Head lice

Definition

An obligate ectoparasitic insect, Pediculus humanus capitis (Phthiraptera: Pediculidae), that completes its entire life cycle on the human scalp, feeding exclusively on human blood. The species is host-specific to Homo sapiens; a closely related species, Pediculus schaeffi, parasitizes chimpanzees and bonobos. Head lice are wingless, dorsoventrally flattened, and possess tarsal claws adapted for gripping hair shafts. Infestations (pediculosis capitis) occur worldwide and are transmitted primarily by direct head-to-head contact.

Etymology

From Middle English hed + lous, louse; the compound distinguishes this scalp-dwelling form from body lice (Pediculus humanus humanus) and pubic lice (Pthirus pubis).

Example

In field surveys of pediculosis, head lice eggs (nits) cemented to hair shafts near the scalp are diagnostic of active infestation, whereas empty nit cases found farther along the hair indicate past exposure.

Synonyms

• Pediculus humanus capitis
• cootie (informal)

Related Terms

• Pediculus humanus humanus
• Pthirus pubis
• Pediculosis
• nit
• Ectoparasite
• Phthiraptera
• host specificity
• vector-borne disease

Usage Notes

The term 'head lice' properly refers to the plural of the insect; 'head louse' is the singular. Distinguish from body lice, which are the same species but a different ecotype (subspecies or strain) adapted to clothing, and from pubic lice (crabs), a distinct genus. 'Lice' alone is ambiguous without context. In clinical and epidemiological usage, 'infestation' is preferred over 'infection' because lice are ectoparasites, not pathogens.