Biology of Pediculus
|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.|
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.
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.
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 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.