Head louse
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{{Otheruses4|the animal|the disease known as head lice|Pediculosis}}
{{Taxobox
| name = Head louse
| image = Pediculus_humanus_var_capitis.jpg
| image_width = 250px
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| classis = [[Insect]]a
| ordo = [[Louse|Phthiraptera]]
| subordo = [[Anoplura]]
| familia = [[Pediculidae]]
| genus = ''Pediculus''
| species = ''P. humanus''
| binomial = ''Pediculus humanus''
| binomial_authority = [[Carolus Linnaeus|Linnaeus]], 1758
| trinomial = ''Pediculus humanus capitis''
| trinomial_authority = [[Charles De Geer]], 1767
| synonyms =
''Pediculus capitis'' <small>(Charles De Geer, 1767)</small><br>
}}
The '''head louse''' (''Pediculus humanus capitis'') is an [[obligate parasite|obligate]] [[parasite|ectoparasite]] of [[humans]].<ref name="Buxton3">{{cite book
|last= Buxton
|first= Patrick A.
|title= The Louse; an account of the lice which infest man, their medical importance and control
|origyear=1947
|edition=2nd edition
|publisher=Edward Arnold
|location=London
|pages=24-72
|chapter=The biology of ''Pediculus humanus''
}}</ref> Head lice are wingless [[insects]] spending their entire life on human [[scalp]] and feeding exclusively on human [[blood]].<ref name="Buxton3"/> Humans are the only known [[host (biology)|host]] of this parasite.<ref name="Buxton3"/>
Head lice are closely related to [[body lice]] (''Pediculus humanus corporis'') which also infest Humans.<ref name="Buxton3"/> A more distantly-related species of louse, the pubic or [[crab louse]] (''Pthirus pubis''), also infests humans.<ref name="Buxton6">{{cite book
|last= Buxton
|first= Patrick A.
|title= The Louse; an account of the lice which infest man, their medical importance and control
|origyear=1947
|edition=2nd edition
|publisher=Edward Arnold
|location=London
|pages=136-141
|chapter=The crab louse ''Phthirus pubis''
}}</ref> Lice infestation is known as [[pediculosis]].<ref name="urlpediculosis - Definition from the Merriam-Webster Online Dictionary">{{cite web
|url=http://www.merriam-webster.com/dictionary/pediculosis
|title=pediculosis - Definition from the Merriam-Webster Online Dictionary
|accessdate=2008-04-23
}}</ref>
The head louse (and lice in general) differ from other [[hematophagic]] ectoparasites such as the [[flea]] in that lice spend their entire life cycle on a host.<ref name="maunder">{{cite journal
|last = Maunder
|first = JW
|title = The Appreciation of Lice
|journal = Proceedings of the Royal Institution of Great Britain
|volume = 55
|pages = 1–31
|publisher = Royal Institution of Great Britain
|location = London
| date = 1983}} '''Although this article is a delightful read, it is entirely devoid of references. At best, it is an unsourced secondary reference, and should probably be avoided for verification purposes.'''</ref> Head lice cannot fly, and their short stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces.<ref name="maunder"/>
==Adult Morphology==
Like other [[insects]] of the suborder [[Anoplura]], adult head lice are small (1-3 mm long), dorso-ventrally flattened (see [[anatomical terms of location]]), and entirely wingless.<ref name="buxton1">{{cite book
|last= Buxton
|first= Patrick A.
|title= The Louse; an account of the lice which infest man, their medical importance and control
|origyear=1947
|edition=2nd edition
|publisher=Edward Arnold
|location=London
|pages=1-4
|chapter=The Anoplura or Sucking Lice
}}</ref> The [[thoracic]] segments are fused, but otherwise distinct from the [[head]] and [[abdomen]], the latter being composed of seven visible [[Segmentation (biology)|segments]].<ref name="buxton2">{{cite book
|last= Buxton
|first= Patrick A.
