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Division of Communicable Disease Control
California Morbidity
March, 1996



Infestation of humans by lice (pediculosis) has been a problem and concern throughout history. The sucking lice, order Anoplura, are obligate, bloodsucking ecto-parasites of mammals. In general, they have evolved with their mammalian host and, consequently, are extremely host specific, rarely infesting even closely related species. The three kinds of lice infesting humans exhibit a set specificity and intimacy: 1) head louse, Pediculus humanus capitis DeGeer, 2) body louse, Pediculus humanus humanus Linnaeus; and 3) pubic or crab louse, Phthirus pubis Linnaeus. All are found world-wide wherever humans exist. The following discussion relates to head lice only:

Head lice are limited to the hair of the head, primarily near the nape of the neck and the ears. There is no apparent correlation with cleanliness or socioeconomic level, except under extreme conditions and neglect, which allow infestations to persist and spread. Their entire life cycle--egg, nymph, and adult--is spent on the host species, usually on the same individual. Those lice that do not transfer directly to a host usually perish within a short time, generally not surviving when off a host for more than a day or two.

The head louse is a small, elongate insect 2-3 mm in a length and is usually found grasping the hair shaft near the scalp with its specially adapted claws. The adult female louse may live for a month, laying 150 or more eggs (nits), up to 10 or more a day. The egg or nit (as it is most frequently called) is oval, about 1 mm long, yellowish opalescent in color, and with an operculum (cap) at its distal end. Each nit is firmly cemented at an oblique angle to the hair close to the scalp, where it takes about a week to hatch. The blood-feeding nymph then passes through three instars (stages) in a little more than a week to become an adult. The entire life cycle takes about three weeks. Although the head louse is seldom incriminated as a vector in disease transmission, the injection of saliva during feeding, as well as the deposition of excreta, usually causes pruritus with resultant scratching that can lead to secondary infection.

Transmission occurs either directly from contact with an h infested person or indirectly by fomites such as shared hair brushes, combs, head gear, scarves, coats, bedding, etc. Transmission via fomites in the school setting has been significantly reduced when students have been assigned individual lockers and adequately spaced coat hooks instead of shared locations. Because head lice require frequent blood meals and a warm, moist scalp environment, the vast majority remain on their host. Consequently, direct transmission, from person to person, is probably more frequent than is indirect transmission from fomites. Lice cannot jump or fly, nor are they transported from one host to another by flying insects. Due to their host specificity, lice are not acquired from dog and cat pets.

Because this is generally not a reportable disease, accurate figures on the magnitude of pediculosis in the United States are unavailable. Based upon the use of pediculicides in the United States, however, the National Pediculosis Association (NPA) estimates that as many as 12 million individuals are treated annually. The head louse appears to be resurgent throughout the United States, especially among school children. In addition, there is circumstantial evidence that head lice are becoming increasingly resistant to commercially available chemical treatments.


The diagnosis of head lice is based primarily on observing nits on hair shafts and, to a lesser extent, adult and nymphal lice on the scalp and hair. On the average, the actual number of adult head lice found on a person is low, seldom more than ten. Nits are readily identified by the trained eye, but they can be easily confused by the untrained. The uniformly oval shape of both the viable nit and empty shell (best seen with a hand-lens or low-power microscope), along with its firm attachment to the hair shaft, is a constant feature. Untrained observers can confuse nits with foreign materials such as solidified globules of hair spray, dandruff, or casts from hair follicles. These latter particles, besides being irregular in shape and size, are not firmly attached to the hair and will usually slide off the hair shaft, unlike the head louse nit. Since the nits are usually cemented to the base of the hair next to the scalp, those found 1/4 inch or more from the scalp (due to hair growth), are generally considered no longer viable.

Treatment and Control

Normal hair grooming and regular shampooing, even with vinegar, can neither eradicate nor prevent infestation by head lice. Fomites, with which the infested person has had contact, should also be treated. Pesticides used for treatment of pediculosis are formulated as shampoos, cream rinses, topical lotions, and sprays. The spray formulations are for use only on fomites and are not for use on humans. The California Department of Pesticide Regulation lists over 100 products, of which less than half are currently available. It is extremely important to use any pediculicides exactly as labeled, since numerous side effects and ineffective treatment can result from improper use.

