Lice Infestation

Lice Infestation



Lice infestations (pediculoses) are infections of the skin, hair, or genital region caused by lice living directly on the body or in hats or other garments. Lice are small wingless insect-like parasites with sucking mouthparts that feed on human blood and lay their eggs on body hairs or in clothing. The name pediculosis comes from the Latin word for louse (singular) or lice (plural). Some anthropologists believe that the different species of head and body lice developed in response to humans' invention and use of clothing about 50,000 years ago.


Lice infestations are not dangerous infections by themselves. It is, however a serious public health problem because some lice can carry organisms that cause other diseases, including relapsing fever, trench fever, and epidemic typhus. Although trench fever is self-limiting, the other two diseases have mortality rates of 5%-10%. Pubic lice are often associated with other sexually transmitted diseases (STDs) but do not spread them.
Lice infestations are frequent occurrences in areas of overcrowding or inadequate facilities for bathing and laundry. They are often associated with homelessness in the general population or with military, refugee, or prisoner camps in war-torn areas. All humans are equally susceptible to louse infestation; the elderly, however, are more vulnerable to typhus and other diseases carried by lice.

Causes and symptoms

The symptoms of lice infestations vary somewhat according to body location, although all are characterized by intense itching, usually with injury to the skin caused by scratching or scraping. The itching is an allergic reaction to a toxin in the saliva of the lice. Repeated bites can lead to a generalized skin eruption or inflammation.

Head lice

This type of infestation is caused by Pediculosis humanus capitis, the head louse. Head lice can be transmitted from one person to another by the sharing of hats, combs, or hair brushes. Epidemics of head lice are common among school-age children from all class backgrounds in all parts of the United States. The head louse is about 1/16 of an inch in length. The adult form may be visible on the patient's scalp, especially around the ears; or its grayish-white nits (eggs) may be visible at the base of the hairs close to the scalp. It takes between three and 14 days for the nits to hatch. After the nits hatch, the louse must feed on blood within a day or die.
Head lice can spread from the scalp to the eyebrows, eyelashes, and beard in adults, although they are more often limited to the scalp in children. The itching may be intense, and may be followed by bacterial infection of skin that has been scratched open. Another common complication is swelling or inflammation of the neck glands. Head lice do not spread typhus or other systemic diseases.

Body lice

Infestations of body lice are caused by Pediculosis humanus corporis, an organism that is similar in size to head lice. Body lice, however, are rarely seen on the skin itself because they come to the skin only to feed. They should be looked for in the seams of the patient's clothing. This type of infestation is associated by wearing the same clothing for long periods of time without laundering, as may happen in wartime or in cold climates; or with poor personal hygiene. It can be spread by close personal contact or shared bedding.
Patients with body lice often have intense itching with deep scratches around the upper shoulders, flanks, or neck. The bites first appear as small red pimples but may cause a generalized skin rash. If the infestation is not treated, the patient may develop complications that include headache, fever, and bacterial infection with scarring. Body lice can spread systemic typhus or other infections.

Pubic lice

Pubic lice are sometimes called "crabs." This type of infestation is caused by Phthirus pubis and is commonly spread by intimate contact. People can also get pubic lice from using the bedding, towels, or clothes of an infected person.
Pubic lice usually appear first on pubic hair, but may spread to other parts of the body, particularly if the patient is very hairy. Pubic lice are also sometimes seen on the eyelashes of children born to infected mothers. It is usually easier for the doctor to see marks from the patient's scratching than the bites from the lice, but pubic lice sometimes produce small bluish spots called maculae ceruleae on the patient's trunk or thighs. Pubic lice also sometimes leave small dark brown specks from their own excreted matter on the parts of the patient's underwear that cover the anal or genital areas.


Doctors can diagnose lice infestations from looking closely at the parts of the body where the patient has been scratching. Lice are large enough to be easily seen with the naked eye or a magnifying glass. The eggs of pubic lice as well as head lice can often be found by looking at the base of the patient's hairs. Pediatricians are most likely to diagnose lice in school-age children.
It is important for doctors to rule out other diseases that can cause scratching and skin inflammation because the medications used to kill lice are very strong and can have bothersome side effects. The doctor will need to distinguish between head lice and dandruff; between body lice and scabies (a disease caused by skin mites); and between pubic lice and eczema. Blood tests or other laboratory tests are not useful in diagnosing lice infestations.