|title= The Louse; an account of the lice which infest man, their medical importance and control
|origyear=1947
|edition=2nd edition
|publisher=Edward Arnold
|location=London
|pages=5-23
|chapter=The Anatomy of ''Pediculus humanus''
}}</ref> Head lice are grey in general, but their precise color varies according to the environment in which they were raised.<ref name="buxton2"/><!-- SECONDARY REFERENCE - Buxton cites Nuttall (1919) Parasitology 11:201-220 --> After feeding, consumed blood causes the louse body to take on a reddish color.<ref name="buxton2"/>
=== Head ===
[[Image:Fig. 1. Male of head louse.jpg|thumb|upright|Male head louse, adult]]
One pair of [[antenna (biology)|antenna]], each with five segments, protrude from the insect's [[head]]. Head lice also have one pair of [[eyes]]. Eyes are present in all species within ''Pediculidae'' (the family of which the head louse is a member), but are reduced or absent in most other members of the Anoplura suborder.<ref name="buxton1"/> Like other members of Anoplura, head lice mouthparts are highly adapted for piercing skin and sucking blood.<ref name="buxton1"/> These mouthparts are retracted into the insect's head except during feeding.<ref name="buxton2"/>
=== Thorax ===
Six [[legs]] project from the fused segments of the [[thorax]].<ref name="buxton2"/> As is typical in Anoplura, these legs are short and terminate with a single [[claw]] and opposing "[[thumb]]".<ref name="buxton2"/> Between its claw and thumb, the louse grasps the hair of its host.<ref name="buxton2"/> With their short legs and large claws, lice are well adapted to clinging to the hair of their host. However, these adaptations leave them quite incapable of jumping, or even walking efficiently on flat surfaces.<ref name="maunder"/>
Like other members of Anoplura, wings are entirely absent.<ref name="buxton1"/>
=== Abdomen ===
There are seven visible segments of the louse [[abdomen]].<ref name="buxton2"/> The first six segments each have a pair of [[spiracles]] through which the insect breathes.<ref name="buxton2"/> The last segment contains the [[anus]] and (separately) the [[genitalia]].<ref name="buxton2"/>
=== Gender differences ===
In male lice, the front two legs are slightly larger than the other four. This specialized pair of legs is used for holding the female during copulation. Males are slightly smaller than females and are characterized by a pointed end of the abdomen and a well-developed genital apparatus visible inside the abdomen. Females are characterized by two gonopods in the shape of a W at the end of their abdomen.
==Louse Eggs==
[[Image:Fig.3.Louse egg.jpg|thumb|upright|Head louse egg attached to hair shaft of host]]
Like most insects, head lice are [[oviparous]]. Louse [[egg (biology)|egg]]s contain a single [[embryo]], and are attached near the base of a host hair shaft.<ref name="pmid11331679">{{cite journal |author=Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA |title=Lice, nits, and school policy |journal=Pediatrics |volume=107 |issue=5 |pages=1011–5 |year=2001 |pmid=11331679 |doi=10.1542/peds.107.5.1011}}</ref><ref name="pmid15965432">{{cite journal |author=Burkhart CN, Burkhart CG |title=Head lice: scientific assessment of the nit sheath with clinical ramifications and therapeutic options |journal=J. Am. Acad. Dermatol. |volume=53 |issue=1 |pages=129–33 |year=2005 |pmid=15965432 |doi=10.1016/j.jaad.2005.01.134}}</ref> Eggs are generally laid within 1 cm of the scalp surface.<ref name="pmid15965432"/> To attach each egg, the adult female secretes a glue from her reproductive organ. This glue quickly hardens into a "nit sheath" that covers the hair shaft and the entire egg except for the operculum -- a cap through which the embryo breathes.<ref name="pmid15965432"/> The glue was previously thought to be [[chitin]]-based <!-- Buxton (1947) implies that this belief may have begun with Schmidt (1939), but that article is in German -->, but more recent studies have shown it to be made of [[protein]]s similar to hair [[keratin]].<ref name="pmid15965432"/>
Each egg is oval-shaped and about 0.8 mm in length.<ref name="pmid15965432"/> They are brown so long as they contain an embryo, but appear white after hatching.<ref name="pmid15965432"/> Viable louse eggs hatch 6 to 9 days after [[oviposition]].<ref name="pmid11331679">{{cite journal |author=Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA |title=Lice, nits, and school policy |journal=Pediatrics |volume=107 |issue=5 |pages=1011–5 |year=2001 |pmid=11331679 |doi=10.1542/peds.107.5.1011}}</ref> After hatching, the louse [[nymph (biology)|nymph]] leaves behind its egg shell, still attached to the hair shaft.
