How do head lice spread?
Head lice are transmitted when head lice, usually adults, crawl from one person to another when heads are held close together (1). Lice cannot jump but they crawl between hair strands. Once lice come away from hair, it is unlikely that they will re-establish themselves on a new head. They quickly become weak and survive for only 1-2 days away from a head (2, 3).
Both adults and children can have head lice (4, 5) regardless of their hair type or personal hygiene. Head lice survive in hair that is washed often or seldom (6).
Symptoms of infestation
An itchy scalp is a common sign of head lice. Louse saliva contains substances that may cause itching when they enter the skin. However, not everyone with lice experiences itching or it can take time for it to start. It is therefore important to check regularly, regardless of whether or not you have itching. Other symptoms of lice include sores and scalp infections caused when lice bites are scratched (7).
How to check for lice
- Use a fine-toothed lice comb (most commonly used method in Norway (8, 9)).
- Comb systematically and thoroughly through all the hair, from scalp to hair tip. In this way, you can find and remove lice that you would not notice with a normal comb, eyes and fingers (10-12).
- Before using the comb on others, wash it throughly at 60 ºC or freeze it for at least four hours to prevent infestation.
Wet or dry hair?
You can check for lice in either wet or dry hair, but we recommend wetting the hair and preferably adding conditioner. The lice do not move as much and they attach more easily to the lice comb (10, 13).
Head lice check in wet hair:
- Lay a white towel over the shoulders when combing wet hair and check the towel and comb for lice. Lice are small and a magnifying glass can be useful.
- Wipe the comb on paper frequently to check for lice (13). Lice will dry within a few minutes, and you will see them move.
Head lice check in dry hair
Some people prefer to check for lice in dry hair. Lice move in dry hair and they may therefore be easier to spot, at least if there are many. However, combing dry hair creates static and lice may be thrown from the comb back into the hair.
- The person being combed should bend over a big white sheet and comb from the scalp and down towards the sheet.
Finding head lice eggs
Combing can also find lice eggs. They stick so well to the hair strand that the hair often breaks during combing, with the egg still attached. Finding eggs alone is not a sure sign of an ongoing lice infestation as they may have hatched or died. Live head lice must be detected for a certain diagnosis. The egg's location on the hair shaft may help to determine the time of infestation. Since eggs are laid mainly near the scalp and hair grows about a centimetre per month, eggs nearer the scalp will have been laid more recently than those further out (13).
What do head lice and their eggs look like?
Head lice are whitish, brown or black. As they are somewhat translucent, they sometimes appear reddish if they have recently fed. Adult head lice are 2-3 mm long (14). Their feet have claws so they can easily grasp and move in the hair. Nymphs resemble adults but they are smaller and lighter in colour. The smallest nymphs are 1 mm long.
The eggs are 0.3 x 0.8 mm and they attach firmly to the hair strands. They are yellowish and translucent before hatching (14) and paper-like white afterwards. Each egg has a lid that falls off during hatching.
Empty eggs lack lids and have a purer white colour. Eggs killed by delousing agents retain their lids and become brown as their contents dry. You can see eggs with the naked eye but a magnifying glass is useful to see details. Both live and dead eggs can stick to the hair strand after the lice are removed from the head. There is no need to remove the eggs.
How often should you check for lice?
A major reason for not being able to get rid of lice in a local environment is that lice checks are performed all too infrequently. If the frequency increases, head lice are detected quicker and fewer will wander round unawares with head lice and potentially infest others (15, 16).
We recommend that families check everyone in the household with a lice comb at least once a month. Only 25 per cent of households in Norway followed this advice in 2008 (8). You should also carry out a thorough lice check as soon as you hear about lice in the contact circle or suspect an infestation.
To prevent infestation, all parents should check their children at the start of the school year and after holidays abroad if the children have been in contact with local residents. It is the parent’s responsibility to check and treat their children for head lice but schools and childcare centres can encourage parents to check their children at the start of the school year.
