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| Head louse | ||||||||||||||||
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| Scientific classification | ||||||||||||||||
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| Binomial name | ||||||||||||||||
| Pediculus humanus Linnaeus, 1758 | ||||||||||||||||
| Trinomial name | ||||||||||||||||
| 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, ectoparasitic, wingless insect spending its entire life on human scalp and feeding exclusively on human blood. Humans are the only known host of this parasite. Humans can also be infested with the pubic or crab louse (Pthirus pubis) and/or with the body louse (Pediculus humanus humanus). Lice infestation is known as pediculosis.
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The dorso-ventrally flattened body of the louse is divided into head, thorax and abdomen. On the head, one pair of eyes and one pair of antennae are clearly visible. The mouthparts are adapted to piercing the skin and sucking blood. The legs, with their terminal claws, are adapted to holding the hair-shaft. In males (Fig.1) 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 (see figure above). Head lice are 1-3 mm in size, varying according to their stage of development. They are usually grayish in color, but can appear reddish-brown soon after a blood-meal.
Fig. 1. Male head louse |
Like most insects, head lice are oviparous. Louse eggs contain a single embryo, and are attached near the base of a host hair shaft.Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA (2001). "Lice, nits, and school policy". Pediatrics 107 (5): 1011–5. PMID 11331679. 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:10.1016/j.jaad.2005.01.134. PMID 15965432. Eggs are generally laid within 1 cm of the scalp surface. 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 -- through which the embyro breathes. 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.
Each egg is oval-shaped and about 0.8 mm in length. They are brown so long as they contain an embryo, but appear white after hatching. Viable louse eggs hatch 6 to 9 days after oviposition.Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA (2001). "Lice, nits, and school policy". Pediatrics 107 (5): 1011–5. PMID 11331679. After hatching, the louse nymph leaves behind its egg shell, still attached to the hair shaft.
Look up nit in
Wiktionary, the free dictionary.
The term nit refers to either a louse egg or a louse nymph.nit - Definition from the Merriam-Webster Online Dictionary. Retrieved on 2008-02-25. With respect to eggs, this rather broad definition includes the following: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.
This has produced some confusion in, for example, school policy (see The "no-nit" policy below) because, of the three items listed above, only eggs containing viable embryos have the potential to infest or reinfest a host.Burgess IF (2004). "Human lice and their control". Annu. Rev. Entomol. 49: 457–81. doi:10.1146/annurev.ento.49.061802.123253. PMID 14651472. 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).
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.
Note that both of these quotations appear to reject the notion that louse nymphs are nits, and may indicate that the nit definition is currently in flux.
During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits). The egg hatches to the first nymphal stage, which after three moltings develop to nymph 2, nymph 3 and eventually to either a male or female louse. Adult lice copulate frequently and the females lay an average of 3-4 eggs daily. Mating can occur during any period of the night or day, with attachment frequently lasting over one hour.Bacot A (1917). "Contributions to the bionomics of Pediculus humanus (vestimenti) and Pediculus capitis". Parasitology 9: 228–258. A generation lasts for about 1 month. 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."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. During oviposition the female excretes a glue-like substance from a gland located at the posterior end of the body and attaches the eggs on the hair of the 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. 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. The most typical means of transmission is head to head contact followed by transfer of some object (a brush, hat, stuffed animal, etc.) from the head of one host to the other.
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.Burgess, Ian (January 2004). "Human Lice and their Control". Annual Review of Entomology 49: 457-481. Annual Reviews. doi:10.1146/annurev.ento.49.061802.123253. Retrieved on 2008-01-02. 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. Retrieved on 2008-01-02. 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. 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.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:10.1111/j.1365-4362.1990.tb04845.x. Retrieved on 2008-01-02.
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).
In order to diagnose infestation, the entire scalp should be combed thoroughly with a lice comb and the teeth of the comb should be examined for the presence of living lice after each time the comb passes through the hair. The use of a louse comb is the most effective way to detect living lice.Mumcuoglu, KY; Friger M, Ioffe Uspensky I, Ben Ishai F, Miller J. (January 2001). "Louse Comb Versus Direct Visual Examination for the Diagnosis of Head Louse Infestations". Pediatric Dermatology 18 (1): 9-12. Society for Pediatric Dermatology, International Society of Pediatric Dermatology, Dutch Pediatric Dermatology Society, Belgian Pediatric Dermatology Society, Latin American Pediatric Dermatology Society and the Spanish Society of Pediatric Dermatology. doi:10.1046/j.1525-1470.2001.018001009.x. Retrieved on 2008-01-02. In cases of children with long and curly/frizzy hair, an alternative method of diagnosis is examination by parting the hair at 2 cm intervals to look for moving lice near the scalp. With both methods, special attention should be paid to the area near the ears and the nape of the neck. The examiner should examine the scalp for at least 5 min. The use of a magnifying glass to examine the material collected between the teeth of the comb could prevent misdiagnosis. The presence of nits alone however (Fig. 4), is not an accurate indicator of an active head louse infestation. Children with nits on their hair have a 35-40% chance of also being infested with living lice and eggs.Williams LK, Reichert A, MacKenzie WR, Hightower AW, Blake PA (2001). "Lice, nits, and school policy". Pediatrics 107 (5): 1011–5. PMID 11331679. If lice are detected, the entire family needs to be checked (especially children up to the age of 13 years) with a louse comb and only those who are infested with living lice should be treated. As long as no living lice are detected, the child should be considered negative for head louse infestation. Accordingly, a child should be treated with a pediculicide ONLY when living lice are detected on his/her hair (not because he/she has louse eggs/nits on the hair and not because the scalp is itchy).