{{-}}
==Nits==
{{wiktionary|nit}}
The term nit refers to either a louse [[egg (biology)|egg]] or a louse [[nymph (biology)|nymph]].<ref name="nitdef">{{cite web |url=http://www.merriam-webster.com/dictionary/nit |title=nit - Definition from the Merriam-Webster Online Dictionary |accessdate=2008-02-25 |format= |work=}}</ref> With respect to eggs, this rather broad definition includes the following:<ref name="pmid10959734">{{cite journal |author=Pollack RJ, Kiszewski AE, Spielman A |title=Overdiagnosis and consequent mismanagement of head louse infestations in North America |journal=Pediatr. Infect. Dis. J. |volume=19 |issue=8 |pages=689–93; discussion 694 |year=2000 |pmid=10959734 |doi=}}</ref>
*Viable eggs that will eventually hatch
*Remnants of already-hatched eggs
*Nonviable eggs (dead [[embryo]]) that will never hatch
[[Image:Anoplura hatching.jpg|thumb|Louse hatching]]
This has produced some confusion in, for example, school policy (see ''[[Pediculosis#The_.22no-nit.22_policy|The "no-nit" policy]]'') because, of the three items listed above, only eggs containing viable embryos have the potential to infest or re-infest a host.<ref name="pmid14651472">{{cite journal |author=Burgess IF |title=Human lice and their control |journal=Annu. Rev. Entomol. |volume=49 |issue= |pages=457–81 |year=2004 |pmid=14651472 |doi=10.1146/annurev.ento.49.061802.123253}}</ref> Some authors have reacted to this confusion by restricting the definition of nit to describe only a hatched egg:
{{quote|Much of the misdiagnosis revolved around whether louse eggs were intact, apparently with a viable embryo, or hatched (what is more correctly called a nit).|I.F. Burgess (2004)<ref name="pmid14651472"/>}}
{{quote|The empty eggshell, termed a nit...|Maunder (1983)<ref name="maunder"/>}}
Others have retained the broad definition while simultaneously attempting to clarify its relevance to infestation:
{{quote|Because nits are simply egg casings that can contain a developing embryo or be empty shells, not all nits are infective.|L.K. Williams and others (2001)<ref name="pmid11331679"/>}}
Note that all these quotations appear to reject the notion that louse nymphs are nits, and may indicate that the nit definition is currently in flux. <!-- I haven't found any reference, other than the dictionary, that says louse nymphs are nits. It'd be nice to include such a reference here. -->
== Development and nymphs ==
[[Image:Pediculus humanus development.jpg|thumb|Development of ''Pediculus humanus corporis'' ([[body lice]]), which is similar to that of head lice (''Pediculus humanus capitis'')]]
Head lice, like other [[insects]] of the suborder [[Anoplura]], are [[hemimetabolous]].<ref name="Buxton3"/> Newly hatched [[nymph (biology)|nymphs]] will [[Ecdysis|molt]] three times before reaching the sexually-mature adult stage.<ref name="Buxton3"/><!-- SECONDARY REFERENCE - Buxton cites Warburton, C. (1910) A preliminary investigation on flock as a possible distributor of vermin, and on the life history of the body-louse. Rep. Local Govt. Bourd, Pub. Health and Medical Subjects. (No. 27) --> Thus, mobile head lice populations contain members of up to four developmental stages: 3 nymphal [[instars]], and the adult ([[imago]]).<ref name="Buxton3"/> [[Metamorphosis]] during head lice development is subtle. The only visible differences between different instars and the adult, other than size, is the relative length of the abdomen, which increases with each molt.<ref name="Buxton3"/><!-- SECONDARY REFERENCE - Buxton cites Buxton, P.A. (1938) Studies on growth of Pediculus (Anoplura). Parasitology 30:65-84 --> Nymph behavior is similar to the adult. Nymphs feed only on human blood ([[hematophagia]]), and cannot survive long away from a host.<ref name="Buxton3"/>
Like adult head lice, the nymph cannot fly or jump. However, there are reports of the light-weighted nymphs being blown by wind.<ref name="Buxton3"/><!-- This is a secondary reference, however, the primary references, Schaefer (1916) and Schilling (1916) are in German --> Similarly, after a molt, the discarded [[exoskeleton]] can be later shed by the host, and may be mistakenly interpreted as a viable louse.<ref name="maunder"/>
