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The life of a head louse

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VOL: 98, ISSUE: 46, PAGE NO: 54

Ian Burgess, MSc, MPhil, FRES, is director, Insect Research and Development Limited, Fulbourn, Cambridge

Little is known about the biology and behaviour of the head louse. There is no published work on most fundamental aspects of its biology, and much of what follows is based on personal observations and some unpublished doctoral theses.

Little is known about the biology and behaviour of the head louse. There is no published work on most fundamental aspects of its biology, and much of what follows is based on personal observations and some unpublished doctoral theses.

Life cycle of the head louse Head lice, Pediculus capitis, belong to the sucking lice, a group of insects found only on mammals. In the case of the head louse it is restricted to the heads of humans - mostly children. Head lice have a short life cycle, with generations about 17-18 days long. Female lice lay five or six eggs a day. Each has a high chance of developing successfully, partly due to the mother's high investment of energy from her diet of blood and also because each is firmly attached to a hair close to the skin.

After only seven days at scalp temperature and humidity the juvenile louse, or nymph, emerges and concentrates all its efforts on feeding and growing. After only three days it moults its skin and the second, slightly larger, second nymph emerges. The same process of growth and moulting follows for the second and third nymphs, the duration of each of which is approximately three days. The final moult gives rise to an adult louse that must then seek out a mate in order to start the cycle again.

Transmission of head lice There are many myths surrounding the transmission of head lice that mislead carers and make the process of control more difficult. Only head-to-head contact can transfer lice from one person to another. Lice are not transmitted on inanimate objects.

Generally only adult lice and third-stage nymphs are mobile. The youngest lice never seem to move between hosts unless contact is greatly prolonged, such as when heads are in contact during sleep. When lice do transfer the contact needs to be relatively prolonged to give the insects time to move from scalp level to the outside of the hair mass and on to the new host. The time this takes depends on how much hair the infested person has. People with lots of thick hair may find that lice can build up to large numbers simply because it is difficult for them to pass them on, as head-to-head contacts are not long enough.

Transmission and school-age children Before school entry most children have few close friends, and their main contacts are with siblings and parents. However, by the age of six most children have acquired an extensive group of friends with whom they play at varying levels of closeness. Inevitably those children who have more friends are more at risk of contracting lice, whereas the socially isolated run fewer risks of becoming infested.

Gender differences Girls are about twice as likely to get head lice as boys, and this seems to be related to differences of behaviour between boys and girls. Boys spend more of their time engaged in so-called open play, kicking a ball, climbing and running around, whereas girls spend a greater proportion of their time in close play.

Seasonal variations The widespread belief that prevalence of lice increases in autumn and winter is open to question. In most cases, risks for transmission probably diminish once schools open in September. However, since many people's minds only turn to head lice when schools reopen it is at that time that they start to look for them and find all the infestations that have been building up over the holidays.

Transmission in the family Although the peak levels of head louse prevalence are found in primary school children, anyone can catch them. Adult women are more likely to, mostly because more women work with children or have more opportunities to catch them from their own children. If lice do get into the household all members should be checked, just in case.

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