Head lice

Author: Dr Steven Lamb, Dermatology Registrar, Department of Dermatology, Waikato Hospital, Hamilton, New Zealand, 1998. Updated by Vanessa Ngan, Staff Writer, 1 February 2014. Updated by Hon A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, October 2015.

What are head lice?

Head lice are small, wingless insects than infest the human scalp. They are the most common of the 3 human lice species.

Head lice infestation is also called pediculosis capitis.

Who get head lice?

Head lice affect people of all ages and walks of life, all over the world. They are a very common problem in children aged 4 to 14 years of age.

Risk factors for infestation include:

Head lice can't jump or fly but spread by crawling along hair shafts by head to head contact.

What causes head lice?

The head louse, Pediculus humanus capitis, is an ectoparasite that feeds on human blood. It is 2–3 mm in length and has a flattened, elongated, grey coloured body that becomes reddish after feeding. The louse clings to the hair shaft by its 6 claws and rapidly moves from hair to hair.

Lice inject anticoagulant saliva into a person's scalp to suck up the blood up to five times a day. They die within one to two days away from the scalp if they are unable to feed.

After mating, the female louse attaches her eggs to the hair shaft close to the scalp in cool climates, and further down the shaft in warmer climates. The female louse can lay 50–100 eggs at a rate of 3–6 per day. The new egg cases are brown and hard to see. They are carried away from the scalp as the hair grows.

The eggs hatch after about 8 days. The empty egg case then appears white and is more easily seen—the nit.

The louse nymph reaches full maturity around 10 days after hatching and the cycle begins again. An adult louse survives on the scalp for about one month, and off the scalp for up to 48 hours.

What are the clinical features of head lice?

Head lice usually cause an itch and irritation in the scalp. This can take several weeks to develop after the initial infestation.

Occasionally the eyelashes can become infested, although this is more frequently due to a different insect, the pubic louse.

Scratching can cause crusting and scaling on the scalp. Hair pulling can lead to small areas of hair loss.

Complications of head lice

The head lice do not carry any other infectious disease. Heavy infestation can lead to:

How are head lice diagnosed?

It is important to identify the lice (or nits) to make a correct diagnosis. The lice can be hard to detect, and there are usually only 10–20 adult lice in each colony.

It is easier to identify (and remove) live lice by wet combing using a lice comb, compared to visual inspection alone.

What is the treatment for head lice?

Treatment of head lice usually consists of at least 2 applications of chemical insecticide and/or physical methods. Note:

Chemical treatment

The most commonly used topical insecticides for head lice is malathion, but resistance has been reported. Resistance to permethrin, phenothrin and other pyrethrins has increased to the degree that they are no longer recommended in the UK. Other, more expensive, topical options include:

Lotions, liquids or cream are preferred to shampoo (which is too weak to be reliable). They are applied directly to the scalp. It is important to follow the manufacturer’s advice on how long to use it and how often to repeat it due to potential toxicity.

Physical methods

Physical methods of removing nits and lice are used in conjunction with insecticide treatments and may be required daily for several weeks.

Expect nit removal to take at least half an hour. Repeat daily until no lice are found on 3 consecutive occasions.

Electrical combs designed to 'zap' lice on the hair shaft are probably not effective.

Other methods

The effectiveness of these treatments has not been extensively studied.

These include:

Why does head lice treatment fail?

Failure to eradicate lice is a common and frustrating problem. It can be due to:

How do you prevent head lice spreading?

It is difficult to prevent head lice infestation in children. Discourage children from sharing or using another child’s hat, comb, or brush.

Community-wide or school-based education programmes informing parents of methods to eradicate lice, and community health teams in schools, are the most effective ways in keeping infestation rates down.

Related information

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