School and childcare centres
There is no need to send children home from childcare or school for having head lice. They have probably had them for a long time, so a few hours make little difference (17). They could wear a headscarf or other headgear to prevent infesting others, and their parents should inform the contact circle that they have found lice. When the child comes home, treatment should begin the same evening. The child can go to childcare or school as normal. No-one should stay away at home because of head lice.
Inform the childcare centre / school and contact circle that your child has head lice, so they can also check for lice and avoid infesting others. The majority of those with head lice in Norway inform their contact circle (8, 9), and this allows others to intensify checks for a period. The school/childcare centre should inform other parents or guardians that lice have been detected within the school grade/department.
Head lice can be combated with delousing agents, by combing or by shaving the hair shorter than 0.5 cm.
Everyone with head lice should be treated as promptly as possible with one of these methods to minimise the infestation risk. If others in your family or contact circle have lice, they must be treated simultaneously to prevent further infestation and re-infestation (18).
Check the hair weekly for three weeks after the last treatment to see if the head lice are gone.
Delousing agents (pediculicides)
In Norway, about 90 per cent of households with head lice use a pediculicide (8, 9). They should only be used on people who have lice.
Pediculicides are divided into two categories: medicines and medical devices. Read the package insert carefully regardless of which preparation is used.
Of lice products classified as medical devices, there are products containing the active ingredient dimethicone and/or plant oils. Dimethicone is a silicone composition that physically kills lice. Dimethicone products are documented to have good effects (18, 19) and are recommended ahead of plant oils, which are less well-documented (20).
In Norway, one medicine is available that contains permethrin. In recent years there have been reports from many countries of a large degree of resistance among head lice to this product. Since it might not be very effective against head lice (18, 20) we do not recommend it.
Delousing agents are less effective against lice eggs (18), so treatment must be repeated after 8-10 days. During this period, any surviving eggs will have hatched but the new lice have not sexually matured yet and have not laid new eggs.
A lice comb can be used to combat lice. Not many people choose this treatment method (8, 9), probably because it is time-consuming. Comb wet hair systematically and thoroughly every day or every second day for 12-14 days. Check the hair weekly for three weeks after the last treatment.
For details about how to use a lice comb, see the paragraph «How to check for lice» above.
Lice cannot survive on hair shorter than 0.5 cm. Shaving is a good treatment option, if having short hair for a period is acceptable. Please note that the lice will crawl among hair clippings after shaving. Some lice will also attempt to cling on to stubble. Use a lice comb to remove them.
Treatment of objects
It is possible for lice to spread via combs and brushes and these should be washed or frozen if there are lice in the household. Freeze combs and brushes to -20 ºC for at least four hours or wash them at 60 ºC.
Transmission via other objects is unlikely (1). It is not necessary to wash clothes, furniture or the house itself if somebody has head lice. After lice treatment, washing bed linen to remove any old lice and their excrement may be done.
Head lice need human blood to survive and therefore live only on human heads. Animals have other species of lice that do not live on humans. Head lice suck blood at least six times a day (3). The chances of head lice transmitting disease-causing bacteria or viruses to humans are minimal, and in recent years, there have been no known cases in Norway.
The head lice development stages include egg, three nymph stages and adult male and female lice. An adult female louse lives for about 20 days, during which time she lays about 100 eggs. The eggs hatch after 8 days. During the next 9 days, the nymphs cast skin three times before they reach adulthood. When the female has been adult for 1-2 days she will begin to lay eggs (3). The development time for a generation from egg to egg is approximately three weeks.
How many head lice are there in Norway?
In a 2008 study involving 42 primary schools in Oslo, Bergen, Trondheim, Bodø and Tromsø, 1.6 per cent of pupils had head lice at any given time. The incidence was somewhat lower among younger and older family members (4). When the same schools were studied again in 2018, the incidence was similar among pupils at 1.7 per cent. Other countries, including several in Europe, have a much higher incidence than Norway (21).
Even though relatively few have head lice at a particular moment, many are affected by head lice through time. In the study mentioned above, 36 per cent of families with primary school children had had head lice once or more earlier. The highest previous prevalence was among families in Bergen and Oslo (45 and 43 per cent), and the lowest was in Bodø (22 per cent) (4).