The time required for head lice to complete their nymph development to the imago depends on feeding conditions. At minimum, eight to nine days are required for lice having continuous access to a human host.<ref name="Buxton3"/><!-- SECONDARY REFERNCE? - Buxton may be citing Buxton, P.A. (1940) The biology of the body louse (Pediculus humanus corpis: Anoplura) under experimental conditions. Parasitology 32:303-312 --> This experimental condition is most representative of head lice conditions in the wild. Experimental conditions where the nymph has more limited access to blood produces more prolonged development, ranging from 12 to 24 days.<ref name="Buxton3"/><!-- SECONDARY REFERNCE? - Buxton may be citing Buxton, P.A. (1940) The biology of the body louse (Pediculus humanus corpis: Anoplura) under experimental conditions. Parasitology 32:303-312 -->
Nymph mortality is reportedly high -- about 38% -- especially within the first two days of life.<ref name="Buxton3"/> Nymph hazards are numerous. Failure to completely hatch from the egg is invariably fatal, and may be dependent on the humidity of the egg's environment.<ref name="Buxton3"/> Death during molting can also occur, although it is reportedly uncommon.<ref name="Buxton3"/> During feeding, the nymph gut can rupture, dispersing the host's blood throughout the insect. This results in death within a day or two.<ref name="Buxton3"/> It is unclear if the high mortality recorded under experimental conditions is representative of conditions in the wild.<ref name="Buxton3"/>
==Reproduction==
[[Image:Nuttall 1917 f11.gif|right|thumb|Copulation in ''Pediculus humanus corporis'' (''Pediculus humanus capitis'' is similar). Female is on top, with the male below. Dilation of the female's vagina has already occurred, and the male's dilator rests against his back (dorsal surface), out of the way. The male vesica, which contains the penis proper (not seen), is fully inserted into the vagina. Note the male's attachment with his specialized claws on the first leg pair to the specialized notch on the female's third leg pair.]]
Adult head lice [[Sexual reproduction|reproduce sexually]], and [[copulation]] is necessary for the female to produce fertile eggs. [[Parthenogenesis]], the production of viable offspring by virgin females, does not occur in ''Pediculus humanus''.<ref name="Buxton3"/><!-- SECONDARY REFERENCE - Buxton cites Buxton (1940) Parasitology 32:296-302 --> Pairing can begin within the first 10 hours of adult life.<ref name="Buxton3"/><!-- Buxton cites Sikora (1915), but the article is in German --> After 24 hours, adult lice copulate frequently, with [[mating]] occurring during any period of the night or day.<ref name="Buxton3"/><ref name="Bacot">
{{cite journal
|author=Bacot A
|title=Contributions to the bionomics of ''Pediculus humanus (vestimenti)'' and ''Pediculus capitis''
|journal=Parasitology
|volume=9
|pages=228–258
|year=1917}}</ref> Mating attachment frequently lasts more than an hour.<ref name="Bacot"/> Young males can successfully pair with older females, and vice versa.<ref name="Buxton3"/><!-- SECONDARY REFERENCE - Buxton cites Buxton (1940) Parasitology 32:303-312 -->
Experiments with ''Pediculus humanus corporis'' (body lice) emphasize the attendant hazards of lice copulation. A single young female, confined with six or more males, will die in a few days, having laid very few eggs.<ref name="Buxton3"/> Similarly, death of a virgin female was reported after admitting a male to her confinement.<ref name="Bacot"/> The female laid only one egg after mating, and her entire body was tinged with red -- a condition attributed to rupture of the [[alimentary canal]] during the sexual act.<ref name="Bacot"/> Old females frequently die following, if not during, [[intercourse]].<ref name="Bacot"/>
Females lay an average of 3-4 eggs daily.{{Fact|date=May 2008}} During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits).{{Fact|date=May 2008}}
==Lifespan and colony persistence==
