The fear of being preyed upon is nearly universal in human beings. Various phobias exist to such intentional
or accidental predators as sharks, spider, snakes, scorpions, tigers and bears. In this section of the
dermatology lectures we explore just such a predator, a parasite which must obtain regular meals of human blood
Stories of obligate human parasites are decades old, with the most well-known being the
vampire mythos. Dracula was written to be a horrifying story, but at least after he drank the victim's blood,
he flew out the window. We are now covering the head louse, another blood-sucking human predator, but there is
an important difference: head lice do not willfully leave the host. These creatures have a perfect existence,
eating and defecating on your head, meeting members of the opposite sex, copulating, and laying eggs on your
hair. They are referred to by some as "mechanized dandruff."
This is not a glamorous topic. In other therapeutics courses you are able to cover the more
compelling human conditions, such as diabetes mellitus, renal failure, and hepatic encephalopathy. However,
throughout this nonprescription products course we have studied the "minor" medical conditions which many
clinicians simply do not have sufficient time to cover (e.g., hemorrhoids, gingivitis, motion sickness,
and dandruff). However, for the sufferer they are obviously important, as sales of products marketed to treat
them continue to grow. Some of these conditions are also extremely embarrassing. For this reason, the
pharmacist must exercise great tact when counseling patients who require assistance.
THE HEAD LOUSE
General Information. There are three
types of human lice, each perfectly evolved for its ecological niche. The most common is the head louse,
Pediculus humanus, var. capitis. The second most common type, the crab louse (Phthirus pubis) is
placed into a separate genus(1). The least common type of human louse in the U.S. is the body louse,
Pediculus humanus, var. humanus or corporis (both terms have been used), which is very
similar to the head louse, differing mainly in its living preferences and some minor anatomic features. Whereas
the head louse lives on the head and lays its eggs on the hair shaft, the body louse lays its eggs in clothing
that is worn for many days without being washed, while obtaining blood meals from the person wearing the
clothing. Unless specifically mentioned, all of the remaining comments will refer to the most common type of
human louse, the head louse.
Paleontologists have yet to locate fossilized head lice, but there is a rich historical lore
surrounding lice, nonetheless. Archeologists are quite familiar with lice, having excavated mummies with lice,
ancient combs with nits, and records indicating that Egyptian priests shaved the heads to treat head lice(2).
Examination of ancient lice reveals that they are identical to the present-day predators.
Head lice belong to the order Phthiraptera, and to the group Anoplura. These
insects have no wings, nor can they jump. The pharmacist counseling a patient with head lice does not have to
fear that the lice will fly over or jump onto the pharmacist like a grasshopper. Their method of movement
relies on six legs, each of which terminates in a claw that is easily able to grasp human hair. Using claws
alone, lice can crawl through hair at an astounding 12 inches per minute, no mean feat for an insect that is
only 2-4 millimeters in length(2).
The head louse found on humans is a human parasite only, and cannot be contracted from the
family dog, cat, or any other animal. While other mammals and birds do indeed have lice, they are
species-specific, so much so that they are placed in different groups, families, or genera(3).
The Life Cycle. The female head louse lives for about 17-22 days, laying about 10
eggs daily, so she could potentially infest the human with approximately 200 eggs, assuming she locates a
sexually mature male whenever fertilization is required(1). She lays eggs (known as nits) on the hair shaft
itself, in the closest proximity possible to the scalp in most cases, at the skin-hair shaft junction. She
attaches the egg to the hair with an extremely strong cement that allows the nit to remain tightly affixed
despite regular combing and washing. Each oval egg is composed of a waxy material that protects the maturing
larva, with a perforated top to allow respiration. The larva normally matures to its birth time in 6-9 days,
using the body heat radiating from the host's scalp as an incubator(4). When it is ready to exit the egg case,
the immature insect begins to swallow air, expelling it rectally. This forms an expanding bubble which pushes
the insect through the top of the egg to the outside. This insect truly flatulates its way to life. Following
its birth, the nymph must obtain a blood meal from the host within 24 hours or it will not survive. After
birth, the nymph matures sexually in 7-10 days, meets a louse of the opposite sex and begins the life cycle
EPIDEMIOLOGY OF HEAD LOUSE INFESTATION
In order to delineate the
population groups most likely to contract head lice infestation, we need to first look at the methods by which
they are transmitted to patients. A primary method of transmission is host-to-host, usually direct head-to-head
contact. For instance, suppose an uninfested child is sleeping with an infested child (such as a sibling or a
friend) during a sleepover. If the hair of the infested child contacts the hair of the uninfested child, the
lice have a handy hair bridge that is approximately body temperature to facilitate colonization of the
unsuspecting new victim.
The mode of transmission is often less direct. Perhaps children's coats are kept in a pile or
on adjacent hooks at school or daycare. Lice on the infested child's coat can crawl onto another coat. Then,
they will enter the new child's scalp when the coat is worn. Similar fomite-to-person spread is seen when items
are shared by successive individuals, such as baseball helmets, computer headphones, portable CD player
headphones. Wearing another person's hats, scarves, or using their combs, brushes or other grooming aids may
When the mode of transmission is one of these less direct avenues, it is usually from
contacting an item that has very recently contacted the host's head. By contrast, lice are seldom contracted
through contact with parts of the environment which are not in intimate contact with a head (e.g.,
tables, couches, chairs, rugs). Lice do not mount expeditions, striking off to find new heads. Instead, since
they are obligate human parasites, they have evolved with the instinct to stay on the human at all costs. Thus,
the perception of one's entire house being contaminated is mistaken. With the above transmission methods in
mind, those most likely to become infested with head lice are patients with a family member or close friend who
Age. The average person thinks of head lice as a problem of childhood, and this
is usually true, in that the most common age infested is 6-11. These children are more likely to engage in
activities that allow prolonged head-to-head contact, such as taking nap breaks during school. Of course, once
a family has an infested member, age is no longer a consideration since families tend to have close contact
with each other regardless of age.
Gender. Females of all ages are more prone to contract head lice than males.
Young girls often enjoy close-contact play, such as "dress-up" and grooming each other and themselves. Boys, on
the other hand, are sociologically more inclined to play games that do not involve close contact with other
boys (chase games, hiding games, war simulation). Even within the family, the mother is more likely to provide
reassurance and nurturing that may involve sustained hugging.