Biology and Control of Head Lice


The head louse, Pediculus humanus capitis De Geer, infests 10 to 12 million people each year in the United States. Pediculosis or "lousiness" is one of the most prevalent communicable conditions in this country. Lice are transferred from person to person by direct contact or by several people using the same combs, brushes, hats, or bedding. Head lice are not found on animals or household pets and are not transmitted from pets to humans. Head louse infestations are normally found on children, but can also be spread to adults. The head louse is not considered to be a serious vector of disease in the United States although severe infestations may cause irritation, scratching and subsequent invasion of secondary infection.

Biology of the Head Louse

Lice have three pairs of legs which makes them true insects. Lice do not have wings or powerful jumping legs so they move about by clinging to hairs with claw-like legs. Head lice prefer to live on the hair of the head although they have been known to wander to other partsof the body. Head lice do not normally live within rugs, carpet, or school buses.

The eggs of lice are called nits. They are oval white cylinders (1/16 inch long). The eggs of head lice are usually glued to hairs of the head near the scalp. The favorite areas for females to glue the eggs are near the ears and back of the head. Under normal conditions the eggs will hatch in seven to 10 days. The young lice which escape from the egg must feed within 24 hours or they will die. Newly hatched lice will periodically take blood meals and molt three times before becoming sexually mature adults. Normally a young louse will mature in 10 to 12 days to an adult (1/8 inch in length). Adults range in color from white to brown to dark gray.

Female lice lay six to seven eggs (nits) per day and may lay a total of 50 to 100 eggs during their life which may last up to 40 days. Adults can only survive one to two days without a blood meal. The nymphs and adults all have piercing-sucking mouthparts which pierce the skin for a blood meal.

The reaction of individuals to louse bites can vary considerably. Persons previously unexposed to lice experience little irritation from their first bite. After a short time individuals may become sensitized to the bites, and may react with a general allergic reaction including reddening of the skin, itching, and overall inflammation.

Prevention of Head Lice

Children should be encouraged not to share combs, hats, and other personal belongings. Daily washing and changing of clothes and keeping hair as short as possible will also help discourage lice; however, head lice should not be solely associated with uncleanness since they may be easily transferred from person to person. Periodic inspections will aid in early detection of any individual lice which are more easily controlled than advanced infestations where dozens of mature lice and possibly hundreds of nits are present. During the early fall months (August through November) children should be inspected weekly because back-to-school seems to be when lice are most commonly transmitted resulting in widespread infestations by December and January. September is National Head Lice Prevention Month.

Control of Lice

Non-Chemical Control

Once an infestation is detected all clothes should be washed in hot soapy water. Pillow cases, sheets, blankets and other bedding material should also be washed and placed in the clothes dryer on "high heat" cycle to kill the lice and their eggs. Any non-washable items such as children's toys should be tightly sealed in plastic bags for at least seven to 10 days to kill adult lice.

Chemical Control

Chemicals are available as prescription or non-prescription drugs to control lice. Over-the-counter products which should be effective include those containing permethrin or pyrethrins (pyrethrum extract) as active ingredients. These drugs are available as creams, lotions, or shampoos. Shampoos are preferred for control of head lice. The application of these insecticidal drugs will kill nymphs, adults, and some eggs. Eggs killed by treatment as well as unaffected eggs may remain attached to hair shafts and should be removed as soon as possible. To remove these eggs it may be necessary to do some "nit-picking" utilizing a special fine-toothed lice comb. Combs and other tools used to remove lice should be soaked in a lice killing solution such as rubbing alcohol after use.

Use of lice sprays to treat objects such as toys, furniture, carpet etc. is not recommended because lice cannot live off the host longer than a couple of days. The same holds true for classrooms Use of these products is considered ineffective and unnecessary.

Cream and Shampoo products** include:

Pyrethrin-Containing Products (kill in 10 to 12 minutes, but have no residual activity which means more than one application may be necessary)

Permethrin-Containing Products (kill in 10 to 12 minutes, activity lasts up to 10 days)

** Careful attention should be given to follow directions on all product labels.

Products Not Recommended due to Reports of Potential Side Effects.

Frequently Asked Questions:

  1. What's the difference between pyrethrins and permethrin?
    Pyrethrins are naturally occurring compounds extracted from chrysanthemum plants. Insects possess mechanisms to break down pyrethrins so a synergist, piperonyl butoxide, is usually added to the formula which inhibits the insect's ability to break down the pyrethrins. Permethrin has the same basic chemical make-up but is synthetically enhanced to be more effective. Permethrin has much longer residual activity and does not require the use of a synergist.
  2. Can lice be transmitted from my dog or cat to my children?
    No. Head lice are specific to humans. They can only survive on a human host. If a head louse is removed from its host, it will die within a day or two.
  3. How big are lice?
    An adult head louse is approximately 2.5 to 3.5 mm long. A louse egg, or "nit" is roughly 0.5 to 1.0 mm long.
  4. I've heard that head lice are resistant to some of the lice treatments; Is this true?
    In some locations there have been reports of potential resistance to several different pediculicides (lice killing agents). Experts are currently researching this issue and are trying to determine to what extent resistance exists. The best way to avoid this situation is to follow the directions on the product label closely. Improper application of pediculicides has significantly contributed to the presence of resistance in lice.
  5. How long do lice live?
    A louse egg hatches about 10 days after being glued to a hair shaft. Upon hatching, a louse will molt three times over a 10 to 12 day period before molting into a mature adult. An adult louse can live up to 40 days.
  6. How long after my child contracts lice will I be able to detect them?
    It only takes one adult female louse to begin an infestation. A female louse can lay several eggs each day. Within a couple of weeks a child may have dozens of immature lice living on his/her head. The more lice present, the more quickly the child will begin feeling an "itchy" scalp which would usually lead to an inspection by a parent or school official. Looking for one louse or one nit might be difficult but dozens should be readily apparent through close inspection.

Authors: Clay Scherer and Dr. Philip Koehler - University of Florida

Photos: University of Florida