Medically reviewed on May 30, 2017
What are they?
Head lice are small, parasitic insects that infest hair on the head of people. Their six legs allow them to grasp and wander between hairs. They feed only on the blood of people, and feed once or more each day. They are more common on children than adults.
A single insect is called a louse. A female louse can lay more than 100 eggs, called nits, in her lifetime. The female louse secretes a glue to attach each egg to a scalp hair, usually close to scalp level. They hatch about 8 days after they are laid, and grow to the adult stage about 9 to 12 days after hatching. Adult head lice survive just a few weeks on the scalp, but die within a day once removed from a person.
In general, a person infested with head lice will have fewer than a dozen live lice on his or her head at a time, although hundreds of developing, dead and hatched eggs may be present.
The most common symptom of a head lice infestation is itching. This is caused mainly by the louse's saliva, which irritates the skin on the scalp. The itching can occasionally be severe enough to prevent sleep, and some people scratch so much that they cause additional skin infections. Head lice are not known to transmit any infections.
Because itching on the scalp may result from many causes other than head lice, a medical professional should diagnose a head-lice infestation, and this should be based on the finding of a live (crawling) louse. Louse infestations may be difficult to identify properly, since lice and their eggs are often hard to distinguish from other materials caught in the hair, such as dandruff, scabs, dirt or insects blown in by wind. In general, only those people with live (crawling) head lice should be treated. The eggs or nits of the louse appear as small white, tan or brown dots attached firmly to individual hairs. If nits are seen, then live lice should be sought before considering treatment.
Head lice sometimes go away on their own because there are not enough insects to maintain the infestation, or they may persist for an indefinite period without treatment. With proper treatment, the infestation usually goes away within about two weeks.
Head lice are acquired by direct contact with an infested person's hair. Only rarely would lice be transferred by shared combs, brushes, hats or other hair accessories.
It is not necessary to isolate infested people or keep children with head lice home from school. Head lice neither are caused by, nor a sign of, poor hygiene or housekeeping.
There are several treatment options to eliminate head lice. In some cases, several methods can be used together to ensure success. First, make sure you have seen a live louse. Do not treat if you've only seen objects you believe to be louse eggs (nits). If you find a live louse on one person in the home, then inspect all other adults and children for lice as well.
If the hair can be combed easily, then you can use a special louse comb or nit comb to remove lice and their eggs from the hair. This sometimes requires combing several times a week until no lice are seen for two or more weeks. Although combing alone may eliminate lice from some people, this method fails for others. If the process requires too much time or causes discomfort, then it is not a good option for treatment.
Pediculicides are special insecticide products (registered by the Food and Drug Administration) that are available over the counter and by prescription in drug stores. Do not apply any other kind of insecticide to the body or hair. Most pediculicides contain a class of chemicals called pyrethroids as the active ingredient. These usually are applied as a shampoo then washed off about 10 minutes later. A second treatment is often necessary, about 10 days later. Although these products work very well for many infestations, some head lice are resistant to them. If live (crawling) head lice are discovered a day after treatment, then you should speak with your doctor about other kinds of pediculicides that are available by prescription. Carefully read and follow the instructions on the product. Do not overtreat. If you have any questions about the use of the product, speak to your doctor or pharmacist.
Do not apply products that are not registered by the FDA as pediculicides. These include many kinds of essential oils, enzyme treatments, antibiotics, vegetable oils, or heat. Such treatments may waste time, money and effort, or may even harm the person being treated. Although head lice are only rarely found on clothes, towels and bed linens, you may want to launder or change any items that were in direct contact with the hair of the infested person during the last day or so. Washing items in hot water or drying them in high heat will kill lice and their eggs. Because head lice do not survive long once they are removed from a person, there is no need to clean or vacuum the home environment intensively.
When To Call a Professional
You should call your physician to confirm the presence of a head-lice infestation and to get help in determining how best to manage the problem. If you are unsure if the objects you see are lice or eggs, then you should have these examined before you treat.
The outlook for recovery from head-lice infestation is excellent. Head lice do not transmit other types of infections, nor do they produce symptoms other than itching. Active infestations that are treated according to directions will soon be eliminated.
Harvard T.H. Chan School of Public Health
Department of Immunology and Infectious Diseases
665 Huntington Ave. FXB 301
Boston, MA 02115
Phone: (617) 432-1023