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INTRODUCTION.
These human-infesting lice receive their common names from the areas of the human body
which they typically infest. It must be realized that an infestation of any of these is a
medical problem, not a pest control problem. However, the PCO can perform a valuable
public service by serving as a source of pertinent information to both institutional
personnel and individuals. Only the body louse is a vector of human diseases, namely
typhus, relapsing fever, and trench fever. These lice are found worldwide and throughout
the United States.
RECOGNITION.
Adults small, about 1116-118" (2-4.2 mm) long. Body flattened dorsoventrally (top to
bottom); wingless. Color dirty white to grayish black. Head small, narrower than thorax;
compound eyes small, ocelli absent; mouthparts sucking, withdrawn into head when not in
use. Antenna short, threadlike, 5-segmented. Leg with tibia short, bearing a thumblike
structure on its inner surface opposite the claw; tarsi 1 -segmented, with 1 large claw;
actoparasites of humans. In particular -
Head louse, Pediculus humanus capitatus DeGeer.
About 1/8" (2.4-3.7 mm) long; body flattened but elongate in form; head only slightly
narrower than thorax; 3 pairs of similar legs; tarsi with I large claw for grasping round
hairs between tibial thumblike process and its opposing claw; abdominal segments without
lateral lobes; occurs almost exclusively on the head and attaches its eggs (nits) to
hairs.
Body louse, Pediculus humanus humanus Linnaeus.
About 1/8" (2.8-4.2 mm) long, body flattened but elongate in form; head only slightly
narrower than thorax; 3 pairs of similar legs; tarsi with I large claw for grasping round
hairs between tibial thumblike process and its opposing claw; abdominal segments without
lateral lobes; occurs primarily in clothing and moves to adjacent body areas to feed, and
attaches its eggs (nits) primarily to clothing fibers.
Crab/pubic louse, Pthirus pubis (Linnaeus). About
1/16" (2 mm) long; body flattened but broadly oval in form; head much narrower than
thorax; front legs much more slender (look smaller) than middle and hind legs; tarsal claw
large, adapted for grasping flat hairs between tibial thumblike process and its opposing
claw; abdominal segments with hairy lateral lobes; occurs primarily where flat hairs are
present (pubic region, arm pits, eyebrows, beards), and attaches its eggs (nits) to body
hairs.
SIMILAR GROUPS.
Chewing lice (order Mallophaga) with chewing
mouthparts, head usually as wide as or wider than thorax, tarsi with 1-2 small claws, and
ectoparasites of birds and mammals (excluding humans).
Psocids or booklice (order Psocoptera) with
chewing mouthparts, antennae long and slender, tarsi 2-3-segmented, not adapted for
grasping, and mold and starch feeders, not parasitic.
SIGNS OF INFESTATION AND INCIDENCE.
Head louse. The presence of eggs/nits is the most
important indication of a problem because they are more numerous than lice, not active,
and not easily removed. Scratching of the back of the scalp or the nape of the neck is
cause to suspect lice. Scratching associated with lice is likely to lead to secondary
infections which are more likely to cause itching than the bites themselves. The
socioeconomic status or income level of the person or family has little to do with who
gets infested. Infestations are particularly common among younger school children.
Caucasian (white) children are more likely than black children to have lice. Girls are
more likely to be infested than boys, up to 5% more likely. Cutting or wearing shorter
hair does not prevent or eliminate an infestation.
Body louse. Other than observing lice on the
outside of clothing, looking for particular living habits or conditions is the key.
Infestations tend to occur when people are forced to constantly wear several layers of
clothing due to inadequate heat. This includes war areas, disaster areas, and homeless
people. Today they are most commonly encountered in accommodations housing the indigent,
hospital emergency rooms, jails that hold vagrants, and in morgues.
Crab/pubic louse. The presence of eggs/nits is the
most important indication of a problem because they are easier to see and inactive whereas
the lice are near the skin feeding. Another indication is general discomfort and
irritation, usually causing scratching. Also, small blue dots (1/64-1/8" or 0.2-3 mm
in diameter) develop at the feeding sites which are often accompanied by swelling and
tenderness.
