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| Figure 1: Pediculus humanus. Illustration by Sharon Belkin. Garcia 547. | Figure 2: Phthirus pubis. Illustration by Sharon Belkin. Garcia 548. | Figure 3: Louse egg (nit) on hair shaft. Photograph by Duane J. Gubler, CDC. Garcia 548. |
Agents:
Pediculosis is caused by organisms of the phylum Arthropoda, class Insecta,
order Anoplura (blood-sucking lice) and genus Pediculus. The three species
that commonly infest humans are Pediculus humanus capitus or head lice,
Pediculus humanus humanus or body lice, and Phthirus pubis or
pubic lice, also known as crabs.
Morphology:
Lice are visible to the naked eye, with P. humanus 2-4 mm long (see Figure
1) and P.pubis 1mm long (see Figure 2). The lice are flattened dorsoventrally,
and are characterized by legs with claws adapted for clinging to hairs and
fibers on the body of the host, as well as mouth parts used for piercing the
skin of the host to attach to it to suck blood. Lice are wingless, so they
can only be transmitted from host to host via direct contact. The eggs laid
by the female louse are oval shaped (see Figure 3) and attached to the hair
shaft of the host.
Life Cycle:
The adult louse attaches to the skin via its mouthparts and sucks blood from
the host to feed itself. Female lice lay eggs at the base of the hair shaft
for up to 30 days, though few live that long, and then die. In body lice and
head lice, females can lay six to eight eggs per day, and the eggs take on
average 7 days to hatch and then another 7 days to reach sexual maturity.
If they do not hatch within 20 days the eggs die. Female pubic lice lay about
five eggs per day. Egg hatching requires 8 days and then it takes another
8 days for the louse to reach maturity. Adult pubic lice live three weeks
longer than body or head lice.
Therefore the incubation period for the infestation is highly variable,
as adult lice may be noticed as early as five days after infestation or as
late as three weeks later.
Transmission:
Transmission of Pediculosis can only occur through parasite transfer
from host to host through direct body contact with lice or lice eggs (nits)
on bodies, clothing or personal articles. P. pubis are transmitted
through sexual contact or other contact with infested external genitalia.
Lice are host specific, and can only survive on the host and briefly (up to
one week) in the environment. Head and pubic lice deposit and cement nits
onto the hair shaft on the scalp or pubic area, while the body louse deposits
eggs primarily on the seams of clothing. Contact with these eggs can also
spread the infestation.
•Reservoir: There is no reservoir for Pediculus other than humans
because the lice are host-dependent (they cannot survive long in the environment)
and host-specific (there are several types of animal lice as well but they
do not typically affect humans).
•Vector: There is no vector for Pediculus because they
are transmitted via direct contact. However, the body lice themselves are
vectors of louse-borne relapsing fever (Borrelia recurrentis) and louse-borne
typhus(Rickettsia prowazeki, R. quintana). Lice become
infected when they feed off the blood of a person infected with either relapsing
fever or typhus. The infected louse dislikes the intense heat of the fevered
body, so leaves the infected person and movees to a new host, where it deposist
the parasites on the body of the new hosts through fecal matter excreted as
it feeds on the new host. This fecal matter is rubbed into the skin by scratching,
and thus the B. recurrentis and Rickettsia parasites are able
to enter the new host's body and infect him.