Understanding Scabies and Lice

Infectious Disease Perspective

Scabies and lice are common public health problems.1 They are classified by infectious disease experts as “ectoparasitic infections,” because they are caused by parasites that live on or within the outer surfaces of the skin rather than on the inside of the body. They thrive and multiply by feeding off the blood of their human hosts.2–4 Both conditions are highly contagious and easily spread by direct physical contact. Scabies and pubic (crab) lice are also common sexually transmitted diseases (STDs).5

What Is Scabies?

Sarcoptes scabiei is a highly-contagious parasitic mite

The scabies mite is white, oval, and too small to be seen by the naked eye without magnification.1

Scabies is caused by 8-legged mites, known as Sarcoptes scabiei, that burrow into the outer layer of the skin.3 They are often called “itch mites” because they can cause intense, unbearable itching.1,6

Scabies live and multiply in the outer layer of the skin:

The female scabies mite digs a shallow tunnel in the outer layer of human skin where she will live and lay 1 to 2 dozen eggs during her 4-week lifespan1
Within 2 months, an infected person may have 25 adult female mites living in his or her skin1
Within 3 to 4 months, there may be as many as 500 mites1

Scabies mites burrow into the outer layer of the skin.

Many parts of the body can be affected by scabies:

In older children and adults, the most common sites of scabies infestation include the genitals, buttocks, fingers, wrists, elbows, armpits, knees, and ankles3
Younger children and infants show more widespread involvement, including the palms and soles of the feet7
Scratching can spread scabies to other areas of the body3

Common sites of scabies infestation
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Scabies is associated with a bumpy red rash and intense itching; scratching can lead to sores and secondary bacterial infection8,10

Scabies is spread by close personal contact

Direct and prolonged skin-to-skin contact is usually how scabies is spread. For example, a brief handshake with an infected person does not typically transmit scabies, and only rarely is it contracted by way of contaminated clothing, towels, or bedding.8,9 However, scabies is easily spread within families and to sexual partners—it is a common STD.5

Who Gets Scabies?

Scabies is easily spread to sexual partners, within families, and among people living in crowded conditions11
Hospitals, nursing homes, and long-term care facilities are likely sites of scabies epidemics11
People with weakened immune systems, such as those with HIV/AIDS, and the elderly are at risk for Norwegian (crusted) scabies, a more severe form of the disease8

What Are Lice?

enlarged microscopic image of head louse

The human head louse (singular) is 1 to 2 mm in length—about the size of a sesame seed. Its sharp claws are used to grasp hair and skin.13,14

Lice are highly-contagious parasitic insects

Lice are six-legged blood-sucking parasitic insects that live near the surface of the skin, often clinging to the shafts of human hair. They can travel quickly, up to 10 inches per minute, which explains why they are so contagious.4,12 Infestation with lice is called “pediculosis.” In the U.S., head lice and pubic (crab) lice are the most common forms of pediculosis.4

Lice mate and feed up to 5 times a day:13,14

The female louse (singular) lays as many as 300 eggs or “nits” during her 3- to 4-week lifespan1,12
Nits, which take about 2 weeks to hatch, become firmly cemented to hair shafts and are difficult to remove12,15
Lice feed by piercing the skin with their sharp claws, injecting saliva, and sucking out blood14
Louse saliva and fecal matter can cause sensitivity reactions, with itching and irritation; scratching can lead to secondary bacterial infection4,13

Lice are typically spread between people who know each other16

Heavy lice infestation of the scalp.

