Head lice are wingless insects approximately the size of a sesame seed that feed on human blood. They are the most common form of lice infestation in the United States. The majority of cases occur in children between the ages of 3 and 10 and their families.

Head lice live approximately 30 days, with the female laying between five and 10 eggs per day. Nits (lice eggs) have a white to brown appearance and are usually found on hair strands near the scalp. They are attached firmly and may be difficult to remove. Nits hatch within approximately six to 10 days as nymphs, becoming adults about 10 days after that.

Infestations of head lice are spread through head-to-head contact with an infested individual or through sharing certain personal items (e.g., hats, helmets, brushes, pillows). Head lice are largely dependent on their host for survival, in most cases dying after 24 hours if separated from the scalp and unable to receive a blood meal. Intense scalp itching (pruritus) is the most common symptom. However, many people with infestations of head lice show no symptoms and are unaware of their presence. Such people may be considered "carriers." 

Treatments for head lice infestation can often be administered at home without a physician’s prescription. More serious or resistant infestations may require prescription treatments. Some common treatment methods include topical pediculicides (e.g., permethrin), wet combing and oral medication (ivermectin).

Many schools have rules forbidding the return of children to school following the discovery of head lice until the child has been initially treated. Outbreaks in schools are often controlled by group examination of teachers and students under direct light and by giving students individual wall hooks or lockers for storing hats and jackets.

If a case of head lice is identified, checking all household members for the insect is recommended to minimize the likelihood of spreading head lice to others. Habitual washing of items that may have been in contact with the head (e.g., furniture, bed sheets, combs, brushes) of someone with head lice can help prevent future infestations as well as reduce the risk of spreading them to others.

Head lice (Pediculus capitis) are bloodsucking, wingless insects approximately the size of a sesame seed. They are the most common form of lice infestation in the United States. Although some estimates put the number of cases between 6 million and 12 million annually, there are no firm statistics.

Children between the ages of 3 and 10, and their families, are most likely to get head lice. Girls are more likely than boys to become infested, but the reasons for this are not known. Lice can appear in the hair of anyone regardless of length, although African Americans tend to have fewer cases of head lice infestation because the coarseness of their hair disrupts the ability of the female to lay her eggs (commonly called nits).

Head lice live approximately 30 days, with the female laying between five and 10 eggs per day. A female louse (the singular form of lice) typically lays her eggs close to the scalp, near the base of a hair shaft, for increased warmth. The nits stick to individual hairs by a cement-like substance, which is similar to the proteins found in human hairs.

The nits are shaped as small capsules, about the size of a pencil-point and resemble dandruff. This similarity often leads to over-diagnosis of head lice in schools when cases of head lice are discovered, and helps account for why there are not accurate statistics on lice infestations. Nits hatch within six to 10 days as nymphs, becoming adults about 10 days after that. They usually die after 24 hours if they are separated from the scalp and are unable to feed. In some warm and humid environments, nits may be able to develop and hatch for up to 10 days away from the host.

Head lice are most often found on the scalp, behind the ears and near the nape of the neck. They are rarely found on the body, eyelashes or eyebrows.

Head lice are not believed to transmit serious infections, although some types of bacteria (e.g., staph, strep) have been found in them. Because the primary symptom of infestation is itching (pruritus), frequent scratching can cause irritation and skin infection (pyoderma).

A person who is diagnosed with head lice may feel extremely embarrassed, because infestation is incorrectly associated with poor hygiene. Contrary to popular misconception, cleanliness is not a factor in head lice infestation. Anyone who comes into close proximity with an infested person can get head lice.

Children diagnosed with nits are often sent home from school and not allowed to return until all evidence of nits and lice are gone. However, lice are often present for a number of weeks before being detected. Furthermore, the presence of nits does not necessarily mean there is an active infestation if lice are not found. Nits may continue to be found for several months following successful therapy.

In addition, some people continue to believe they are infested even when the lice have been eradicated. Although some lice may be resistant, treatment is very often successful when completed as directed by a physician.

In addition to head lice, other types of lice can infest humans, including:

• Body lice (Pediculus corporis). Occurs most frequently in underdeveloped nations and among homeless people. Individuals with body lice tend to be impoverished, have poor hygiene and live in overcrowded areas. Infestations occur when clothes are not washed or changed frequently. Lice tend to live and lay eggs in the seams of clothing and feed on areas of the back, shoulder and waist. Body lice, and the environmental and social factors that contribute to them, are responsible for the spread of epidemic typhus, trench fever and relapsing fever.
  
