About Lice

What are lice? Adult head lice ( Pediculosis capitis) are parasites that are 2 to 3 millimeters long, about the size of a sesame seed and usually pale gray. Nits, their eggs, are about 1 millimeter long -- hard to see with the naked eye. A female louse lays about 10 eggs per day, attaching them to hair with a glue. A nymph hatches from its egg after about eight days. Lice feed only on humans, not pets or other animals.

Who gets them? An estimated 6 million to 12 million children get head lice each year, according to the American Academy of Pediatrics; Harvard public health entomologist Richard Pollack estimates the number is much smaller. African Americans are less likely than others to be infested with lice. Prevalence is not related to cleanliness. Head lice should not be confused with body lice or pubic lice.

What are the symptoms? Primarily itching. As the lice suck blood from the scalp, they inject saliva into the wound, which causes an allergic reaction.

How are they spread? Lice cannot hop or fly; they crawl. Transmission mostly occurs by head-to-head contact. Much less frequently, lice are spread by combs, brushes and hats.

Standard treatment Over-the-counter treatments include A-200, Pronto, R&C, Rid and Triple X with pyrethrins, often combined with piperonyl butoxide, and Nix with permethrin. Prescription treatments include Ovide, which contains malathion (for ages 6 and older) and lindane shampoo (prescribed with caution in patients less than 110 pounds; seizures reported after excess dosage).

Alternative treatments Pollack says the oils commonly used in natural treatments may have insecticidal qualities but haven't been carefully studied for lice treatment in humans, and concentrations aren't regulated. There's no clinical evidence that petroleum jelly, mayonnaise, margarine or olive oil smother lice on human heads. Studies on effectiveness of frequent combing alone have found most combers fail to get all the nits. A second treatment is often necessary, about 10 days after the first, because some eggs may survive the first treatment.

For more information

¿ Centers for Disease Control and Prevention, http://www.cdc.gov/ncidod/dpd/parasites/lice/default.htm.

¿ Harvard School of Public Health, http://www.hsph.harvard.edu/headlice.html.

¿ American Academy of Pediatrics, http://www.aap.org. For information on the effectiveness on treatments and resistance, see the AAP's clinical report on head lice published in the journal Pediatrics in 2002.

-- Elizabeth Agnvall

SOURCES: Harvard School of Public Health, Centers for Disease Control and Prevention, American Academy of Pediatrics