Head lice (Pediculus humanus var. capitis):

Direct and prolonged head-to-head contact is the usual mode of head lice transmission; as such, lice are typically spread between people who know each other4,14
Sharing of inanimate objects that come in contact with the hair or scalp, such as hats, scarves, or hairbrushes, may also cause the spread of head lice, though this is less common4
Contrary to popular belief, lice cannot jump from one head to another, and they cannot survive for long off of the human body4

Pubic (crab) lice attach themselves to pubic hair, but can spread to the upper thighs, abdominal area, armpits, chest and beard.4

Pubic (crab) lice (Pthirus pubis):

Pubic lice (or crab lice) are usually transmitted sexually and are among the most contagious of STDs 4
Condoms do not prevent the spread of pubic lice1
Nonsexual transmission of pubic lice is also possible1,17
Young children can get crab lice of the eyelashes from contact with an individual affected by pubic lice17

Infestation of the eyelashes with pubic lice.

Social and Economic Impact of Lice

Lice infestation carries a painful social stigma due to the misconception that it is related to poor hygiene and only occurs among the poor4,13
In a single year, children in the U.S. are estimated to lose as many as 24 million days of school because of lice; parents are often forced to miss work or pay for child care18,19
Direct and indirect costs of lice, such as lost work productivity, are estimated to total hundreds of millions of dollars in the U.S. alone19

References:

Orion E, Matz H, Wolf R. Ectoparasitic sexually transmitted diseases: scabies and pediculosis. Clin Dermatol. 2004;22(6):513–519.
Roos TC, Alam M, Roos S, et al. Pharmacotherapy of ectoparasitic infections. Drugs. 2001;61(8):1067–1088.
Habif TP. Scabies. In: Clinical Dermatology, 4th edition. New York: Mosby; 2004:497–505.
Habif TP. Pediculosis. In: Clinical Dermatology, 4th edition. New York: Mosby; 2004:506–510.
U.S. Centers for Disease Control and Prevention (CDC). Ectoparasitic infections. Sexually transmitted diseases treatment guidelines. MMWR Recomm Rep. 2002, May 10;51(No. RR-6):67–69. Web version available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5106a1.htm.
McCarthy JS, Kemp DJ, Walton SF, et al. Scabies: more than just an irritation. Postgrad Med J. 2004;80:382–387.
Paller AS, Mancini J. Insect bites and parasitic infestations. In: Hurwitz Clinical Pediatric Dermatology: A Textbook of Skin Disorders of Childhood and Adolescence. London: Elsevier; 2006, Ch. 18, p. 479.
U.S. Centers for Disease Control and Prevention (CDC). Parasitic Disease Information: Scabies Fact Sheet. 2005. Available at: http://www.cdc.gov/ncidod/dpd/parasites/scabies/factsht_scabies.htm.
Johnston G, Sladden M. Scabies: diagnosis and treatment. British Med J. 2005;331:619–622.
Walton SF, Holt DC, Currie BJ, et al. Scabies: new future for a neglected disease. Adv Parasitol. 2004;57:309–376.
Wendel K, Rompalo A. Scabies and pediculosis pubis: an update of treatment regimens and general review. Clin Infect Dis. 2002;35:S146–S151.
Frankowski BL, Weiner LB. Head lice: Guidance for the clinician in rendering pediatric care. Pediatrics. 2002;110(3);638–643.
Guenther L, Maguiness S, Austin TW. Pediculosis. 2005. Available at: http://www.emedicine.com/med/topic1769.htm.
Hansen RC. Overview: the state of head lice management and control. Am J Manag Care. 2004;10(9 Suppl):S260–S263.
Ko CJ, Elston DM. Pediculosis. J Am Acad Dermatol. 2004;50(1):1–12; quiz 13–4.
Nash B. Extracts from “best treatment”: Treating head lice. British Med J. 2003;326:1256–1257.
U.S. Centers for Disease Control and Prevention (CDC). Parasitic Disease Information: Pubic Lice Infestation Fact Sheet. 2005. Available at: http://www.cdc.gov/ncidod/dpd/parasites/lice/factsht_pubic_lice.htm.
Roberts RJ. Clinical practice. Head lice. N Engl J Med. 2002;346(21):1645–50.
West DP. Head lice treatment costs and the impact on managed care. Am J Manag Care. 2004;10(9 Suppl):S277–282.