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Head lice (Pediculus capitis) are blood sucking ectoparasites of humans from the family of lice Pediculidae, and they have world wide distribution. This common human parasite is strictly host specific and does not affect other animals. An adult head louse is a small six legged insect, 2.5-3.5mm in length, with well developed eyes, small antennae and a flattened light brown body which is slightly lobed at the margins. The claws on each leg enable the lice to hold on to hairs, and they can run quickly over the scalp through hair. Head lice live their entire life (about a month) on the head of their host, and are often concentrated towards the back of the head and above/behind the ears. All nymphs (immature stages) and adults blood feed on the surface of the scalp until fully engorged, and can feed at any time of the day or night. A female head louse can lay 6-8 pale colored oval shaped eggs (or nits) a day, and may lay 300 eggs within her lifespan. The eggs are glued to the base of the hair shaft and grow out with the hair. The further away from the scalp that eggs are found can be an indicator of the length of the infestation (and most eggs found further than 1cm from the scalp should be hatched or are dead). Most eggs hatch within 7-10 days but some can remain unhatched for up to 3-4 weeks. Individuals who have a head louse infestation will have on average 10-20 lice at any given time. Without a bloodmeal and the humid environment their human host provides, head lice will only survive a few days. They cannot infest furniture, bedding, pets or other household situations - they must have a human host.
Transfer of head lice to individuals is by hair to hair, head to head, close bodily contact. Sharing combs, brushes, ribbons, hair bands, hats, pillows and similar personal articles is the other main way that lice can spread. Lice will not voluntarily leave the scalp. Head lice tend to be more common among children than adults, and do not discriminate between sex or hair color - although lice in lighter colored hair may be more difficult to detect. Head lice do not prefer dirty or unkempt hair, but may go unnoticed and proliferate in such an individual compared with one who pays attention to their hair. However, an infestation should not be seen as a reflection on personal hygiene, home environment or social status, and should not be seen as a social stigma.
An infestation of head lice may not be noticed initially, but with time will produce irritation leading to scratching of the scalp. Persistent scratching can cause development of lesions which may give rise to secondary infections and, in some cases, even swollen lymph glands. Lice are a nuisance and can disrupt peoples rest, but they are not responsible for the spread of any infectious disease-causing organisms.
The detection of eggs of the shaft of the hairs is the most common
method of identifying a head louse infestation, and it usually indicates the lice have
been established for some time. In addition to observing the lice throughout the hair,
further evidence of head lice may be seen on pillows or bed linen in the form of dark
powder (faeces of the louse) or cast skins. Heavy infestations of head lice, if left
untreated, result in matted hair interspersed with lice, their cast skins and feces, plus
attached eggs and egg shells, and may eventually eventually develop a putrid odor.
Pediculus humanus, or the body/clothing louse is a blood sucking species of louse that can live between the skin and clothing of humans. This host specific ectoparasite of humans is thought to have evolved from head lice (Pediculus capitis) but migrated to the body in association with the wearing of clothes. Body lice are small flattened insects with a slightly elongated lobed abdomen, a distinct head, small eyes, a pair of short antennae and six legs, each terminating in a strong claw. Each of these stout claws has a small thumb-like spine for grasping, enabling the louse to move quickly around the clothes utilizing the fibers of the fabric or body hair for support. Adult lice are 2-4mm in length, grey in color, but redden after blood feeding. The mouth parts are tube-like, armed with minute teeth and sharp stylets for piercing the skin, and when not in use, are telescoped within the head. Adult lice and the three nymphal (immature) stages live their entire life within the clothing of humans. Lice may only leave the clothes briefly or hold onto the fibers of clothes or body hair while blood feeding. The lice blood feed frequently, at any time, day or night, but usually when the person is at rest. These parasites prefer to feed where the skin is soft and folded and the clothing fabric is in close contact with the body.
Female body lice will lay their eggs (or nits) along the seams or hems of clothes (especially underwear) that are adjacent to the surface of the skin. Each egg is firmly glued to fibers of the clothes, but occasionally body hair maybe used. A mature female louse will lay 200-300 eggs within her life span of a month, laying between 6-9 eggs a day. The eggs are white and oval in shape and rounded at the top. Eggs hatch within 5-10 days, but if the clothing is removed each night from the warmth of the body, development time is increased and the eggs may take up to 2 weeks before hatching. Louse eggs can remain viable for up to 14 days. Body lice are extremely sensitive to change in temperature and humidity and have been known to abandon a dead person or people with elevated temperatures. Without a constant source of blood, the lice perish within 2-5 days. In hot weather, when several layers of infested clothing are worn, the lice may move to an outer layer where the temperature is cooler. Lice are very rarely seen crawling on the outside of infested clothes, if they are visible it is an indication the individual is heavily infested. Normally body lice are sensitive to light and if disturbed will quickly move to a seam or crease for cover.
Transmission of body lice occurs when living conditions are crowded,
personal hygiene is neglected, clothes are not changed and facilities for laundering
clothes are not available. Lice can spread rapidly through homeless people or victims of
war and natural disasters, when people sleep in their clothes and huddle together for
warmth. Bedding and furniture have also been implicated as a source of infestation in
The pubic louse is grey in color, and smaller (1.25 -2.00mm) than the head and body louse. Their body shape is oval and broader than long, with four distinct lobe-like protuberances on each side of their abdomen. The lice have a small head with short antennae and simple eyes. Each of the six legs of the louse terminates in a claw, but claws on the second and third pair of legs are huge compared to the first pair which are slender. Within each claw there is an associated thumb-like projection which enables the louse to grasp body hair, securing them whilst they feed on blood using their mouthparts which are especially adapted for piercing and sucking. The lice blood feed intermittently over several hours. Adults and the three smaller nymphal stages usually remain and feed in a settled position. Pubic (and other lice) lice cannot burrow into the skin and do not live under the skin.
The life span of adult lice is less than a month. A mature female louse will lay a total of 30 eggs (nits), laying up to 3 eggs a day. The eggs are smaller than the eggs of the other human lice, and are a darkish-brown with an opalescent sheen. Each egg is cemented to the shaft of coarse hair, and at skin temperature will hatch within 6-8 days. If the lice are forced off the host they will die within 24-48 hours. Pubic lice cannot infest the rooms or carpets of an infected person.
Pubic lice are usually transmitted by sexual
contact, and although this is the most common method, it is incorrect to assume this is
the only means of transmission. Shared bath-towels and clothing, discarded clothing
hanging in overcrowded locker rooms, children sleeping with an infected parent, or bedding
that has recently been vacated by an infected individual can lead to infestations. Pubic
louse infestations cannot be transmitted from animals.