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Tick

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Not to be confused with Tic.
For other uses, see Tick (disambiguation).

Tick
Castor bean tick, Ixodes ricinus
Scientific classification
Kingdom:
Animalia
Phylum:
Arthropoda
Class:
Arachnida
Subclass:
Acari
Superorder:
Parasitiformes
Order:
Ixodida

Leach, 1815
Superfamily:
Ixodoidea

Leach, 1815
Families
Diversity
18 genera, c. 900 species

Ticks are small arachnids in the order Ixodida. Along with mites, they constitute the subclass Acarina. Ticks are ectoparasites (external parasites), living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks are vectors of a number of diseases, including Lyme disease, Q fever (rare; more commonly transmitted by infected excreta),[1] Colorado tick fever, Rocky Mountain spotted fever, African tick bite fever, tularemia, tick-borne relapsing fever, babesiosis, ehrlichiosis, Tick paralysis and tick-borne meningoencephalitis, as well as bovine anaplasmosis.

Taxonomy

Of the three families of ticks, one – Nuttalliellidae – comprises a single species, Nuttalliella namaqua. The remaining two families contain the hard ticks (Ixodidae) and the soft ticks (Argasidae).[2][3]

Ixodidae (>700 species) are distinguished from the Argasidae by the presence of a scutum or hard shield. This shield is hard enough to resist crushing with a shoe, or footwear of a person. However, an engorged tick, fat with blood can easily be killed by stepping on it. In Ixodidae nymphs and adults, a prominent capitulum (head) projects forwards from the body; in the Argasidae, conversely, the capitulum is concealed beneath the body.[4]

The Argasidae contain 193 species, although the composition of the genera is less certain, and more study is needed before the genera can become stable.[2] The currently accepted genera are Antricola, Argas, Nothaspis, Ornithodoros, and Otobius.[2] Though common in North America, they feed rapidly, primarily on birds, and are very rarely found to parasitize land animals or humans.[5]

The family Nuttalliellidae contains only a single species, Nuttalliella namaqua, a tick found in southern Africa from Tanzania to Namibia and South Africa,.[2][6] It can be distinguished from ixodid ticks and argasid ticks by a combination of characteristics, including the position of the stigmata, lack of setae, strongly corrugated integument, and form of the fenestrated plates.[7]

Fossilized ticks are common. Recent hypotheses based on total-evidence approach analysis place the origin of ticks in the Cretaceous (65 to 146 million years ago), with most of the evolution and dispersal occurring during the Tertiary (5 to 65 million years ago).[8] The oldest example is an argasid (bird) tick from Cretaceous New Jersey amber. The younger Baltic and Dominican ambers have also yielded examples, all of which can be placed in living genera.

Range and habitat

Tick species are widely distributed around the world,[9] but they tend to flourish more in countries with warm, humid climates, because they require a certain amount of moisture in the air to undergo metamorphosis, and because low temperatures inhibit their development from egg to larva.[10] Ticks of domestic animals are especially common and varied in tropical countries, where they cause considerable harm to livestock by transmission of many species of pathogens and also causing direct parasitic damage.

For an ecosystem to support ticks, it must satisfy two requirements: the population density of host species in the area must be high enough, and humidity must be high enough for ticks to remain hydrated.[11] Due to their role in transmitting Lyme disease, ixodid ticks, particularly I. scapularis, have been studied using geographic information systems (GIS), 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.[5]

Anatomy and physiology

Ticks, like mites, have bodies which are divided into two primary sections: the anterior capitulum (or gnathosoma), which contains the head and mouthparts; and the posterior idiosoma which contains the legs, digestive tract, and reproductive organs.[11]

Diet and feeding behaviors

A questing tick

Ticks satisfy all of their nutritional requirements on a diet of blood, a practice known as hematophagy. They extract the blood by cutting a hole in the host's epidermis, into which they insert their hypostome, likely keeping the blood from clotting by excreting an anticoagulant.[12] Blood is a requirement for ticks surviving and moving from one stage of their life to the next. As such, ticks unable to find a host to feed on will die.[13]

Ticks find their hosts by detecting animals' breath and body odors, or by sensing body heat, moisture and vibrations. They are incapable of flying or jumping, but many tick species wait in a position known as "questing". While questing, ticks hold onto leaves and grass by their third and fourth pair of legs. They hold the first pair of legs outstretched, waiting to climb on to the host. When a host brushes the spot where a tick is waiting, it quickly climbs onto the host. Some ticks will begin to feed immediately while others may wander around and look for areas where the skin is thinner.[13]

Legs

Ticks have eight legs.

