Head lice are tiny wingless insects that are grey-brown in colour. They are the size of a pinhead when they hatch and 3mm long (the size of a sesame seed) when fully grown.
Head lice cannot fly, jump or swim. They are spread by head-to-head contact and climb from the hair of an infected person to the hair of someone else.
Life cycle of head lice
A female head louse lays eggs by cementing them to hairs (often close to the root) where they will be kept warm by the scalp. The eggs are pinhead size and difficult to see.
When the baby lice hatch 7 to 10 days later, the empty eggshells (nits) remain glued in place. Nits glisten white and become more noticeable as the growth of the hair carries them away from the scalp.
Head lice feed by biting the scalp and sucking blood through it. They take 6 to 10 days to become fully grown. Once mature, a head louse can transfer from head to head.
After mating, a female may start to lay eggs as early as the seventh day after she has hatched. So to break the life cycle and stop head lice spreading, they need to be removed from the head before the sixth day after hatching.
How common are head lice?
Children are most commonly affected by head lice, although anyone with hair can catch them.
Children are often affected by head lice because they tend to have more head-to-head contact while at school or during play. Head lice are most common in children between 4 to 11 years old.
A head lice infestation is not the result of dirty hair or poor hygiene. Head lice can affect all types of hair irrespective of its condition and length.
Head lice often cause the scalp to itch, although this is not always the case. Itching is not caused by the lice biting the scalp but by an allergy to the lice.
Some people are not allergic to head lice, so they may not notice that they have a head lice infestation. Even if someone with head lice is allergic to them, itching can take up to three months to develop.
In some cases of head lice, a rash may appear on the back of the neck. This is caused by a reaction to louse droppings.
Looking for head lice
Head lice are difficult to detect on the head, even when the head is closely inspected. Unhatched eggs or nits (empty eggshells) alone are not enough to diagnose an active head lice infestation.
This is because it can be difficult to distinguish between eggs and nits that are dead or alive. Nits also often remain glued to hairs after successful treatment.
In order to confirm an active infestation, a louse must be found through a reliable method, such as detection combing. Detection combing can be used to check more accurately for head lice. See Head lice – diagnosis for further information about this detection method.
Getting rid of head lice
Head lice can be effectively treated using medicated lotions or by wet combing, using a specially designed head lice comb.
Always seek advice from a healthcare professional before using medicated head lice lotions on the following groups:
• young babies (under six months old)
• pregnant women
• people with asthma or allergies
Always read the instructions carefully before using medicated head lice lotions.
There is no need to wash or fumigate clothing or bedding that comes into contact with head lice.
Head lice can only survive on humans and they die after a day or two of being away from the human scalp.
Head lice that fall off the scalp are likely to be close to death (their life span is about three weeks).