Cases of head lice are usually treated with sham-poos or rinses containing either lindane (Kwell) or permethrin (Nix). Permethrin is considered preferable, however, because lindane is absorbed through the skin and may produce symptoms of neurotoxicity. The person applying the treatment should wear rubber gloves and rinse the patient's hair or body completely after use. Following the treatment, nits should be removed from the hair with a fine-toothed comb or tweezers. Lindane is also effective for treating infestations of body or pubic lice, but it should not be used by pregnant women. In most cases one treatment is sufficient, but the medication can be reapplied a week later if living lice have reappeared.
Another drug that appears to be effective in treating lice is ivermectin (Stromectol), a strong antiparasite drug that is usually given to treat intestinal worms. Two doses of the drug, however, cured 99% of cases of head lice as well as intestinal worms in a poor population with high rates of infestation with both types of parasites.
Infestations of body lice can also be treated by washing the patient's clothes or bedding in boiling water, ironing seams with an iron on a high setting, or treating the clothes with 1% malathion powder or 10% DDT powder.
If the patient's eyelashes have been infested, the only safe treatments are either a thick coating of petroleum jelly (Vaseline) applied twice daily for eight days, or 1% yellow oxide of mercury applied four times a day for two weeks. Any remaining nits should be removed with tweezers.
Patients with pubic lice should be examined and tested for other STDs.

Alternative treatment

For pubic lice, some practitioners of holistic medicine recommend a mixture of 25% oil of pennyroyal (Mentha pulegium), 25% garlic (Allium sativum) oil, and 50% distilled water applied three times in a three-day period, followed by removal of dormant eggs to prevent reinfestation.
Another alternative treatment for head lice is tea tree oil, sometimes called melaleuca oil. Tea tree oil appears to work well in treating head lice that have developed resistance to other preparations.


Lice can be successfully eradicated in almost all cases, although a growing number of cases of drug-resistant lice have been reported. As of 2004, some researchers are concerned about the rapid but unpredictable spread of these resistant lice. Ovide, a newer medication containing malathion, appears to be effective in treating patients with permethrin-resistant lice.
In general, patients are more at risk from typhus, trench fever, rickettsial infections, and other diseases spread by lice than from the lice themselves.


There are no vaccines or skin treatments that will protect a person against lice prior to contact. In addition, lice infestation does not provide immunity against reinfection; recurrences are in fact quite common. Prevention depends on adequate personal hygiene at the individual level and the following public health measures:
  • Teaching school-age children the basics of good personal hygiene, including the importance of not lending or borrowing combs, brushes, or hats.
  • Notifying and treating an adult patient's close personal and sexual contacts.
  • Examining homeless people, elderly patients incapable of self-care, and other high-risk individuals prior to hospital admission for signs of louse infestation. This measure is necessary to protect other hospitalized people from the spread of lice.



Beers, Mark H., MD, and Robert Berkow, MD., editors. "Pediculosis." In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.


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American Academy of Dermatology (AAD). 930 East Woodfield Road, Schaumburg, IL 60173. (847) 330-0230.

Key terms

Crabs — An informal or slang term for pubic lice.
Lindane — A benzene compound that is used to kill body and pubic lice. Lindane works by being absorbed into the louse's central nervous system, causing seizures and death.
Maculae ceruleae — Bluish or blue-grey skin eruptions often seen on the trunk or thighs of patients with pubic lice. The Latin words mean blue spots.
Malathion — An insecticide that can be used in 1% powdered form to disinfect the clothes of patients with body lice.
Nits — The eggs produced by head or pubic lice, usually grayish-white in color and visible at the base of hair shafts.
Pediculosis (plural, pediculoses) — The medical term for infestation with lice.
Permethrin — A medication used to rid the scalp of head lice. Permethrin works by paralyzing the lice, so that they cannot feed after hatching within the 24 hours required for survival.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.
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