The most characteristic symptom of infestation is pruritus (itching) on the head which normally intensifies 3 to 4 weeks after the initial infestation. The bite reaction (Fig. 5) is very mild and it can be rarely seen between the hairs. Bites can be seen, especially in the neck of long-haired individuals when the hair is pushed aside. In rare cases, the itch scratch cycle can lead to secondary infection with impetigo and pyoderma. Swelling of the local lymph nodes and fever are rare. Head lice are not known to transmit any pathogenic microorganisms.
Fig. 4. Louse nits |
Fig. 5. Louse bites |
The "no-nit" policy, which is implemented in approximately 80% of schools in the United States[citation needed] and in parts of Canada and Australia, requires the dismissal of a child from a school, camp or childcare setting until all head lice, eggs and nits have been removed from the hair of an infested individual. The efficacy of the no-nit policy has been called into question by different groups of scientists and by several agencies, including The American Academy of Pediatrics and National Association of School Nurses (USA).[citation needed] There are no convincing studies proving that enforced exclusion policies are effective in reducing the transmission of lice. Therefore, some scientists and policy makers argue that the "no-nit" policy is ineffective and harmful and should be discontinued.Mumcuoglu, Kosta Y.; Meinking, Terri A; Burkhart, Craig N; Burkhart, Craig G. (August 2006). "Head Louse Infestations: The "No Nit" Policy and Its Consequences". International Journal of Dermatology 45 (8): 891-896. Palm Coast, FL: International Society of Dermatology. doi:10.1111/j.1365-4632.2006.02827.x. Retrieved on 2008-01-04.
In Australia, the National Health and Medical Research Council\'s Guidelines for Infectious Diseases warranting school exclusion exclude head lice if treatment begins before the child\'s next day of school or care.Staying Healthy in Child Care: Preventing infectious diseases in child care (4th ed.), Canberra: National Health and Medical Research Council, December 2005, <http://www.nhmrc.gov.au/publications/synopses/_files/ch43.pdf>
Dead eggs or empty egg-shells (nits) can remain attached to the hair for up to 6 months. In order to prevent a false-positive diagnosis and for aesthetic reasons, the hair could be treated with a regular conditioner and combed with a louse comb once or twice a week for a period of 2–3 months.Mumcuoglu, Kosta Y. (Jul-Sep 1999). "Prevention and Treatment of Head Lice in Children". Paediatric Drugs 1 (3): 211-218. Yardley, Pennsylvania: Adis International. Retrieved on 2008-01-03. Mumcuoglu, Kosta Y.; Miller, Jacqueline (2004). "The Relevance of Head Louse Comb and Nits to Head Louse Infestations.". Trends in Entomology 3: 113-117. Retrieved on 2008-01-04. The electronic nit comb makes nit identification easier, as it emits a beep when a nit is detected.Michigan Head Lice Manual (PDF) 16-17. Michigan Department of Community Health & Michigan Department of Education (July 2004). Retrieved on 2008-01-24.
Examination of the child’s head at regular intervals using a louse comb allows the diagnosis of louse infestation at an early stage. Early diagnosis makes treatment easier and reduces the possibility of infesting others. In times and areas when louse infestations are common, weekly examinations of children, especially those 4–13 yrs old, carried out by their parents will aid control. Additional examinations are necessary, if the child came in contact with infested individuals, if the child frequently scratches his/her head, or if nits suddenly appear on the child’s hair. Keeping long hair tidy could be helpful in the prevention of infestations with head lice. In order to prevent new infestations, the hair of the child could be treated with 2–4 drops of concentrated rosemary oil every day, before he/she leaves for school or kindergarten. The oils can be combed through the hair using a regular comb or brush.Mumcuoglu, Kosta Y.; R. Galun, U. Bach, J. Miller, and S. Magdassi (1996). "Repellency of Essential Oils and Their Components to the Human Body Louse, Pediculus humanus humanus". Entomologia Experimentalis et Applicata 78 (3): 309-314. Wezep: The Netherlands Entomological Society. Clothes, towels, bedding, combs and brushes, which came in contact with the infested individual, can be disinfected either by leaving them outside for at least 3 days or by washing them at 60°C for 30 minutes. An insecticidal treatment of the house and furniture is not necessary.
The number of cases of human louse infestations (or pediculosis) has increased worldwide since the mid-1960s, reaching hundreds of millions annually.Gratz, N. (1998). "Human lice, their prevalence and resistance to insecticides.". Geneva: World Health Organization (WHO). Retrieved on 2008-01-02. There is no product or method which assures 100% destruction of the eggs and hatched lice after a single treatment. However, there are a number of treatment modalities that can be employed with varying degrees of success. These methods include chemical treatments, natural products, combs, shaving, hot air, and silicone-based lotions.
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This section does not cite any references or sources. (January 2008) Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. |
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.Anderson, Andrea (February 8, 2008). DNA from Peruvian Mummy Lice Reveals History. GenomeWeb Daily News. GenomeWeb LLC. Retrieved on 2008-02-21.
Look up head louse in
Wiktionary, the free dictionary.
| Human lice and pediculosis | |
|---|---|
| Species |
Head louse · Crab louse · Body louse |
| Infestation |
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| Treatment |
Treatment of human head lice · Nitpicking · Lindane · Permethrin · Delphinium |
| Other terms of interest |
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