HEAD LICE (PEDICULUS HUMANUS CAPITIS):
NITPICKY AND LOUSY PROBLEM
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
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
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
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
- 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
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infections: review of the English-language literature,
1982-1992.Clin Infect Dis; 1995, 20 (Suppl 1): S104-9.
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pediculosis. Calif Vector Views; 1979, 26(5/6): 51-2.
- Orkin M, et al. Scabies and pediculosis. JB Lippincott,
Phila; 1977, 203 pp.
- Poorbaugh JH. Head lice infestation - treatment failures
with 1% lindane (Kwell®). Calif Morbidity; April 17, 1987, #14.
- Poorbaugh JH. Head lice infestation - update on control
measures. Calif Morbidity; November 30, 1990, #47/48.