  
• Pubic lice (Pthirus pubis). Usually transmitted by intimate or sexual contact. Pubic lice bear a resemblance to the common crab, with infestations sometimes referred to as a case of “crabs.” They are most often located around the genitals but may spread to other areas (e.g., abdomen, armpits).

Lice are wingless insects. They do not fly or jump. Infestations of head lice (Pediculus capitis) are spread primarily through head-to-head contact with an infested individual. It can also be spread by sharing clothing (e.g., hats), helmets, pillows, combs, brushes and other objects. Other types of close contact can also cause infestation.

Age is a risk factor for head lice, with the majority of infestations occurring among children between the ages of 3 and 10 in the United States. Children at these ages often play closely with one another in school or daycare, and they frequently share objects or clothing as well. Close contact with family members can also spread head lice in rare cases. Household pets do not spread lice.

For unknown reasons, females are also more likely to be infested than males.

The most common symptom of head lice infestation is intense itching (pruritus), although the severity can vary from person to person. Itching is caused by the body's reaction to the lice's saliva or feces (similar to an allergic reaction), not to the bites themselves. Some people may not itch.

When itching occurs, it may take two to six weeks after the initial infestation for pruritus to develop due to the body's immune response. Itching may occur more quickly (sometimes between 24 and 48 hours) if a person becomes infested again. Frequent scratching can cause sores, which may become infected. Signs of infection include redness and tenderness. Patients with head lice may also experience a low-grade fever and swollen lymph nodes.

Children with head lice may display irritability as the result of discomfort. They may also describe a "tickling" sensation on the scalp or a feeling that something is moving around on their head.

A diagnosis of head lice (Pediculus capitis) is made by spotting nits (lice eggs) and/or adult lice in the hair of the scalp. Eggs that are still in development are usually a tan or brown color, while hatched eggs are clear or white and may be difficult to distinguish from other common findings on the scalp.

For instance, dandruff, dried hair gel and hair casts (ringlike parts of hair follicles) can appear as nits. However, unlike these materials, nits cannot be slid along a strand of hair or easily brushed out of the hair. In addition, itching can be caused by many scalp-related conditions. This difficulty has sometimes led to over-diagnosis of head lice.

The U.S. Centers for Disease Control and Prevention (CDC) notes that it is difficult to spot nymphs (newly hatched lice) or adults because they move quickly. There are also usually only a few present at a time. Although there can be hundreds of nits present on the scalp, there are usually only about a dozen hatched lice at any given time.

According to the CDC, finding nits within 1/4 inch (6.5 mm) of the scalp confirms that an infestation is present and should be treated. Finding nits more than 1/4 inch from the scalp usually indicates that the infestation is an old one and does not require treatment.

It is important to note that infestations are active only if there are lice. Nits can persist for several months, even after successful treatment. Because of the risk of over-diagnosis, some physicians recommend using a fine toothed comb to detect lice and nits. People who are not sure if an infestation is active should consult their physician, health care provider, a school nurse or a professional from the local health department.

Once a child has been diagnosed with head lice, their parent should notify the staff at the child's school.

Treating head lice may include the use of a specialized comb to remove nits or medications that may be available over-the-counter or by prescription, depending on the severity of the infestation.

All medicines used to kill hatched head lice are called pediculicides. Treatments are often effective at killing nymphs (newly hatched lice) and adults, but not nits (lice eggs). The appearance of nits alone without lice does not by itself indicate an active infestation. Nits may remain for several months following successful treatment. In addition, patients who have been successfully treated with lice medication may continue to experience itching for a few days following the treatment.

Common methods used to treat head lice include:

• Topical pediculicides. The most common and first choice of treatment for head lice. Topical preparations (e.g., permethrin, pyrethrin, malathion) are effective in treating head lice in the vast majority of cases. However, resistance to these treatments by lice is an increasing concern.
  
  After the scalp has been washed with shampoo, rinsed with water and towel dried, the scalp is saturated with the appropriate topical pediculicide. It remains on the hair for 10 minutes before it is rinsed off with water. A second application is usually recommended within seven to 10 days after the initial treatment due to increased resistance by head lice to these preparations.
  
  Products containing permethrin and pyrethrin are available in over-the-counter medicines. Products containing malathion must be prescribed by a physician. Lotions with malathion may not be preferable, as they must be left in place for eight to 12 hours before being washed off. Lindane shampoo may be effective in some instances, but it is generally not used due to the risk for serious, though rare, side effects (e.g., seizures) and growing resistance from head lice to the drug.
  
