Biology
Lice are a large family of insects, phthiraptera, that includes head lice, body lice and pubic lice. Head lice are small parasitic insects that only live on the human head. Head lice have been found on "mummies" in Egypt and South America.
Head lice are supremely adapted to living on the human head. The human head is the only place to find head lice. They do not live on any other part of the body nor on any other animal.
Head lice have six legs designed to grasp the hair shaft and to enable rapid movement backward and forward to escape detection.
Like many insects, head lice are sensitive to temperature and humidity. The human head makes a perfect humidicrib for these parasites, and depending on where they are on the head, the lice can control these factors by moving to different parts of the scalp.
Head lice cannot fly or jump. Looking through an electron microscope, it's easy to see why: each of their long legs has a claw at the end, which makes it difficult to jump or to walk on flat surfaces.
Head lice move between heads via a hair; they grasp a hair from another head by swinging between heads on a hair shaft. Their grasp is very strong which makes them hard to dislodge from the scalp and hair. Head lice do not burrow into the skin.
Head lice feed only on human blood, so they cannot survive for very long away from the head. They need to feed several times a day.
Eggs (nits) are laid by a female close to the scalp on the hair shaft, usually no more than 1.5 centimetres from the scalp. These eggs are attached to the hair with an incredibly strong glue. There is currently no product on the market that can effectively treat all eggs.
Unless manually removed, egg shells and unhatched eggs grow out with the hair. It is quite common to detect a badly infested head by the lines of eggs that have grown out with the hair.
Depending on the temperature and humidity of the head, eggs hatch after five to seven days. Small nymphs emerge, and providing they can feed, begin to grow into adult-sized lice after about seven or eight days.
It can be about 14 days after the eggs are laid that the mature louse can begin breeding.
Adults live for up to 14 days. The lifecycle of a head louse is only about one month.
This is important to remember. As no product has been shown to kill eggs, any chemical treatment must be reapplied after any eggs have hatched, ie. five to seven days after the first treatment.
A female louse can lay about 100 eggs in a lifetime, laying around eight eggs at a time. Only those eggs laid by an inseminated female will hatch.
Do head lice cause illness or disease?
Head lice have not been shown to transmit any infectious agents and rarely cause any harm.
Adults more than children do suffer from considerable outrage at the presence of these parasites. This outrage usually outweighs any public health significance head lice may present.
The lice's saliva and faeces may cause some irritation, though rarely. This irritation may, in turn, cause scratching and thereby increase the chance of a secondary infection. However this is rare.
The most likely harm caused by head lice is from inappropriate use of chemicals in an attempt to treat them.
The continued application of chemicals to the scalp can cause severe reactions on some heads.
Parents, in their frustration, can resort to applying products not tested for human use and not shown to have any effect on reducing head lice.
How do you catch head lice?
Head lice are caught from another human head. Head lice cannot survive off the human head for any length of time. Depending on the temperature and humidity levels, healthy adult lice can survive for only a few hours off the head. Like many insects, lice are sensitive to dry and/or cold conditions.
Spring cleaning your home, washing bedding and toys and rigorous vacuum cleaning do not affect the head lice population on a human head.
Head lice move from human hair to human hair. They do not crawl along furniture or hop between car seats. Focussing on the environment does not reduce head lice, merely takes up time that could be spent combing the head.
It is thought that increased human contact and changes in hair styles and behaviour of young children, particularly girls, may have contributed to an increase in head lice. While these changes in society have been undoubtedly for the better, they may have brought with them increased transmission opportunities for head lice.
Research in NSW and Queensland has shown that head lice incidence is greatest in primary school children and peaks in girls in middle years of primary school. Current data show approximately 23 per cent of primary school children screened through the Nitbusters Program in NSW public schools have head lice.
This web page is managed and authorised by Environmental Health of Centre for Health Protection of the NSW Department of Health. Last updated: 31 March, 2009