What is 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. They cannot live without a host to feed on for more than 48 hours. Head lice live for about month, and their spans are categorized into three stages; nit (egg), nymph, and adult. Adult female lice attach nits to the hair follicle base via a naturally secreted adhesive. The nits are laid very close to the scalp (generally within six millimetres), as to provide heat for the incubation of the eggs. Nits are yellowish-white, oval-shaped eggs, about 0.8×0.3mm in size, and they usually hatch within a week to ten days, becoming nymphs. After the nit hatches, the shell remains attached to the hair follicle, and it’s color dampens to a darker shade of yellow. Nymphs look identical to adult lice, except they are slightly smaller and may also be slightly lighter in color. A nymph will shed it’s exoskeleton three times before reaching adulthood within a week of hatching.

Once reaching adulthood, the louse will require about 5 blood-feedings per day. The adult louse uses it’s claws to pierce the skin, after which it injects saliva and then sucks the blood through it’s mouth. The saliva that injects is primarily responsible for the irritation and itching sensation experienced. Although lice do not swell like many other ectoparasites, they do turn to a darker rust-colored shade after feeding. Despite their small size they can travel fairly quickly, and an adult female louse can lay up to eight nits per day. Since the adult lifespan of a louse is about 3 weeks, this means that a female can lay more than 150 nits in her lifespan, resulting in rapid reproduction and severe infestation if not treated.

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.

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.