A generation lasts for about 1 month. {{Fact|date=May 2008}}
The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school) and socio economic status were found to be significant factors in head louse infestation . Girls are 2-4 times more frequently infested than boys. Children between 4 and 13 years of age are the most frequently infested group.<ref name="IJD">{{cite journal| last =Mumcuoglu | first =Kosta Y. | coauthors =Miller J, Gofin R, Adler B, Ben-Ishai F, Almog R, Kafka D, Klaus S. | title =Epidemiological studies on head lice infestation in Israel. I. Parasitological examination of children | journal =International Journal of Dermatology | volume =29 | pages =502–506 | publisher =[[International Society of Dermatology]] | location =[[Palm Coast, FL]] |date =[[1990]] | url =http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1365-4362.1990.tb04845.x | doi =10.1111/j.1365-4362.1990.tb04845.x | accessdate =2008-01-02 }}</ref>
==Behavior==
===Feeding===
All stages are blood-feeders and they bite the skin 4-5 times daily to feed. "''To feed, the louse bites through the skin and injects saliva which prevents blood from clotting; it then sucks blood into its digestive tract. Bloodsucking may continue for a long period if the louse is not disturbed. While feeding, lice may excrete dark red feces onto the skin.''"<ref name="UF">{{cite web|last =Weems, Jr. |first =H. V. |coauthors =Fasulo, T. R. |title =Human Lice: Body Louse, Pediculus humanus humanus Linnaeus and Head Louse, Pediculus humanus capitis De Geer (Insecta: Phthiraptera (=Anoplura): Pediculidae) |publisher =[[University of Florida]], Institute of Food and Agricultural Sciences | date=[[June 2007]] |url =http://edis.ifas.ufl.edu/in261 |accessdate =2008-02-21}}</ref>
===Position on host===
Although any part of the scalp may be colonized, lice favor the nape of the neck and the area behind the ears, where the eggs are usually laid. Head lice are repelled by light, and will move towards shadows or dark-colored objects in their vicinity.<ref name="Bacot"/> Lice have no wings and no powerful legs for jumping, so they move by using their claw-like legs to transfer from hair to hair.<ref name="UF"/> The most typical means of transmission is head to head contact; the next most typical is via transfer of some object (a brush, hat, stuffed animal, etc.) from the head of one host to the other.
===Migration===
Normally head lice infest a new host only by close contact between individuals, making social contacts among children and parent child interactions more likely routes of infestation than shared combs, brushes, towels, clothing, beds or closets. Head-to-head contact is by far the most common route of lice transmission.
==Distribution==
About 6-12 million people, mainly children, are treated annually for head lice in the United States alone. High levels of louse infestations have also been reported from all over the world including Israel, Denmark, Sweden, U.K., France and Australia.<ref name="pmid14651472">{{cite journal| last =Burgess | first =Ian | title =Human Lice and their Control | journal =Annual Review of Entomology | volume =49 | pages =457–481 | publisher =[[Annual Reviews]] | date =[[January 2004]] | url =http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.ento.49.061802.123253 | doi =10.1146/annurev.ento.49.061802.123253 | accessdate =2008-01-02 }}</ref><ref name="JDD">{{cite journal| last =Mumcuoglu | first =Kosta Y. | coauthors =Barker CS, Burgess IF, Combescot-Lang C, Dagleish RC, Larsen KS, Miller J, Roberts RJ, Taylan-Ozkan A. | title =International Guidelines for Effective Control of Head Louse Infestations | journal =Journal of Drugs in Dermatology | volume =6 | pages =409–414 | date =[[2007]] | accessdate =2008-01-02 }}</ref>
==As a disease vector==
[[Head lice]] (''Pediculus humanus capitis'') are not known to be vectors of diseases, unlike body lice(''Pediculus humanus humanus''), which are known vectors of epidemic or louse-borne typhus (''Rickettsia prowazeki''), trench fever (''Rochalimaea quintana'') and louse-borne relapsing fever (''Borrellia recurrentis'').