  It is important for people to apply these medications exactly as stated on their labels. Applying these drugs too often or in too large of an amount can irritate the scalp. Improper use of topical pediculicides also decreases the likelihood of the treatment being successful. It should also be noted that topical lice treatments, whether prescription or over-the-counter, should never be used on the eyebrows or eyelashes. In addition, individuals should only use one form of medication at a time.
  
  
• Wet combing. Removal of head lice by running a fine toothed wet nit comb through the hair under direct light. Wet combing is generally used on young children under 2 years of age, women who are pregnant or breastfeeding, and people at risk for complications from pediculicides (e.g., allergic reaction).
  
  The hair is first brushed or combed to remove tangles that may be present. The wet comb is then placed near the top of the head (crown) until it touches the scalp. It is then passed firmly down through the hair repeatedly and examined closely after each stroke until lice are no longer found.
  
  The process may be repeated as often as every three to four days for several weeks if necessary. Lubricants (e.g., hair conditioner, vinegar) may increase the effectiveness of wet combing, though it is still less effective than medical therapy. Cutting the hair short before wet combing may make it easier to find and remove lice.
  
  
• Medication. Some oral medications (e.g., ivermectin) may be effective in cases where treatment with topical pediculicides does not successfully eradicate head lice infestation. In such cases, a single dose of an oral agent is given followed by a repeat dose two weeks later. In addition, combination therapy with topical pediculicides and oral medications may be recommended in instances where head lice show resistance.

Failure to properly follow application instructions and continued contact with other infested and untreated individuals are the most common reasons for unsuccessful treatment. However, resistance to treatment by head lice is an increasing medical concern. Most cases of head lice resistance can be successfully treated by applying different medicines to the scalp.

Alternative, non-pesticidal treatments (e.g., petroleum jelly, olive oil) may be effective at slowing the movements of adult head lice so that they may be combed out more easily. It should be noted, however, that these substances do not eradicate infestations of head lice and are typically not encouraged as specific treatments. Patients are encouraged to consult a physician if symptoms of skin or scalp infections (e.g., redness, swelling) develop from scratching. The infection may require treatment with an antibiotic.

Patients should also contact their physician if home treatment using over-the-counter medicines is ineffective or use of any medication more than three times on one person, or after two weeks of starting treatment, is ineffective. The physician may recommend another form of over-the-counter medication or a prescription medication.

Head lice are spread by head-to-head contact or through sharing certain personal items (e.g., hats, helmets, brushes, pillows) with infested individuals. Many adults or children may have head lice and never show any signs of infestation (e.g., itching), making prevention of additional infestations rather difficult.

If a case is identified, checking all household members for head lice is recommended to minimize the chances of spreading head lice to others. However, only those found to be infested should be treated. Lice medications should not be used as a preventive measure.

Looking for lice in the hair regularly combined with the habitual washing of items that may have been in contact with the head (e.g., furniture, bed sheets, combs and brushes) can help prevent infestations from later recurring. Steps to prevent infestation include:

• Washing clothing, bed sheets and towels that the infested person had contact with during the two days prior to their diagnosis. The items should be laundered in the washing machine’s hottest setting. They should then be placed in the dryer for at least 20 minutes in the hot cycle. Non-washable items (e.g., pillows, coats, stuffed animals, helmets) can be dry cleaned or sealed in a plastic bag for two weeks.
  
  
• Soaking combs, brushes and hair accessories (e.g., barrettes, headbands) in rubbing alcohol or medicated shampoo for one hour. They can also be washed with hot water and soap or simply thrown out and replaced.
  
  
• Vacuuming mattresses, as well as the floors and furniture in the home. Cars should also be thoroughly vacuumed as well.

Preparing questions in advance can help parents have more meaningful discussions with their child's physicians. Parents may wish to ask their child's doctor the following questions related to head lice:

1. How did my child get head lice?
   
   
2. Do head lice pose a danger to my child?
   
   
3. What are my child’s treatment options? How effective are they?
   
   
4. When can I expect my child’s symptoms to subside?
   
   
5. How long should I keep my child out of school or away from other children?
   
   
6. How can I prevent other members of my family from becoming infested?
   
   
7. How can I keep my child from getting another infestation?
   
   
8. How prevalent is head lice in our community?
   
   
9. How can I distinguish head lice from other materials that may be caught in the hair (e.g., dandruff, dirt)?
   
   
10. How do I know if I have gotten rid of all of the head lice?
    
    
11. What can I do to keep my child from constantly scratching?