Biology and Life Cycle of Lice

What are head lice?

The head louse, or Pediculus humanus capitis, is a parasitic insect that can be found on the head and, more rarely, the eyebrows and eyelashes of people. Head lice feed on human blood several times a day and live close to the scalp to maintain their body temperature.

What does head lice look like?

Head lice have a life cycle with three stages:

	Egg/Nits: These are lice eggs laid by the adult female louse at the base of the hair shaft near the scalp. The eggs are firmly attached to the hair shaft and are oval-shaped and very small and hard to see. Eggs vary in color from clear to light brown to yellowish-white. They are often confused with dandruff, scabs, or hair spray droplets. Eggs are usually located no more than 1/4in (.635cm) from the base of the hair shaft.
	Nymphs: A nymph is the immature louse that has recently hatched from the egg. Nymphs look like adult lice, but are smaller. Nymphs mature after 3 molts. Like adult lice, nymphs must feed regularly on human blood.
	Adults: The fully grown adult louse is about the size of a sesame seed, has six legs, and is tan to grayish-white in color. To survive, adult lice must feed on blood. An adult head louse can live about 30 days on a person’s head, but will die within 24-48 hours if it falls off a person.

Life span of head lice

• Eggs: Eggs usually take about 8-9 days to hatch.
• Nymphs: Nymphs mature into adults about 9-12 days after hatching from the egg.
• Adults: Adult lice can live about 30 days on a person’s head. If they come off the host, they will usually die within 24-48 hours.

Symptoms and Diagnosis of Lice

Because head lice and lice eggs are very small and hard to see, a louse infestation is often difficult to detect and diagnose – even for many health care professionals. At the time of detection, an average infestation often includes only 10-15 hatched lice and 20-30 eggs.

Symptoms and Signs of Lice Infestation

• Tickling feeling of something moving in the hair.
• Excessive itching, usually caused by an allergic reaction to the bites of the head louse.
• Irritability and difficulty sleeping; head lice are most active in the dark
• Sores on the head caused by scratching. These sores can sometimes become infected with bacteria found on a person’s skin.

Detection and Diagnosis of Lice

Head lice and eggs are found almost exclusively on human head hair and the scalp, frequently around and behind the ears and near the neckline at the back of the head. Head lice and eggs are sometimes also found on the eyelashes or eyebrows, but this is uncommon. Misdiagnosis of head lice is common.

The best diagnosis is by finding a live nymph or adult louse on the scalp or hair of a person. Adult and nymphal lice are very small, move quickly, and avoid light, so they may be difficult to find. Using a fine toothed louse comb helps in correctly diagnosing head lice.

Eggs that are attached within 1/4in (.635cm) of the base of hair shafts suggest the person may have an active infestation. Eggs that are found more than 1/4in (.635cm) from the base of hair shafts are almost always non-viable eggs (dead or empty egg casings).

If no live nymphs or adult lice are seen, and the only eggs found are more than 1/4in (.635cm) from the scalp, the infestation is probably old and no longer active.