Like all arachnids, ticks have eight legs. The legs of Ixodidae and Argasidae are similar in structure. Each leg is composed of six segments: the coxa, trochanter, femur, patella, tibia, and tarsus. Each of these segments is connected by muscles which allow for flexion and extension, but the coxae have limited lateral movement. When not being used for walking, the legs remain tightly folded against the body.[14][15]

In addition to being used for locomotion, the tarsus of leg I contains a unique sensory organ, the Haller's organ, which can detect odors and chemicals emanating from the host, as well as sensing changes in temperature and air currents.[14][15][16]

Life cycle and reproduction

Ixodidae

Both ixodid and argasid ticks undergo three primary stages of development: larval, nymphal, and adult.[17] Ixodid ticks require three hosts, and their life cycle takes at least one year to complete. Up to 3,000 eggs are laid on the ground by an adult female tick. When larvae emerge, they feed primarily on small mammals and birds. After feeding, they detach from their host and molt to nymphs on the ground, which then feed on larger hosts and molt to adults. Female adults attach to larger hosts, feed, and lay eggs, while males feed very little and occupy larger hosts primarily for mating.[5]

Argasidae

Argasid ticks, unlike ixodid ticks, may go through several nymphal stages, requiring a meal of blood each time.[18] Their lifecycles 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. Larvae feed very quickly and detach to molt to nymphs. Nymphs may go through as many as seven instars, each requiring a blood meal. 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 which is unique to argasid ticks.[5]

Researcher collecting ticks using the "tick dragging" method

Medical issues

A sign in a Lithuanian forest warning about a high risk of tick-borne encephalitis infection

Tick-borne disease

Main article: Tick-borne disease

Tick-borne illnesses are caused by infection with a variety of pathogens, including Rickettsia and other types of bacteria, viruses, and protozoa. Because ticks can harbor more than one disease-causing agent, patients can be infected with more than one pathogen at the same time, compounding the difficulty in diagnosis and treatment. Major tick-borne diseases include Lyme disease, Rocky Mountain spotted fever, relapsing fever, tularemia, tick-borne meningoencephalitis, Colorado tick fever, Crimean-Congo hemorrhagic fever, babesiosis, and cytauxzoonosis. A recent find is Candidatus neoehrlichia mikurensis, a bacterium which causes blood clots; present in 9% of rodents, it mainly affects persons with lowered immune defense, and can be cured with antibiotics.[19]

Tick bites may also induce a delayed allergy to red meat, involving the oligosaccharide, galactose alpha-1,3-galactose: the food-induced reactions, including anaphylaxis, characteristically present several hours after eating in subjects who have experienced a large local reaction to tick bites up to six months earlier.[20][21]

Eggs can be infected with pathogens inside the ovaries, meaning the larval ticks can be infectious immediately at hatching, before feeding on their first host.[18]

First aid

Engorged tick attached to back of toddler's head. Adult thumb shown for scale.

In general, the best way to remove an adult tick is mechanically. To facilitate prompt removal, fine-tipped tweezers can be used to grasp the tick as close to the skin as possible and detach it by applying a steady upward force without crushing, jerking or twisting, in such a way as to avoid leaving behind mouthparts or provoking regurgitation of infective fluids into the wound.[22][23][24] Proprietary tick removal tools are also available.[22][23] It is important to disinfect the bite area thoroughly after removal of the tick.[24] The tick can be stored and, in case of signs or symptoms of a subsequent infection, shown to a clinician for identification purposes together with details of where and when the bite occurred.[22] If the tick's head and mouthparts are no longer attached to its body after removal, a punch biopsy may be necessary to remove any parts left inside the patient.[25]

Population control measures

With the possible exception of widespread DDT use in the Soviet Union, attempts to limit the population or distribution of disease-causing ticks have been very unsuccessful.[26]

The parasitoid chalcid wasp Ixodiphagus hookeri has long been investigated for its potential to control tick populations. It lays its eggs into ticks; the hatching wasps kill their hosts.

Another natural form of control for ticks is the guineafowl, a bird species which consumes mass quantities of ticks.[27] Just two birds can clear 2 acres (8,100 m2) in a single year.

