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Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Head lice

What are head lice?

Head lice are tiny insects that infest human scalps, and are a common problem in children. The lice attach themselves to hair shafts and bite the skin to feed on blood. Itching and irritation results from the louse feeding. Head lice don't jump or fly but spread by crawling from the hair of one person to another who are in close contact, such as family or school classmates.

Head lice infestation is also called pediculosis capitis. The proper name for the head louse is Pediculus humanus capitis.

Head lice are the most common of the 3 human lice species that have infested humans for thousands of years. Infestation with head lice is not related to poor hygiene and can affect people of all ages and walks of life. The head lice do not carry any other infectious disease.

Head louse
Louse
Head lice and dermatitis
Dermatitis
Head lice eggs. Nits.
Eggs
Head lice

What do head lice look like and how do they grow?

The head louse is about 1-2mm in length and has a flattened, elongated body. It is wingless and white to grey coloured. Head lice grip on to hair by their claws and rapidly move from hair to hair. Using their mouthparts, lice inject saliva into a person's scalp and suck the blood which provides their nutrition. This can occur up to five times a day. One to two days away from the scalp means the lice are unable to feed and therefore die.

Lice lay eggs (nits) on the hair shaft close to the scalp where the temperature is best for incubation. The nits are cemented on to the hair and are carried away from the scalp as the hair grows. They hatch at around 8 days. The empty egg case then turns white and becomes more easily seen. The louse reaches full maturity at around 10 days after hatching. If mating occurs the female louse can lay 50-100 eggs at a rate of six per day.

In most infestations there are usually only a dozen or so lice at any one time, but there can be hundreds of eggs.

What are the symptoms of head lice infestation?

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

Scratching can cause crusting and scaling on the scalp. Occasionally secondary bacterial infection of the scalp results in small sores on the scalp with tender glands in the neck. Dermatitis can also occur with a heavy infestation of lice. Fortunately head lice are not known to carry any diseases which can affect humans.

It is important to identify the lice (or nits) to make a correct diagnosis. Lice are on average 1-3 mm in length and can be seen moving from hair to hair. Unhatched eggs are within a few millimetres of the scalp and have a dark area within the shell while hatched eggs are transparent.

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

Occasionally the eyelashes can become infested.

What is the treatment for head lice?

Treatment of head lice usually consists of chemical treatments (pesticides) and/or physical methods (combs). Important points to remember when treating head lice are listed below:

Chemical Treatment

Insecticides used to treat head lice include:

Lotions, liquids or cream are preferred to shampoo (which is too weak to be reliable). All are topical applications; therefore they are applied directly to the scalp. Even so, a small portion may be absorbed into the body and for this reason it is important to follow the manufacturer's advice on how long to use it and how often to repeat it.

Physical methods

Physical methods of removing nits and lice, often a neglected part of treatment, can be effective on their own. They are however more reliable used in conjunction with insecticide treatments. Physical removal of nits, although effective, is very time consuming.

Eggs are cemented strongly to the hair shaft and simple washing usually doesn't remove them. Nit combs are the most effective way of physically removing the nits. They are available from chemists and pet stores. Metal combs are much more effective than plastic. Electrical combs designed to 'zap' lice on the hair shaft are not effective.

Cutting the hair short (i.e. No. 1 cut) may be useful in difficult cases. This makes searching and removing lice easier but won't prevent reinfestation.

What other methods are used to get rid of head lice?

There are numerous other methods that have been used to eradicate lice. The effectiveness of these treatments has not been extensively studied.

These include:

What happens when head lice treatment fails?

Failure to eradicate lice is a common and frustrating problem. There may be a number of reasons for this

There is no information on the resistance patterns of lice in New Zealand. Recent studies in the USA suggest that resistance has developed where pyrethroids have been used as the main treatment for lice. This usually occurs in chronically infested individuals that have been treated many times over a short period. Lice resistant to malathion have also been detected in a recent UK study.

How do you prevent head lice spreading?

It is difficult to prevent head lice infestation in children. Do not allow children to share or use other children’s hat, comb, or brush. It may be necessary to stop children from using head gear such as in dress-up areas of play centres.

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

References

On DermNet NZ:

Other websites:

Author: Steven Lamb MBChB
Department of Dermatology, Health Waikato. Updated by Vanessa Ngan, 1 February 2014.

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