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You can have head lice without knowing it, as there are often no symptoms.

This means it can take weeks or even months before you realize that you or someone in your family has head lice.¹

But you may occasionally notice some of the following symptoms.

Some people get an itchy scalp when they have head lice. We don't know exactly how many people with head lice get an itchy scalp because there hasn't been much research.^{2 3} This is difficult to study because just the thought of head lice makes some people itch.

It's the louse's saliva that can cause itching. Head lice inject saliva into your blood as they feed. The saliva contains chemicals that stop your blood from clotting.

Your body's immune system (which protects your body against infection) detects the saliva in your blood. Because the saliva is foreign to your body, your immune system responds by releasing chemicals into your bloodstream to attack the saliva and get rid of it.

Some of the chemicals act at the site where the saliva got into your body, and it's these chemicals that make you itch. It can take up to three months for the itching to start.

On most scalps, bites from head lice are almost impossible to spot. But if you know what you're looking for, you may be able to see them. They are small and round (about the size of a pinprick) and red from the scab that has formed over the bite. Occasionally, bites can become infected (see below) and the scabs can become crusty and itchy.

You may notice nits in the hair. These are the shells (also called casings) of the head lice eggs, and they are usually near the scalp, where the eggs are laid for warmth and moisture. Unhatched eggs are colorless and therefore very difficult to spot. But once the baby head lice hatch, the casings that are left behind look white and are easier to see.

As the hair grows, the eggs or their casings move away from the scalp. If you see nits more than a half inch from the scalp, it means either they have hatched already or the eggs are not going to hatch.

Seeing nits is not a sign that you currently have head lice, since they may be empty shells or dead eggs. But they are a sign that you need to check your hair for head lice. You have to find a live head louse on your hair to be sure you have them.⁴

You might see a head louse crawling through your child's hair or your own hair. If you have not seen head lice before, you will find it difficult to spot one. You're more likely to come across a head louse or its discarded skin when you comb the hair. But unless you know what you're looking for, you might miss these signs. To learn about what head lice look like, see What are head lice?

Head lice are rarely more than a nuisance. But, occasionally, the bites can become infected. This is more likely to happen if you scratch your head and pull the scabs off bites. If the head lice in your hair are carrying bacteria (germs), these can get into the wound.

One of the most common infections from head lice is impetigo of the scalp. This is a skin infection that head lice can pick up from one person, carry on their body and transmit to someone else. Impetigo causes crusty and very itchy scabs. Impetigo needs to be treated with antibiotics.

Impetigo is also easily passed on (highly contagious). If you have crusty or itchy scabs on your scalp, you should see your doctor right away. You should get rid of the head lice at the same time you're taking antibiotics for the infection.

If head lice go untreated for a long time (say, 12 months to 18 months), they can make you feel generally low and tired.² This is because your immune system becomes worn down after this time. Since head lice are usually treated these days, this is now rare.

Some people say that head lice leave a gritty black powder on their pillow. But there is no evidence for this. The head lice do leave tiny bits of blood they've digested, but the amount is very small (unless the infestation is very heavy).

Any waste from the head lice will normally be dissolved in sweat, especially at night. Children sweat more than adults, and this means their pillowcases may become dirty more quickly than usual.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.