About 3% of the public is infested. In the 15-19
year old group, females are infested more often than males but above 20 years males are
more commonly infested. Whites are more likely to have crab lice than blacks. Clinics
dealing with sexually transmitted diseases (STD's) see far more infestations than do
general physicians or dermatologists.
BIOLOGY.
Head louse. These lice live continuously on the
host. Each female lays 50-100 oval eggs which are pearly white, about 1/32" (0.8 mm)
long by 1/64" (0.3 mm) broad, and fitted with a cap to facilitate respiration and
emergence. Each egg is individually securely cemented to a head hair near the scalp and is
commonly called a nit. Eggs require high relative humidity and warmth in order to develop
and hatch. Eggs hatch in 5-1 0 days and there are 3 nymphal instars. The life cycle (egg
to egg) requires about 3 weeks. Adults live for about 22-23 days. Head lice can survive no
more than 48 hours off the host.
Body louse. These lice occur primarily in the
clothing and especially in parts of the clothing that are in contact with the body. They
move to adjacent body areas to feed. The female lays about 200 oval eggs which are pearly
white, about 1/32" (0.9 mm) long by 1/64" (0.3 mm) broad, and fitted with a cap
to facilitate respiration and emergence. Females lay about 5-8 eggs/24 hours, usually
during periods of host inactivity. Each egg (nit) is individually securely cemented to a
clothing fiber and only occasionally to body hair. Eggs (nits) tend to be laid in large
numbers along the seams of underwear and in places where clothing is in contact with the
body such as the neck, shoulder, armpit, waist, and the crotch of trousers. Because eggs
are removed with the clothing, development and hatch time are dependent on room
temperature and occur between 75- 99'F (24-37'C), taking 17-21 and 6-7 days respectively.
There are 3 nymphal instars. The life cycle (egg to egg) requires 8-9 days if clothing is
not removed nightly to 2-4 weeks if clothing is removed at night. They feed only when the
infested person is resting. If clothing is not worn for several days, the lice usually
die. Adults live up to 29 days. Body lice serve as vectors of typhus, relapsing fever, and
trench fever.
Crab/pubic louse. These lice live continuously on
the host. The female lays 2-3 whitish eggs/24 hours, for a lifetime total of 15-50 eggs.
Each egg (nit) is firmly cemented to a coarse hair near the skin. Eggs hatch in 7-8 days
with feeding beginning in 1-2 hours. The lst instar nymph feeds off and on for 5-6 days
and then molts, the 2nd instar requires 9-10 days, and the 3rd instar 13-17 days. The life
cycle (egg to egg) requires about 5-6 weeks. Adults live for about 15-25 days. Crab lice
can survive no more than about 24 hours off the host. There is no evidence that crab lice
are involved in the transmission of AIDS.
HABITS.
Head louse. For all practical purposes head lice
occur only on the head. They are most commonly found above the ears and on the back of the
scalp, less often on the entire scalp; rarely are they found on other body hairs such as
in the eyelashes. Head lice live continuously on the head area unless dislodged by
scratching, hats, comb, brush, towel, etc. They are commonly transmitted by the direct
contact of 2 heads, or by combs, hair brushes, or hats being shared, or by hats being
temporarily stored in contact with other hats.
Body louse. Females usually occur along clothing
seams where they lay eggs. Males tend to be generally distributed over the inside of the
clothing but further away from the body skin. Adults migrate further from the body than
nymphs. The infestation must be v" heavy/severe before lice can be seen on the
outside of clothing. Body lice prefer wool clothing. When feeding, they usually retain
their hold on the adjacent clothing and only occasionally will they crawl on the body.
Body lice are commonly transmitted by the sharing of infested clothing and/or bedding.
Crab/pubic louse. These lice live continuously on
the host. Once a louse settles it does not move much. They feed off and on for several
hours at a time and deficate frequently both blood and body wastes while feeding. Crab
lice are transmitted by intimate or sexual body contact. They are found primarily in the
pubic and perianal areas. Prior to puberty, children are usually not infested but if crab
lice are present, they are usually in the eyelashes. If infants are infested, the mother
should be examined.
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