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.
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?
- Regularly examine your children's scalps. Look for nits close to the skin, behind and above the ears and on the back of the neck.
- Treat all members of the family at the same time.
- Not all eggs are killed with one application of insecticide, therefore a second application is recommended 7 days later.
- The lice may not be killed immediately and may take a day or so to die.
- The presence of nits doesn't mean ACTIVE infection. Hatched nits (empty eggshells) will remain attached to the hair shaft until the hair grows out, unless they are actively removed or the hair is cut.
- Machine-wash all bed linens, clothes, towels in hot water.
- Items that can't be washed such as soft toys and helmets should be placed in an airtight plastic bags for two weeks.
- Vacuum pillows, etc.
- Spray hairbrushes, combs with fly spray
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 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.
- Using a nit comb is easiest when the hair is wet. Combing may be easier if a conditioner is applied first. It is best done after treating with insecticides. It should take about half an hour to comb the entire scalp.
- Use a good light.
- Work through the hair in sections and comb down the hair shaft towards the scalp to try and remove the stubborn nits.
- It may be easier if this is done while the child is kept occupied (for example watching television).
- Repeat the combing at least twice more on consecutive nights if possible and then at least weekly until no lice are found on 3 consecutive occasions.
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.
- New pediculocides. Spinosad topical suspension is a prescription medicine recently approved in the USA for topical treatment of head lice infestation (2011). It is a natural product derived from a soil actinomycte, Saccharopolyspora spinosa. A clinical trial showed it to be significantly more effective than permethrin with nit combing. A single treatment kills lice and eggs.
- Suffocating agents: Recently, suffocants containing dimethicone, petroleum jelly, benzyl alcohol and other ingredients have been marketed for head lice. These have been shown to be as or more effective than chemical treatments, and are safe and well tolerated. They should be applied to dry hair from the scalp to the ends, covered with a shower cap for 20 minutes, and then washed out. It is recommended treatment be done once weekly on 3 occasions, to catch newly hatched lice.
Many home remedies have been used to smother head lice. These include mayonnaise (full fat) and olive oil, and should be left on the scalp for at least two hours. This process can be messy. These methods only kill active lice and have no effect on nits. Therefore the treatment needs to be repeated.
- Natural oils: There are several products available that are based on natural oils. These may also be more effective than standard pediculicides based on pyrethrins.
- Antibiotics: Co-trimoxazole (sulfamethoxazole and trimethoprim) is sometimes prescribed as a second line treatment for head lice. The belief is that the bacteria in the gut of the lice, which are essential for the digestion of nutrients, are killed when the lice feed on the blood of a person taking this antibiotic. They then starve to death. It should be reserved as second-line treatment as co-trimoxasole may sometimes cause rare serious adverse reactions.
- Ivermectin: Oral ivermectin 400mcg/kg has occasionally been tried off-label to treat head lice and appears effective in the majority of cases. Two treatments one week apart are advised. Topical invermectin was licensed in the USA for use in head lice in 2012 but is not yet generally available.
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
- Reinfestation from another person or from contaminated clothes, hats, etc.
- Resistance of lice to insecticides or improperly applied treatment. If a course of one kind of insecticide treatment fails, a different insecticide may be selected for the next course.
- Misdiagnosis of inactive infection. It is important to look for active lice not just nits.
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.
- Trying to Keep Ahead of Lice: A Therapeutic Challenge – Medscape, from Skin Therapy Lett. 2006;11(10):1-6.
- Greive KA, Lui AH, Barnes TM, Oppenheim VM. A randomized, assessor-blind, parallel-group, multicentre, phase IV comparative trial of a suffocant compared with malathion in the treatment of head lice in children. Australas J Dermatol. 2010 Aug;51(3):175-82.
- Barker SC, Altman P M. A randomised, assessor blind, parallel group comparative efficacy trial of three products for the treatment of head lice in children - melaleuca oil and lavender oil, pyrethrins and piperonyl butoxide, and a "suffocation" product. BMC Dermatology 2010, 10:6doi:10.1186/1471-5945-10-6
On DermNet NZ:
- Head Lice – Medline Plus
- National Pediculosis Association
- Harvard University
- James Cook University – North Queensland Australia
- Head Lice – The Childcare Setting, CDC
- Best treatments – clinical evidence for patients from the BMJ: Head lice
- Lice – Medscape Reference
- Lice.ca: Louse Detection and Lice Treatment – SkinCareGuide
- Head Lice – British Association of Dermatologists
- Patient information: Head lice (Beyond the Basics) – UpToDate (for subscribers)