Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Arthropoda (arthropods, segmented body, exoskeleton, jointed appendages)
Uniramia (with antennae, first mouthparts mandibles)
Insecta (insects, 3 body parts, 6 legs, many with wings)
Anoplura (sucking lice, wingless, ectoparasites, incomplete metamorphosis)

Family: Peliculidae
Lice are small wingless dorsoventrally flattened insects with three body parts, head, thorax and abdomen. The head has two antennae and the thorax has six legs arranged in three bilateral pairs. All lice undergo gradual metamorphosis and there are no free-living stages. Eggs are cemented to host hairs whereas nymphs and adults cling to hairs using enlarged tarsal claws. Over 500 species of sucking lice parasitize mammals. The sucking mouthparts are retracted in the narrow head when not feeding. The mouthparts are introduced directly into host blood vessel (solenophage mode of feeding).

Pediculus capitus [this species causes head lice infestation in humans]

Parasite morphology: Head lice form three developmental stages: eggs, nymphs and adults. Eggs (commonly called nits) appear as white ellipsoidal operculate bodies (0.8 x 0.3mm) which are glued to hair shafts. Nymphs are similar in appearance to adults, but are smaller measuring 1-2mm in length. Adult lice have elongate dorsoventrally flattened bodies (2-4mm long) which appear opaque although darker internal organs can be seen mainly in the abdomen. Head lice are known colloquially as cooties, greybacks, or mechanized dandruff.

Host range: P. capitus is highly host-specific for humans and will not infest other animals. Some authorities regard head lice as a unique species (P. capitus) while others consider it to be a subspecies (P. humanus capitus) closely related to body lice (P. humanus corporis). Only body lice colonies can be bred in the laboratory after their adaptation to feeding on rabbits. Body lice are 2-4mm in size and spends most of the time in host clothing. Their life-cycle is completed in 2-4 weeks. Eggs attached to fibres in clothes hatch in 7 days and there are 3 nymphal moults taking 8-9 days. Pubic lice (or ‘crabs’) are also found on humans. These lice belong to a separate species (Phthirus pubis) which have grasping tarsi reminiscent of crab pincers. Infections are not confined to the pubic region, but may also involve the armpits, beard, moustache, eyebrows and eyelashes. The lice remain in position for some time with mouthparts inserted in skin and the bites cause intense pruritus. The life-cycle is completed in less than 1 month and infestations are transmitted mainly venereally, but can be passive especially in crowded situations.

Site of infection: All developmental stages of head lice can be found attached to, or grasping, hairs on the head, especially at the back of the neck and behind the ears. They are highly site-specific and head lice transplanted to other body regions attempt to migrate back to the head.

Pathogenesis: Nymphs and adults of both sexes feed by piercing the skin and sucking blood about every 2-3 hours. Light infestations may only cause moderate itching of the scalp exacerbated by sensitization to louse saliva. Heavy infestations, however, may cause considerable discomfort as the bites produce red papules, fever, aches and intense pruritus which induces scratching leading to dermatitis and secondary infections. Heavy louse infestation is known as pediculosis and is often associated with crowded conditions and poor sanitation.

Mode of transmission: Once hatched, head lice undergo gradual metamorphosis whereby nymphs moult several times before forming adults. No free-living stages are formed and lice do not survive long off their hosts. Infestations are therefore transmitted between hosts by direct physical contact, although some transmission via contaminated clothing or bedding cannot be entirely dismissed. The complete life-cycle takes 2-3 weeks, and louse populations often exhibit pronounced seasonal fluctuations, apparently linked to crowding during winter housing, particularly in temperate regions. Female head lice lay around 90 eggs which are cemented singly onto hair shafts.

Differential diagnosis: Infestations are diagnosed by finding live lice or empty eggs shells in the hair either by direct visual examination or using a fine-toothed nit comb (using hair conditioner to untangle hairs and trap lice).

Treatment and control: Many insecticides (e.g. malathion, carbamyl and pyrethrins) can be used to control lice and they are available in many hair care products (shampoos or lotions). Repeat washing are required within 10 days as most insecticides have limited activity against eggs. Over recent years, mounting problems with insecticide resistance have been encountered, and researchers are currently exploring herbal remedies. During infestations, daily grooming with nit combs is recommended to remove eggs and lice. Some countries still enforce home quarantine of infested school-children to curtail outbreaks. Inter-personnel hygiene must be improved and clothing and bedding should be well laundered.