Head louse
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| Head louse | ||||||||||||||||
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| Pediculus humanus Linnaeus, 1758 |
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| Pediculus humanus capitis Charles De Geer, 1767 |
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Pediculus capitis (Charles De Geer, 1767) |
The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans.[1] Head lice are wingless insects spending their entire life on human scalp and feeding exclusively on human blood.[1] Humans are the only known host of this parasite.[1]
Head lice are closely related to body lice (Pediculus humanus corporis) which also infest Humans.[1] A more distantly-related species of louse, the pubic or crab louse (Pthirus pubis), also infests humans.[2] Lice infestation is known as pediculosis.[3]
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.[4] Head lice cannot fly, and their short stumpy legs render them quite incapable of jumping, or even walking efficiently on flat surfaces.[4]
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[edit] 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.[5] The thoracic segments are fused, but otherwise distinct from the head and abdomen, the latter being composed of seven visible segments.[6] Head lice are grey in general, but their precise color varies according to the environment in which they were raised.[6] After feeding, consumed blood causes the louse body to take on a reddish color.[6]
[edit] Head
One pair of 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.[5] Like other members of Anoplura, head lice mouthparts are highly adapted for piercing skin and sucking blood.[5] These mouthparts are retracted into the insect's head except during feeding.[6]
[edit] Thorax
Six legs project from the fused segments of the thorax.[6] As is typical in Anoplura, these legs are short and terminate with a single claw and opposing "thumb".[6] Between its claw and thumb, the louse grasps the hair of its host.[6] 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.[4]
Like other members of Anoplura, wings are entirely absent.[5]
[edit] Abdomen
There are seven visible segments of the louse abdomen.[6] The first six segments each have a pair of spiracles through which the insect breaths.[6] The last segment contains the anus and (separately) the genitalia.[6]
[edit] 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.
[edit] Louse Eggs
Like most insects, head lice are oviparous. Louse eggs contain a single embryo, and are attached near the base of a host hair shaft.[7][8] Eggs are generally laid within 1 cm of the scalp surface.[8] 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.[8] The glue was previously thought to be chitin-based , but more recent studies have shown it to be made of proteins similar to hair keratin.[8]
Each egg is oval-shaped and about 0.8 mm in length.[8] They are brown so long as they contain an embryo, but appear white after hatching.[8] Viable louse eggs hatch 6 to 9 days after oviposition.[7] After hatching, the louse nymph leaves behind its egg shell, still attached to the hair shaft.
[edit] Nits
The term nit refers to either a louse egg or a louse nymph.[9] With respect to eggs, this rather broad definition includes the following:[10]
- Viable eggs that will eventually hatch
- Remnants of already-hatched eggs
- Nonviable eggs (dead embryo) that will never hatch
This has produced some confusion in, for example, school policy (see 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.[11] Some authors have reacted to this confusion by restricting the definition of nit to describe only a hatched egg:
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)[11]
The empty eggshell, termed a nit...
—Maunder (1983)[4]
Others have retained the broad definition while simultaneously attempting to clarify its relevance to infestation:
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)[7]
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.
[edit] Development and nymphs
Head lice, like other insects of the suborder Anoplura, are hemimetabolous.[1] Newly hatched nymphs will molt three times before reaching the sexually-mature adult stage.[1] Thus, mobile head lice populations contain members of up to four developmental stages: 3 nymphal instars, and the adult (imago).[1] 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.[1] Nymph behavior is similar to the adult. Nymphs feed only on human blood (hematophagia), and cannot survive long away from a host.[1]
Like adult head lice, the nymph cannot fly or jump. However, there are reports of the light-weighted nymphs being blown by wind.[1] Similarly, after a molt, the discarded exoskeleton can be later shed by the host, and may be mistakenly interpreted as a viable louse.[4]
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.[1] 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.[1]
Nymph mortality is reportedly high -- about 38% -- especially within the first two days of life.[1] 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.[1] Death during molting can also occur, although it is reportedly uncommon.[1] During feeding, the nymph gut can rupture, dispersing the host's blood throughout the insect. This results in death within a day or two.[1] It is unclear if the high mortality recorded under experimental conditions is representative of conditions in the wild.[1]
[edit] Reproduction
Adult head lice 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.[1] Pairing can begin within the first 10 hours of adult life.[1] After 24 hours, adult lice copulate frequently, with mating occurring during any period of the night or day.[1][12] Mating attachment frequently lasts more than an hour.[12] Young males can successfully pair with older females, and vice versa.[1]
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.[1] Similarly, death of a virgin female was reported after admitting a male to her confinement.[12] 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.[12] Old females frequently die following, if not during, intercourse.[12]
Females lay an average of 3-4 eggs daily.[citation needed] During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits).[citation needed]
[edit] Lifespan and colony persistence
A generation lasts for about 1 month.[citation needed]
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.[13]
[edit] Behavior
[edit] 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."[14]
[edit] 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.[12] 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.[14] 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.
