What are head lice?
Head lice are small, wingless insects that infest the human scalp. They are the most common of the 3 human lice species.
Head lice infestation is also called pediculosis capitis.
Head lice (microscopic appearance) and eggs
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 4–14 years of age.
Risk factors for infestation include:
- Female sex
- Greater number of children in a family
- Sharing beds, clothing and hair brushes.
Head lice can't jump or fly but spread by crawling along hair shafts by head to head contact.
More about 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 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 away from 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.
- Lice favour the nape of the neck and the skin behind the ears.
- Nits are generally easy to see after the eggs have hatched, as adherent white grains on the hair shaft.
- Red-brown spots on the skin are due to excreted digested blood.
Occasionally the eyelashes can become infested, although this is more frequently due to a different insect, the pubic louse.
Scratching can cause crusting and scale 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. A heavy infestation can lead to:
- Dermatitis: red scaly itchy plaques
- Secondary bacterial infection (impetigo): crusted sores
- Tender swollen lymph nodes.
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.
- Look for lice and nits behind and above the ears and on the back of the neck.
- They may be observed scurrying to hide from the light to dwell in dark shadows.
- Unhatched eggs are mostly within a few millimetres of the scalp and have a dark area within the shell.
- Hatched eggs are transparent or whitish. These may persist after successful treatment unless physically removed. They are not infectious.
- White scale encircling the base of the hair shaft at the scalp surface is not due to head lice.
It is easier to identify (and remove) live lice by wet combing using a lice comb, compared to visual inspection alone.
For information on trichoscopic findings of head lice (dermoscopy of the scalp and hair), see Trichoscopy of scalp infestations.
What is the treatment for head lice?
Treatment of head lice usually consists of at least 2 applications of an insecticide and/or physical methods. Note:
- Treat all members of the family at the same time. Inform the daycare or school.
- The second application of insecticide is required 7–10 days after the first one because the eggs may survive, allowing new louse nymphs to hatch out.
- Physical methods are required to remove the nits, as simple washing is not effective.
The main suffocating agent used to kill the adult and nymph head lice is 4% dimethicone (also spelt dimeticone). Lice are unlikely to develop resistance due to the mechanism of action, and the product is safe and well tolerated. Cure rates are reported to be 69–92% after two applications one week apart.
The most commonly used topical insecticides for head lice is malathion, but resistance has been reported and cure rates are reported to be as low as 33%. 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 of removing nits and lice are used in conjunction with insecticide treatments and may be required daily for several weeks.
- Nit combs used in wet hair are the most effective way of physically removing the lice and nits. Metal combs are best for thick hair, and plastic is kinder on fine or long hair. Apply a conditioner to the hair.
- Work through the scalp in sections
- Comb down the hair shaft towards the scalp
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.
The effectiveness of these treatments has not been extensively studied.
- Shaving the head or cutting the hair very short is effective, but rarely necessary.
- Daily 30-minute exposure to hot air dryer over a period of one month destroys live eggs.
- Treatments using natural oils may be as effective as the chemical treatments that have been approved for use in head lice.
- Oral ivermectin 400 mcg/kg (off label)
- Oral sulfamethoxazole/trimethoprim (off label) intended to kill the bacteria in the gut of the lice, which are essential for the digestion of nutrients, so they starve to death. It should be reserved as second-line treatment as trimethoprim + sulphamethoxazole may sometimes cause rare serious adverse reactions.
Why does head lice treatment fail?
Failure to eradicate lice is a common and frustrating problem. It can be due to:
- Inactive infestation (white nits do not contain live eggs)
- Reinfestation from another person
- Less likely, reinfestation from contaminated clothes, hats, etc
- Improperly applied treatment
- The resistance of lice to insecticides
How do you prevent head lice from 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.