==Ancient lice - Use in archaeogenetics==
{{SectOR|date=January 2008}}
Lice are also important in the field of [[Archaeogenetics]]. Because most "modern" human diseases have in fact recently jumped from animals into humans through close agricultural contact, and also given fact that [[Neolithic]] human populations were too scattered to support contagious "crowd" diseases, lice (along with such parasites as intestinal [[Cestoda|tapeworms]]) are considered to be one of the few ancestral disease infestations of humans and other [[Hominidae|hominid]]s. As such, analysis of [[mitochondrial]] lice [[DNA]] has been used to map early human and archaic human migrations and living conditions. Because lice can only survive for a few hours or days without a human host, and because lice species are so specific to certain species or areas of the body, the evolutionary history of lice reveals much about human history. It has been demonstrated, for example, that some varieties of human lice went through a [[population bottleneck]] about 100,000 years ago (supporting the [[Single origin hypothesis]]), and also that hominid lice lineages diverged around 1.18 million years ago (probably infesting ''[[Homo erectus]]'') before re-uniting around 100,000 years ago. This recent merging seems to argue against the [[Multi-regional origin]] of modern human evolution and argues instead for a close proximity replacement of archaic humans by a migration of anatomically modern humans, either through inter-breeding, fighting, or being more fit to use available resources.
Analysis of the DNA of lice found on [[Peru]]vian [[Mummy|mummies]] have led to some surprising findings. The research indicates that some diseases (like [[typhus]]) may have passed from the [[New World]] to the [[Old World]], instead of the other way around.<ref name="Peru">{{cite web|last =Anderson |first = Andrea |title =DNA from Peruvian Mummy Lice Reveals History |work=GenomeWeb Daily News |publisher =GenomeWeb LLC |date =[[February 8]], [[2008]] |url =http://www.genomeweb.com/issues/news/144957-1.html |accessdate =2008-02-21}}</ref>
==See also==
*[[Treatment of human head lice]]
*[[Pediculosis]]
*[[Nitpicking]]
*[[Body louse]]
*[[Crab louse]]
*[[Lindane]]
*[[Delphinium]]
==References==
{{reflist|2}}
==External links==
{{wiktionary|head louse}}
* [http://www.cdc.gov/ncidod/dpd/parasites/lice/ Centers for Disease Control and Prevention: Division of Parasitic Diseases]
* [http://www.hsph.harvard.edu/headlice.html Harvard School of Public Health: Head Lice Information]
* [http://www.jcu.edu.au/school/phtm/PHTM/hlice/hlinfo1.htm James Cook University, Australia: Head Lice Information Sheet]
* [http://www.medicinenet.com/head_lice/article.htm MedicineNet.com: Head Lice Infestation (Pediculosis)]
* [http://www.phthiraptera.org/ Phthiraptera Central: Bibliography of Lice]
* [http://lancaster.unl.edu/pest/lice/ University of Nebraska: Head Lice Resources You Can Trust]
<br>
{{Human lice}}
[[Category:Lice]]
[[br:laou]]
[[de:Kopflaus]]
[[es:Phthiraptera]]
[[eo:Pediko]]
[[fr:Pediculus humanus]]
[[id:Kutu]]
[[it:Pediculus humanus capitis]]
[[lt:Galvinė utėlė]]
[[mk:вошка]]
[[nl:Hoofdluis]]
[[fi:Päätäi]]
[[sv:Huvudlus]]
[[zh:頭蝨]]