Related concepts:

Pediculosis, Head lice

Introduction to lice:

My head starts to itch when I even write about lice. Lice are a common problem wherever children gather. If you are concerned about lice, you are not alone. Each year, many day-care centers, schools, neighborhoods, extended families, and small family units face this problem.

What is lice?

Adult lice (also called Pediculus humanus capitis) are six-legged, wingless insects 2-4 mm long. They have translucent grayish-white bodies, and look a bit like a grain of rice with six legs. Their heads have two tiny eyes (too small to be seen without magnification) and two small antennae (usually visible). Six pairs of hooks that surround the mouth allow them to attach themselves to the skin of the scalp for feeding. The mouth contains two retractable, needle-like tubes that pierce the scalp. Salivary juices are injected into the scalp to prevent blood from clotting, and then the lice feed happily, sucking blood through these same tubes. Their translucent bodies turn reddish brown when engorged with blood. Lice completely depend on the blood extracted from humans for existence, and thus will starve to death after 55 hours without blood.

Adult lice can freely move around a head of hair and travel to another person, clothing, plastic combs or brushes, or upholstered furniture. Adult lice usually live for about a month on a human host. During this time, the females generally lay from three to 10 eggs per day (although some female lice have been known to lay up to 5,000 eggs in their lives when in an environment to their liking).

Lice eggs are called nits. These white, translucent, pinpoint-sized eggs are laid near the base of hair shafts, and move outward as the hair grows (nits found near the tips of long hairs suggest a longstanding infestation). Nits are glued tightly to the side of the hair shafts, and cannot be moved along the shafts or knocked off with fingers. The eggs hatch between ten to fourteen days after they are laid. The empty eggs remain attached to the hair shaft. The newborn larvae must feed on human blood within 24 hours, or they will starve to death. The larvae become sexually mature adult lice within about one week. Adult head lice can survive up to two days away from the scalp, which is how they are transmitted by things like combs, brushes, and hats.

During this whole life cycle, larvae and adult lice deposit their feces in the scalp, which eventually causes itching as the person develops an allergic reaction to the lice stool.

Who gets lice?

Lice seem to prefer children to adults, long hair to short hair, and they particularly like the hair of females. Interestingly, lice only rarely afflict African Americans living in North America. The lice in Africa and South America have adapted, however, and cases are common in every group on those continents. Cases of lice are most common in children 3 and 10 years old, but can occur at any age.

Lice have been a nuisance to humans since ancient times. Having head lice is not a sign of poverty or poor hygiene. They have thrived almost wherever humans have been in prolonged close contact with each other. One notable exception to this has been in areas where the pesticide DDT is in widespread use. In the United States, for a period of about 30 years, lice outbreaks were uncommon.