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Lice Infestation (Pediculosis)

Lice Infestation (Pediculosis)


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Of the many parasites that can infest humans, one of the most common is the louse, a wingless insect. There are several types of lice that infect humans, usually classified as one of three species. These are the head louse (Pediculus humanus capitis), which infests only the head; the body louse Pediculus humanus corporis), which lives in clothing near the skin; and the crab louse or pubic louse (Phthirus pubis), which mostly infests the groin. Infestation with lice is called pediculosis (ped-ih-q-LO-sis). Recently some biologists have argued that the head louse and body louse may be different varieties of a single species. Head and body lice can interbreed in captivity, but do not do so on the human body.

Disease History, Characteristics, and Transmission

Human lice can exist only on human beings; they die in about 24 hours if they are removed from the body. Lice also infest humans’ nearest evolutionary cousins, the chimpanzees and gorillas, but these lice belong to a different species than those that infest humans. As apes and humans continued to evolve over the last few million years, their lice evolved along with them.

The female head or body louse lays several eggs a day. Lice eggs are called nits (the source of the word “nitpick”) and are cemented to the hair or, in the case of body lice, to clothing fibers. The eggs take 7–10 days to hatch. A female louse can start laying eggs 7–10 days after hatching. A louse bites through the skin to suck blood from its host about five times a day.

On a healthy host, lice can cause itching, rash, fever, headaches, and fatigue, but are rarely life-threatening. However, body lice can act as carriers of more serious diseases. Three types of disease can be transmitted by body lice to the humans that they bite, namely relapsing fever (caused by the bacterium Borrelia recurrentis), trench fever (caused by the bacillus Bartonella quintana), and—most seriously—typhus (caused by Rickettsia prowazekii).

Lice spread by crawling from one host to another or through the transfer of eggs. The majority of head lice are spread by head to head contact or, more rarely, by coming into contact with objects that have picked up eggs from the hair, such as combs, pillows, hats, hair ties, and the like. Body lice are spread through body contact or shared clothing and bedding. Pubic lice are spread primarily through sexual contact or other body contact.


PARASITE: An organism that lives in or on a host organism and that gets its nourishment from that host. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.


The Division of Parasitic Diseases (DPD), Centers for Disease Control and Prevention (CDC) recommends that “for children under 2 years old, remove crawling bugs and nits by hand. If this does not work, ask your child's health care provider for treatment recommendations. The safety of head lice medications has not been tested in children 2 years of age and under.”

SOURCE: Centers for Disease Control and Prevention

Scope and Distribution

Throughout history, lice infestation has been common in most populations. Dead lice and eggs have been found on Egyptian mummies and Roman bodies buried under volcanic ash at Pompeii. About 6–12 million people acquire head lice in the United States each year; smaller numbers acquire body or pubic lice.

The head louse is still found worldwide at all levels of society. Head lice are extremely common in developing countries. In Western industrialized countries, outbreaks are often associated with schoolchildren.

With the Industrial Revolution and the spread of bathing technology through much of the modern world—indoor plumbing, soap, shampoo, laundry machines, and detergent—the body louse has become less common. Today human body lice are found mostly in situations where poor hygiene, overcrowding, and wearing the same clothing for extended periods are more common—whether these be entire countries or impoverished groups, such as the homeless, in richer countries.

Treatment and Prevention

Prevention of lice infestation is accomplished by treating those who are infested and by environmental control (cleaning objects that may have picked up eggs). The two methods of treating lice infestation are pesticides (chemicals that kill insects or other pests) and physical removal of the lice via lice combs. The pesticides most often used are permethrin and pyrethrins, followed by malathion or lindane. However, as is common with pesticides, heavily exposed populations of lice have evolved resistance to these chemicals. The U.S. National Pediculosis Association advises against the use of pesticides on any person with pre-existing illnesses such as severe asthma, epilepsy, cancer, or AIDS. Fine-toothed steel combs can also be used to remove head lice and nits from hair.

Impacts and Issues

According to studies reported in the Annals of the New York Academy of Sciences in 2006, lice and louse-borne disease were being increasingly reported among homeless and poor inner-city populations in industrialized countries such as the United States, France, Holland, and Russia.

Wars and social breakdown can lead to large outbreaks of lice, as can any condition where people exist in crowded areas without acess to sanitation and clean clothing. A large outbreak of typhus occurred in several refugee camps in Burundi in 1997 where most of the inhabitants were louse-infested.

The safety of the pesticides used in standard anti-lice products is questioned by the American Pediculosis Association, which has campaigned for the use of finetoothed combs as the treatment of choice in removing lice. Some individuals can have allergic reactions to the pesticides used to treat pediculosis.


Schools and daycare facilities often exclude children for 24 hours after treatment for head lice, or they maintain a “no-nit policy” that excludes treated children until nits are not visible upon inspection of the scalp. Evidence shows these policies often result in the needless loss of instructional time. Usually by the time a case of head lice is discovered, the possibility of transmission to others has already existed for at least a month, and pesticide treatment quickly kills both adult lice and nits. Also, killed nits may temporarily remain in the hair after treatment.

See AlsoParasitic Diseases; Typhoid Fever; Typhus.



Elston, Dirk M. “Drugs Used in the Treatment of Pediculosis.” Journal of Drugs in Dermatology 4.2 (March-April 2005): 207–211.

Raoult, Didier, and Véronique Roux. “The Body Louse as a Vector of Reemerging Human Diseases.” Clinical Infectious Diseases 29 (1999): 888–911.

Wade, Nicholas. “What a Story Lice Can Tell.” New York Times (October 5, 2004).

Witkowski, Joseph A., and Lawrence Charles Parish. “Pediculosis and Resistance: The Perennial Problem.” Clinics in Dermatology 20 (2002): 87–92.

Web Sites

The National Pediculosis Association. “Welcome to” 2007. <> (accessed January 22, 2007).

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