Sydney University Department of Medical Entomology Westmead Hospital
Head Lice

Head lice

modified from: Kettle, D.S. (1995). Medical and Veterinary Entomology. CAB International. Wallingford

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Natural History

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 Live head licehost, 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 coloured 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 anNit on hair shaft 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 colour - although lice in lighter coloured 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.


Clinical Presentation

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 people’s 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 faeces, plus attached eggs and egg shells, and may eventually eventually develop a putrid odour.


Laboratory Diagnosis

Identification of louse specimens is by light microscopy. Details from the patient on exactly what part of the body the specimens were collected is important in establishing the identity of the louse. The body louse, Pediculus humanus (see Body Lice), is taxonomically impossible to differentiate from the head louse, but body lice are never found on the head (they are most often in the clothing where they lay their eggs attached to cloth fibres) and head lice are generally not found on other parts of the body. Pubic lice, Pthirus pubis (see Pubic Lice), are typically found in the pubic and perianal areas but are occasionally found in hair on the trunk and head, including the beard and eyelashes.


Treatment & Control

Head lice are contagious and infect a large number of school age children annually. When a case of head lice is detected the person should be treated immediately with a preparation obtained from a pharmacy. A wide range of liquid products (and fine toothed combs) are available and a prescription is not necessary. Products with permethrin or malathion are usually recommended. One or perhaps two applications are advised and all family members, and other close contacts, should be treated at the same time. The presence of unhatched nits or eggs after the preparation has been applied is not necessarily a sign of treatment failure - the treatment will not separate eggs from hairs, they must be physically removed - but the presence of living lice indicates either a treatment failure or a reinfestation.

A combined effort by school authorities requesting parents to treat all children in affected classes over a weekend will decrease the incidence of re-infestation. The control measures should be widespread to be effective and limit the risk of re-infestation. If re-infestation occurs it may indicate that not all infestations have been found and treated. Repeated finding of lice in some individuals following treatment could be a sign of resistance of head lice towards the chemical preparation, but there is little evidence for the presence of resistance in Australia, and inadequate treatment and opportunities for reinfestation should first be considered as an explanation. Some people may experience itchiness after a treatment but they should be careful to not automatically suspect persistence of lice, and should not automatically reapply the anti-lice products to relieve their discomfort. Routine or repetitious use of these preparations is unwarranted and expensive, and can lead to further irritation and other skin problems.


Confirmation and Enquiries

Identification of head and other lice, and all other medically important insects, is performed through the Medical Entomology Department at ICPMR, Westmead Hospital.

See 'Contacts' for further information.


Related Links

<http://www.hsph.harvard.edu/headlice.html> (Extensive review on headlice and its treatment from Richard Pollack, Harvard University).

<http://www.headlice.org> (The American National Pediculosis Association, headlice information).

<http://www.ifas.ufl.edu/~schoolipm/> (School IPM WWW site; details methods of pest management relevant to schools with an excellent presentation on headlice in Powerpoint format).

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