Piojos y liendres

What do head lice look like?​

• Piojos are tiny grey/brown insects. They are about the size of a sesame seed - the seeds on burger buns. Head lice cling to hairs but stay close to the scalp which they feed off. Head lice lay eggs which hatch after 7 to 10 days. It takes about 7 to 10 days for a newly hatched louse to grow into an adult and start to lay eggs.

• Nits are the empty yellow-white eggshells which are left when the lice hatch. Nits look like dandruff but stick strongly to hair. Unlike dandruff, you cannot easily brush out nits.

Where do head lice come from​?

Male and female head lice size comparison

© Pediculosis, CC BY-SA 3.0, via Wikimedia Commons

Head lice look like tiny little ants, and the eggs look like white spots clinging to hair strands. Nits look a lot like dandruff; in fact it's more or less impossible to tell the difference.

Lice feed from the blood on your scalp, and it is the bites - and your skin's reaction to the bites - which make you itch.

The more time you spend with your head in close contact with someone who has nits, the more likely you are to catch them yourself. While the eggs (nits) cannot move, lice can walk from one head to another. The more hair available for lice transfer, the more likely this transfer is. So it's more common in kids with longer hair, and more likely if hair isn't tied up.

Head lice don't jump and they don't fly, so you do have to have hair-to-hair contact. Fortunately, lice don't live for very long off a human head, but they do survive for a day or two, so you can also catch them by sharing a comb or hairbrush with someone with lice.

Head lice symptoms

• Itchy scalp. Not all scalp itching is caused by head lice, and head lice don't always cause an itch. Many people with head lice do not have any symptoms. However, an itchy scalp occurs in some cases. This is due to a skin reaction to the lice bites or saliva, not due to their biting as such, or their movement on your head. It can take about three months for an itch to develop after you are infested with lice. Therefore, you may not notice that you have head lice for a while and you may have passed them on to others by the time you are aware of them. Head lice do not wash off with normal shampoo.

• Rash on the scalp. Head lice do not usually cause any other medical problems. Very occasionally, the scratching can cause a rash on the scalp, and occasionally this can cause an infection.

• Glándulas linfáticas inflamadas. Glands in the neck can get bigger in response to this infection.

It can be embarrassing for children or parents to admit they have found head lice. Sometimes parents can be too embarrassed to tell the school, or their children's friends. This may encourage spread, as others may not be aware they have been exposed. Sometimes repeat infestations can be frustrating for children and their families.

The number of lice that may be on one person can vary greatly. There may be just a few in some; in other cases there are more than 100. Very rarely in very severe cases there may be over 1,000. Typically in the UK, there are around 30 lice present on each affected person's head.

How to know if you have nits?

The only way of telling for sure whether you have head lice is to see a live louse. If someone has quite a lot of lice, you may sometimes be able to see them just by looking through their hair. The best way to check for lice, however, is with a special nit detection comb. These combs have very fine teeth designed to pull out lice. If you have head lice, when you comb through hair, one or more black nits come out on the comb. You can buy the special comb from your chemist.

Some people advise that you do this to children's hair regularly, about once a week. This may not always be necessary but can be helpful if people you are in contact with have head lice, or if it is a recurring problem.

Detection combing: wet hair method

This will take 5-15 minutes to check each head, depending on hair length and thickness. It is also used as a head lice treatment .

• Wash the hair in the normal way with ordinary shampoo.

• Rinse out the shampoo and put on lots of ordinary conditioner.

• Comb the hair with a normal comb to get rid of tangles.

• When the hair is untangled, switch to a detection comb. This is a special fine-toothed comb to remove head lice and nits. (The teeth of normal combs are too far apart and the teeth of 'nit combs' are too close together.) Most pharmacies stock detection combs.

  • Bug Buster® detection combs are available on prescription. You can also get them by UK mail order from Community Hygiene Concern.

  • The Hedrin® detection comb is not available on prescription but can be bought from pharmacies.

• Slot the teeth of the detection comb into the hair at the roots so it is touching the scalp.

• Draw the detection comb through to the tips of the hair.

• Make sure that all parts of the hair are combed by working around the head.

• Check the comb for lice after each stroke. A magnifying glass may help.

• If you see any lice, clean the comb by wiping it on a tissue or rinse it before the next stroke.

• After the whole head has been combed, rinse out the conditioner.

• Repeat the combing procedure in the wet hair to check for any lice that might have been missed the first time.

Detection combing: dry hair method

This will take at least 3-5 minutes to check each head, depending on hair length and thickness. Although dry combing is an option to detect head lice, it is reportado useful as a treatment.

• Untangle the dry hair, using an ordinary brush and comb.

• Once the comb moves freely through the hair without dragging, switch to a detection comb, as described above.

• Starting from the base or the side of the scalp, comb the hair from the scalp down to the end of the hair. Comb each section of hair 3-4 times before moving to an adjacent section.

• Look for lice as the comb is drawn through the hair.

• If a possible louse is seen, trap it against the face of the comb using your thumb. This avoids the risk of the louse being repelled by static electricity as the comb is pulled out of the hair.

• Continue combing the hair section by section until the whole head of hair is combed through.

See the separate article called How to check your child for head lice and nits for more details.

Head lice treatment

The main ways of treating head lice and nits are:

• Medicated lotion and spray treatments.

• Wet combing.

Most insecticides need to be used twice, and combing has to be done several times over at least two weeks. Everything that you need to treat head lice can be bought over the counter - NHS England has advised doctors not to prescribe for this problem and so it is best to see a pharmacist rather than your doctor.

It is almost impossible to prevent head lice entirely, although if you have a child with long hair, tying it back will reduce the risk of catching them. Detection combing once a week will allow you to pick up and treat head lice early.

Read more detail on treating and preventing head lice.

Who gets head lice?

Head lice are common all over the world.

They are not a sign of dirty hair or poor hygiene. Close hair-to-hair contact is usually needed to pass lice on. Head lice cannot jump or fly but walk from one head to another. They soon die when away from hair and they do not live for long in clothes, bedding, etc. However, they can occasionally be passed by sharing items such as brushes, combs, hats or bedding. Most head lice infections are caught from family or close friends who are not aware that they have head lice.

Head lice are more common in young children with long hair and, partly, for this reason, tend to be more common in girls. The more children there are in any one family, the more common head lice are.