Topical (drops/dust) flea/tick medicines may be toxic to animals and humans. Phenothrin (85.7%) in combination with methoprene was a popular topical flea/tick therapy for felines. Phenothrin kills adult fleas and ticks. Methoprene is an insect growth regulator that interrupts the insect's lifecycle by killing the eggs. However, the US Environmental Protection Agency required at least one manufacturer of these products to withdraw some products and include strong cautionary statements on others, warning of adverse reactions.[28]

Deer ticks

An adult tick found on a German shepherd dog, ballpoint pen is shown for scale

The black-legged or deer tick (Ixodes scapularis) is dependent on the white-tailed deer for reproduction. Larval and nymphal stages (immature ticks that cannot reproduce) of the deer tick feed on birds and small mammals. The adult female tick needs a large three-day blood meal from the deer before she can reproduce and lay her 2000 or more eggs. Deer are the primary hosts for the adult deer ticks, and are key to their reproductive success.[29]

Numerous studies have shown the abundance and distribution of deer ticks are correlated with deer densities.[29][30][31][32]

When the deer population was reduced by 74% at a 248-acre (100-ha) study site in Bridgeport, Connecticut, for example, the number of nymphal ticks collected at the site decreased by 92%.[29] The relationship between deer abundance, tick abundance, and human cases of Lyme disease was well documented in the Mumford Cove Community in Groton, Connecticut, from 1996 to 2004. The deer population in Mumford Cove was reduced from about 77 to about 10 deer per square mile (four deer per square kilometer) after two years of controlled hunting. After the initial reduction, the deer population was maintained at low levels. Reducing deer densities to 10 deer per square mile was adequate to reduce by more than 90% the risk of humans contracting Lyme disease in Mumford Cove.[33]

A 2006 study by Penn State's Center for Infectious Disease Dynamics indicated reducing the deer population in small areas (but not large areas) may lead to higher tick densities, resulting in more tick-borne infections in rodents, leading to a high prevalence of tick-borne encephalitis and creating a tick hot-spot.[34]

See also

Wikimedia Commons has media related to Ixodida.

References

Notes

  1. ^ "Q fever". Centers for Disease Control. Retrieved November 7, 2010.
  2. ^ a b c d Guglielmone et al. (2010)
  3. ^ Goddard (2008): p. 80
  4. ^ Molyneux (1993) p. 6
  5. ^ a b c d Allan (2001)
  6. ^ Keirans et al. (1976)
  7. ^ Roshdy et al. (1983)
  8. ^ de la Fuente (2003)
  9. ^ Magnarelli (2009)
  10. ^ Nuttall (1905)
  11. ^ a b Wall & Shearer (2001): p. 55
  12. ^ Goddard (2008): p. 82
  13. ^ a b "Life cycle of Hard Ticks that Spread Disease". Centers for Disease Control and Prevention. Retrieved 22 June 2013.
  14. ^ a b Sonenshine (2005): p. 14
  15. ^ a b Nicholson et al. (2009): p. 486
  16. ^ For Haller's organ, see also: Mehlhorn (2008): p. 582.
  17. ^ Dennis & Piesman (2005): p. 5
  18. ^ a b Aeschlimann & Freyvogel, 1995: p. 182
  19. ^ Andersson & Råberg (2011)
  20. ^ Bianca Nogrady (May 10, 2008). "One tick red meat can do without". The Australian. Retrieved October 17, 2011.
  21. ^ Saleh et al. (2012)
  22. ^ a b c "Correct tick removal". Borreliosis and Associated Diseases Awareness UK. Retrieved March 30, 2012.
  23. ^ a b "Tick removal". LDA. Retrieved March 30, 2012.
  24. ^ a b "Tick removal". Centers for Disease Control and Prevention. Retrieved March 30, 2012.
  25. ^ Zuber & Mayeaux (2003), p. 63
  26. ^ Dennis & Piesman, 2005: p. 3
  27. ^ Duffy et al. (1992)
  28. ^ "Hartz flea and tick drops for cats and kittens to be phased out". Environmental Protection Agency. Archived from the original on January 11, 2011. Retrieved September 16, 2011. {{cite news}}: |archive-date= / |archive-url= timestamp mismatch; January 11, 2010 suggested (help)
  29. ^ a b c Stafford (2007)
  30. ^ Rand et al. (2004)
  31. ^ Walter et al. (2002)
  32. ^ Wilson et al. (1990)
  33. ^ Kilpatrick & LaBonte (2007)
  34. ^ "Deer-free areas may be haven for ticks, disease". Science Daily. September 4, 2006.

Bibliography

Further reading

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