Pediculus capitis

Head louse
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Insecta Order: Phthiraptera Suborder: Anoplura Family: Pediculidae Genus: Pediculus Species: P. humanus
Binomial name
Pediculus humanus Linnaeus, 1758
Trinomial name
Pediculus humanus capitis Charles De Geer, 1767
Synonyms
Pediculus capitis (Charles De Geer, 1767)

This page is about microbiologic aspects of the organism(s).  For clinical aspects of the disease, see Head Lice Infestation.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

The head louse (Pediculus humanus capitis) is an obligate, ectoparasitic, wingless insect spending its entire life on human scalp and feeding exclusively on human blood. Humans are the only known host of this parasite. Humans can also be infested with the pubic or crab louse (Pthirus pubis) and/or with the body louse (Pediculus humanus humanus).Head lice infect hair on the head. Tiny eggs on the hair look like flakes of dandruff. However, instead of flaking off the scalp, they stay put. Head lice can live up to 30 days on a human. Their eggs can live for more than 2 weeks. Head lice spread easily, particularly among school children. Head lice are more common in close, overcrowded living conditions. Pediculus humanus capitis is most commonly found on the scalp, behind the ears and near the neckline at the back of the neck. Head lice hold on to hair with hook-like claws found at the end of each of their six legs. Head lice are rarely found on the body, eyelashes, or eyebrows.

Morphology

The dorso-ventrally flattened body of the louse is divided into head, thorax and abdomen. On the head, one pair of eyes and one pair of antennae are clearly visible. The mouthparts are adapted to piercing the skin and sucking blood. The legs, with their terminal claws, are adapted to holding the hair-shaft. They cannot jump from head to head and being wingless insects they also cannot fly. In males (Fig.1), the first pair of legs are slightly larger and also used for holding the female during copulation. Males are slightly smaller than females and are characterized by the pointed end of the abdomen and the well-developed genital apparatus visible inside the abdomen. Females are characterized by two gonopods in the shape of a W at the end of their abdomen (see figure above). The eggs (Fig.3) are oval-shaped and ca. 0.8 mm in length. Immediately after oviposition they are shiny, round, and transparent. Head lice are 1-3 mm in size, varying according to their stage of development. They are usually grayish in color but depending on the time since their previous blood-meal they can be also reddish-brown.

• Fig. 1. Male of head louse

  Fig. 1. Male of head louse

• Fig.3.Louse egg

  Fig.3.Louse egg

Biology

During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits). The egg hatches to the first nymphal stage, which after three moltings develop to nymph 2, nymph 3 and eventually to either a male or female louse. Adult lice copulate frequently and the females lay an average of 3-4 eggs daily. A generation lasts for about 1 month. All stages are blood-feeders and they bite the skin 4-5 times daily to feed. During oviposition the female excretes a glue-like substance from a gland located at the posterior end of the body and attaches the eggs on the hair of the host. Although any part of the scalp may be colonized, lice favor the nape of the neck and the area behind the ears, where the eggs are usually laid.

See also

• Lindane
• Pediculosis
• Nitpicking
• Delphinium
• Body louse
• Crab louse

External links

• Centers for Disease Control and Prevention: Division of Parasitic Diseases

• Harvard School of Public Health: Head Lice Information

• James Cook University, Australia: Head Lice Information Sheet

• MedicineNet.com: Head Lice Infestation (Pediculosis)

• Phthiraptera Central: Bibliography of Lice

• University of Nebraska: Head Lice Resources You Can Trust