It is possible to have head lice for years without knowing it, especially if the person does not experience itching. Generally by the time someone identifies head lice, he/she has had head lice for four to eight weeks.
Itching is caused by an allergic reaction to the saliva that lice secrete when feeding. The saliva keeps the blood from clotting, making the feeding process easier. About half of those who get head lice are not allergic to the saliva and therefore don’t itch. Even if someone is allergic, it takes approximately two weeks to build up enough lice saliva to cause itching
Check your child’s head on a regular basis and run a lice comb through the hair once or twice a week. Be aware of your child's behavior and watch for telltale signs.
Besides itching, symptoms of head lice may include a low-grade fever, swollen glands, a rash at the nape of the neck, bags under the eyes, and feeling tired. Head lice are nocturnal, meaning that they are more active at night than during the day.