[edit] 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.
[edit] 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.[11][15]
[edit] 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).
[edit] Ancient lice - Use in archaeogenetics
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This section may contain original research or unverified claims. Please help Wikipedia by adding references. See the talk page for details.(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 tapeworms) are considered to be one of the few ancestral disease infestations of humans and other hominids. 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 Peruvian 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.[16]
[edit] See also
[edit] References
- ^ a b c d e f g h i j k l m n o p q r s t u v Buxton, Patrick A. [1947]. "The biology of Pediculus humanus", The Louse; an account of the lice which infest man, their medical importance and control, 2nd edition, London: Edward Arnold, 24-72.
- ^ Buxton, Patrick A. [1947]. "The crab louse Phthirus pubis", The Louse; an account of the lice which infest man, their medical importance and control, 2nd edition, London: Edward Arnold, 136-141.
- ^ pediculosis - Definition from the Merriam-Webster Online Dictionary. Retrieved on 2008-04-23.
- ^ a b c d e Maunder, JW (1983). "The Appreciation of Lice". Proceedings of the Royal Institution of Great Britain 55: 1-31. London: Royal Institution of Great Britain. 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.
- ^ a b c d Buxton, Patrick A. [1947]. "The Anoplura or Sucking Lice", The Louse; an account of the lice which infest man, their medical importance and control, 2nd edition, London: Edward Arnold, 1-4.
- ^ a b c d e f g h i j Buxton, Patrick A. [1947]. "The Anatomy of Pediculus humanus", The Louse; an account of the lice which infest man, their medical importance and control, 2nd edition, London: Edward Arnold, 5-23.
- ^ a b c Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA (2001). "Lice, nits, and school policy". Pediatrics 107 (5): 1011–5. doi:. PMID 11331679.
- ^ a b c d e f Burkhart CN, Burkhart CG (2005). "Head lice: scientific assessment of the nit sheath with clinical ramifications and therapeutic options". J. Am. Acad. Dermatol. 53 (1): 129–33. doi:. PMID 15965432.
- ^ nit - Definition from the Merriam-Webster Online Dictionary. Retrieved on 2008-02-25.
- ^ Pollack RJ, Kiszewski AE, Spielman A (2000). "Overdiagnosis and consequent mismanagement of head louse infestations in North America". Pediatr. Infect. Dis. J. 19 (8): 689–93; discussion 694. PMID 10959734.
- ^ a b c Burgess IF (2004). "Human lice and their control". Annu. Rev. Entomol. 49: 457–81. doi:. PMID 14651472.
- ^ a b c d e f Bacot A (1917). "Contributions to the bionomics of Pediculus humanus (vestimenti) and Pediculus capitis". Parasitology 9: 228–258.
- ^ Mumcuoglu, Kosta Y.; Miller J, Gofin R, Adler B, Ben-Ishai F, Almog R, Kafka D, Klaus S. (1990). "Epidemiological studies on head lice infestation in Israel. I. Parasitological examination of children.". International Journal of Dermatology 29: 502-506. Palm Coast, FL: International Society of Dermatology. doi:.
- ^ a b Weems, Jr., H. V.; Fasulo, T. R. (June 2007). Human Lice: Body Louse, Pediculus humanus humanus Linnaeus and Head Louse, Pediculus humanus capitis De Geer (Insecta: Phthiraptera (=Anoplura): Pediculidae). University of Florida, Institute of Food and Agricultural Sciences. Retrieved on 2008-02-21.
- ^ Mumcuoglu, Kosta Y.; Barker CS, Burgess IF, Combescot-Lang C, Dagleish RC, Larsen KS, Miller J, Roberts RJ, Taylan-Ozkan A. (2007). "International Guidelines for Effective Control of Head Louse Infestations". Journal of Drugs in Dermatology 6: 409-414.
- ^ Anderson, Andrea (February 8, 2008). DNA from Peruvian Mummy Lice Reveals History. GenomeWeb Daily News. GenomeWeb LLC. Retrieved on 2008-02-21.
[edit] External links
- Centers for Disease Control and Prevention: Division of Parasitic Diseases
- Harvard School of Public Health: Head Lice Information
- James Cook University, Australia: Head Lice Information Sheet
- MedicineNet.com: Head Lice Infestation (Pediculosis)
- Phthiraptera Central: Bibliography of Lice
- University of Nebraska: Head Lice Resources You Can Trust
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