lowering tick-borne disease risk by lowering the density and/or tick burden of reservoir-competent hosts. A study in the Netherlands found that the number of larval ticks on bank voles and wood mice was lower at sites with significant red fox (Vulpes vulpes) and stone marten (Martes foina) activity.[103]
This supports the results of a study from thenortheastern United States, in which the incidence of Lyme borreliosis was negatively correlated with the density of red fox, possibly because foxes decrease the density ofwhite-footed mice (Peromyscus leucopus), the most important reservoir-competent host forBorrelia burgdorferi.[103][104]
Another natural form of control for ticks is thehelmeted guineafowl, a bird species that consumes mass quantities of ticks.[105]Opossums groom themselves, swallowing many ticks; they are net destroyers of ticks, killing around ninety percent of the ticks that attempt to feed on them.[106] More generally, highanimal diversity has a strongly protective effect against tick-borne disease.[75]
Topical tick medicines may be toxic to animals and humans. Thesynthetic pyrethroid insecticidephenothrin in combination with the hormone analoguemethoprene was a popular topical flea and tick therapy for felines. Phenothrin kills adult ticks, while methoprene kills eggs. Some cat products were withdrawn in the US due to adverse reactions,[107] and others are known to cause adverse reactions.[citation needed]
In 2020, the world's first monument to a tick was erected in the Russian city ofUfa on a stone base from the Ural Mountains with the inscription: "Same as you I also want to live".[108][better source needed]
^Micrographs of the wasp laying eggs into a tick, and the hole by which the young wasps emerge from the tick's dead body, are available in Plantard et al 2012.[101]
^"Tick Allergy"(PDF).Australasian Society of Clinical Immunology and Allergy. 21 May 2019. Archived fromthe original(PDF) on 17 July 2023. Retrieved17 July 2023.
de la Fuente J (2003). "The fossil record and the origin of ticks (Acari: Parasitiformes: Ixodida)".Experimental & Applied Acarology.29 (3–4):331–44.doi:10.1023/A:1025824702816.PMID14635818.S2CID11271627.
Keirans JE, Clifford CM, Hoogstraal H, Easton ER (1976). "Discovery ofNuttalliella namaqua Bedford (Acarina: Ixodoidea: Nuttalliellidae) in Tanzania and redescription of the female based on scanning electron microcopy".Annals of the Entomological Society of America.69 (5):926–932.doi:10.1093/aesa/69.5.926.
Magnarelli LA (2009). "Global importance of ticks and associated infectious disease agents".Clinical Microbiology Newsletter.31 (5):33–37.doi:10.1016/j.clinmicnews.2009.02.001.
Molyneux DH (1993). "Vectors". In Cox FE (ed.).Modern Parasitology: a Textbook of Parasitology (2nd ed.).Wiley-Blackwell. pp. 53–74.ISBN978-0-632-02585-5.
