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Crab louse

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Crab louse
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Psocodea
Family: Pthiridae
Genus: Pthirus
P. pubis
Binomial name
Pthirus pubis
  • Pediculus pubis Linnaeus, 1758
  • Phthirus pubis Leach, 1817[1]

The crab louse or pubic louse (Pthirus pubis) is an insect that is an obligate ectoparasite of humans, feeding exclusively on blood.[2] The crab louse usually is found in the person's pubic hair. Although the louse cannot jump, it can also live in other areas of the body that are covered with coarse hair, such as the perianal area, the entire body (in men), and the eyelashes (in children).[3][4]

Humans are the only known hosts of the crab louse, although a closely related species, Pthirus gorillae, infects gorillas. The human parasite is thought to have diverged from Pthirus gorillae approximately 3.3 million years ago.[5] It is more distantly related to the genus Pediculus, which contains the human head and body lice and lice that affect chimpanzees and bonobos.


An adult crab louse is about 1.3–2 mm long (slightly smaller than the body louse and head louse), and can be distinguished from those other species by its almost round body. Another distinguishing feature is that the second and third pairs of legs of a crab louse are much thicker than the front legs and have large claws.[6]

Life cycle

The eggs of the crab louse are laid usually on the coarse hairs of the genital and perianal regions of the human body. The female lays about three eggs a day. The eggs take 6–8 days to hatch, and there are three nymphal stages which together take 10–17 days before the adult develops, making a total life cycle from egg to adult of 16–25 days. Adults live for up to 30 days.[6] Crab lice feed exclusively on blood, and take a blood meal 4–5 times daily. Outside the host they can survive for 24–48 hours. Crab lice are transmitted from person to person most commonly via sexual contact, although fomites (bedding, clothing) may play a minor role in their transmission.[7]

Infestation of humans

Crab lice on eyelashes
Live louse magnified about 10x
Live louse magnified about 10x

Infestation of the eyelashes is referred to as pediculosis ciliaris or phthiriasis palpebrarum.[8]

The main symptom of infestation with crab lice is itching, usually in the pubic-hair area, resulting from hypersensitivity to louse saliva, which can become stronger over two or more weeks following initial infestation. In some infestations, a characteristic grey-blue or slate coloration appears (maculae caeruleae) at the feeding site, which may last for several days.[citation needed]

Crab louse egg on human body hair

Current worldwide prevalence has been estimated at 2% of the human population, but accurate numbers are difficult to gauge because crab louse infestations are not considered a reportable condition by many health authorities, and many cases are self-treated or treated discreetly by physicians.[9]

It has been suggested that an increasing percentage of humans removing their pubic hair, especially in women, has led to reduced crab louse populations in some parts of the world.[10][11]

Crab lice

Crab lice are not known to transmit disease. In infested individuals an average of a dozen lice can be found. Although they are typically found attached to hair in the pubic area, sometimes they are also found on coarse hair elsewhere on the body (for example, eyebrows, eyelashes, beard, moustache, chest, armpits, etc.). They do not generally occur on the finer hair of the scalp.[6][12] Crab lice attach to pubic hair that is thicker than other body hair because their claws are adapted to the specific diameter of pubic hair and other thick hairs of the body.[13] Crab louse infestations (pthiriasis) are usually spread through sexual contact and are most common in adults.[13][3][4] The crab louse can travel up to 25 cm (10 in) on the body. Crab louse infestation is found worldwide and occurs in all races and ethnic groups and in all socio-economic levels. Occasionally they may be also transmitted by close personal contact or contact with articles such as clothing, bed linen, and towels that have been used by an infested person.[7][14]

Crab lice found on the head or eyelashes of children may be an indication of sexual exposure or abuse.[15] Symptoms of crab louse infestation in the pubic area include itching, redness and inflammation. Crab lice are not known to transmit disease; however, secondary bacterial infection can occur from scratching of the skin.[citation needed]

Crab louse infestation can be diagnosed by identifying the presence of active stages of the louse, as well as of eggs (nits) on the pubic hair and other hairs of the body. When infestation is diagnosed, other family members and contact persons should also be examined. A magnifying glass or dermoscope could be used for better identification.[13]


  1. ^ "Taxonomy of Human Lice". Phthiraptera.info. Archived from the original on April 14, 2020.
  2. ^ Rapini, Ronald P.; Bolognia, Jean L.; Jorizzo, Joseph L. (2007). Dermatology: 2-Volume Set. St. Louis: Mosby. ISBN 978-1-4160-2999-1.
  3. ^ a b CDC (Centers for Disease Control and Prevention) (2017-05-02). "Parasites: lice". www.cdc.gov. Retrieved 2017-12-04.Public Domain This article incorporates text from this source, which is in the public domain.
  4. ^ a b CDC (Centers for Disease Control and Prevention) (2017-05-02). "Parasites: pubic "crab" lice". www.cdc.gov. Retrieved 2017-12-04.Public Domain This article incorporates text from this source, which is in the public domain.
  5. ^ Weiss RA (10 February 2009). "Apes, lice and prehistory". J Biol. 8 (2): 20. doi:10.1186/jbiol114. PMC 2687769. PMID 19232074.
  6. ^ a b c Service, Mike (2012). Medical Entomology for Students (5th ed.). Cambridge: Cambridge University Press. ISBN 978-1-107-66818-8.
  7. ^ a b CDC (Centers for Disease Control and Prevention) (2017-05-02). "Pubic "crab" lice: biology". www.cdc.gov. Retrieved 2017-12-04.Public Domain This article incorporates text from this source, which is in the public domain.
  8. ^ Manjunatha NP, Jayamanne GR, Desai SP, Moss TR, Lalik J, Woodland A. Pediculosis pubis: presentation to ophthalmologist as phthriasis palpebrarum associated with corneal epithelial keratitis. Int. J. STD AIDS 2006; 17: 424–426
  9. ^ Anderson AL, Chaney E (February 2009). "Pubic lice (Pthirus pubis): history, biology and treatment vs. knowledge and beliefs of US college students". Int J Environ Res Public Health. 6 (2): 592–600. doi:10.3390/ijerph6020592. PMC 2672365. PMID 19440402.
  10. ^ Armstrong, N. R.; Wilson, J. D. (2006). "Did the "Brazilian" kill the pubic louse?". Sexually Transmitted Infections. 82 (3): 265–266. doi:10.1136/sti.2005.018671. PMC 2564756. PMID 16731684.
  11. ^ Bloomberg: Brazilian bikini waxes make crab lice endangered species, published 13 January 2013, retrieved 14 January 2013
  12. ^ Nuttall, GHF (1918). "The biology of Phthirus pubis". Parasitology. 10 (3): 383–405. doi:10.1017/s0031182000003954.
  13. ^ a b c Hoffman, Barbara L.; Williams, J. Whitridge (2012). Williams gynecology (2nd ed.). New York: McGraw-Hill Medical. ISBN 9780071716727. OCLC 779244257.
  14. ^ CDC (Centers for Disease Control and Prevention) (2017-05-02). "Pubic "crab" lice: epidemiology & risk factors". www.cdc.gov. Retrieved 2017-12-04.Public Domain This article incorporates text from this source, which is in the public domain.
  15. ^ Klaus S, Shvil Y, Mumcuoglu KY (1994). "Generalized infestation of a 3 1/2-year-old girl with the pubic louse". Pediatric Dermatology. 11 (1): 26–8. doi:10.1111/j.1525-1470.1994.tb00068.x. PMID 8170844. S2CID 5989980.

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