The current treatment of choice utilizes permethrin, a synthetic pyrethroid, in a cream rinse formulation. In addition to its low toxicity, it generally requires only a single application because of its residual activity on the hair. This residual activity is effective against any nymphs emerging from viable nits. The next best treatment is pyrethrin-based (with piperonyl butoxide as a synergist) which is normally formulated as a shampoo. These require a second application 7 to 10 days later to kill the lice that have hatched since the first application. Both of the above pediculicides are over-the-counter items sold under various trade names. It should also be noted that the concurrent use of other hair care products, conditioners, gels, etc., can readily decrease the effectiveness of treatments.

Lindane, used since the 1950s, is both the least effective and, by far, the most toxic. Even though the "old is standby" Kwell® is no longer produced, other lindane-based products are still available and still require a prescription. Since 1983, the NPA has maintained that the potential toxicity of lindane far outweighs its possible benefits as a pediculicide. Among the adverse effects reported to the NPA's register are seizures, behavioral changes, neuromuscular complaints, attention deficit disorders, chronic skin eruptions, and death. It was stated in California Morbidity (April 17, 1987, #14) that: "Given that 1% lindane shampoo (Kwell®, etc.) is less effective and has more potential toxicity than the easily available alternatives, there is no reason to continue prescribing this material for the control of head lice in California."

In addition to pesticide treatment. nits should also be removed from the hair. Special fine-toothed combs are available that remove nits from the hair. Metal combs are preferable and are generally available through most pharmacies. Since the head louse nit has a diameter of about 0.3 mm, it is essential that the teeth of the comb be spaced around 0.1 mm apart to effectively remove the nits. As with the pesticides, the directions for the use of these combs and their sanitation must be clearly observed. Metal flea combs, available from most pet stores, are also effective and can be boiled in water after each use.

The household contacts of an infested person and the playmates of infested children are at great risk and should be carefully examined for lice. However, they should be treated only if an infestation is found. Prophylactic treatment should be encouraged only for bed mates. In order to destroy any possible lice or nits in clothing and bedding, they should be laundered in hot, soapy water, and then dried in a clothes dryer on the hot cycle. Nonwashable clothing can be dry-cleaned or isolated for several days. Spraying and fumigating schools and homes are not recommended and are strongly discouraged by most health professionals, including the Centers for Disease Control and Prevention, as an unnecessary potential health risk.

It is frequently recommended that children with head lice be kept out of school until treated. In addition, some schools require the removal of all nits before children are allowed to return. Concerned parents should consult their child's school regarding its lice policy.

Failure to discover and treat lice infestations promptly and correctly, not filth or suspected pesticide resistance, is responsible for the persistence and increased prevalence of head lice. Consequently, an increased awareness of pediculosis at all socioeconomic levels is essential, with an emphasis on following the directions for treatment exactly as recorded.

It is no disgrace to have lice,
but it is to keep them

Reported by: James C. Hitchcock, Sc.D., Richard M. Davis, D.Sc., and Vicki Kramer, Ph.D., Vector-Borne Disease Section, Disease Investigations and Surveillance Branch, Division of Communicable Disease Control, Department of Health Services.


  1. Altschuler DZ, Menditto L. Shampooing with poison: NPA issues, lindane alert to public. Press Release; January 1995, 5 pp. (NPA - Natl. Pediculosis Assn., PO Box 149, Newton, MA 02161; 1-800-446-4672)
  2. Barnes AM, Keh B. The biology and control of lice on man. Calif Vector Views; 1959, 6(2):7-11.
  3. Brown S, Becher J, Brady W. Treatment of ectoparasitic infections: review of the English-language literature, 1982-1992.Clin Infect Dis; 1995, 20 (Suppl 1): S104-9.
  4. Keh B. Answers to some questions frequently asked about pediculosis. Calif Vector Views; 1979, 26(5/6): 51-2.
  5. Orkin M, et al. Scabies and pediculosis. JB Lippincott, Phila; 1977, 203 pp.
  6. Poorbaugh JH. Head lice infestation - treatment failures with 1% lindane (Kwell®). Calif Morbidity; April 17, 1987, #14.
  7. Poorbaugh JH. Head lice infestation - update on control measures. Calif Morbidity; November 30, 1990, #47/48.


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