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Head Lice

The Life Cycle, Symptoms, and Spread of Head Lice

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Updated July 03, 2014

Head lice are parasitic insects found most commonly behind the ears and near the neckline at the back of the neck. The medical term for head lice infestation is Pediculus humanus capitis. There are 6 – 12 millions cases of head lice in the United States every year more commonly in children than adults. Head lice infestations are actually rare in African Americans possibly because the louse prefers a specific hair shape and width. A head lice infestation can be psychologically disturbing, but head lice are not a health hazard and are not responsible for the spread of any disease.

Head Lice Life Cycle

The life cycle of the head louse has three stages: nit, nymph, and adult. The life cycle of a single louse from egg to egg is about 1 month.

Nits: Head lice eggs are called nits. Nits are laid by the adult female louse and are attached to the base of the hair shaft by strong cement. They are located close to the scalp because they require body heat for incubation. More nit characteristics:

  • Size - 0.8mm by 0.3mm
  • Shape – Oval
  • Color - Yellow to white
  • Location – Within 6 mm of the scalp
  • Duration - Nits take 6 – 9 days to hatch.
  • Nit Picture

Nymphs: When an egg hatches, it releases a nymph. The nit shell stays attached to the hair and turns a dull yellow color. The nymph looks like an adult louse but is smaller. As the nymph grows, it molts out of its exoskeleton 3 times before it becomes an adult. More nymph characteristics:

  • Size – Variable but about the size of a pinhead
  • Shape – Same shape as the adult louse
  • Color – Yellow to rust-colored
  • Location – Close to the scalp, especially behind the ears and at the neckline
  • Duration – Nymphs become adults after about 7 days
  • Nymph Picture

Adult: The adult louse feeds about 5 times a day by piercing the skin with its claws, injecting irritating saliva, and sucking blood. Lice do not become engorged like ticks, but their color changes to a rust color after feeding. Head lice hold onto the hair with hook-like claws found at the end of each of their 6 legs. Adult lice are active and can travel quickly. More adult characteristics:

  • Size – The size of a pinhead. The female louse is larger than the male.
  • Shape – Oval with 3 legs and claws at the end of each leg
  • Color – Rust
  • Location – Close to the scalp, especially behind the ears and at the neckline
  • Duration – Adult lice can live up to 30 days on a person’s head
  • Reproduction – An adult female can lay up to 8 nits per day
  • Adult Louse Picture

Symptoms of Head Lice

  • Scalp itching
  • Sensation of something moving in the hair
  • Red or pustular sores on the scalp caused by scratching
  • Swollen lymph nodes in the front and back of the neck

Transmission of Head Lice

Head lice can be spread by direct contact with the head or hair of an infested person. The adult louse can survive up to 1-2 days without feeding on the scalp; therefore can be found on objects that have been in contact with an infested person. Examples of objects that commonly spread head lice are:
  • Clothing such as hats, scarves, coats, and sports uniforms
  • Hair care instruments such as combs, brushes, or towels
  • Other objects such as pillows, beds, carpet, and stuffed animals

Diagnosis of Head Lice

A head lice infestation is diagnosed by direct visualization of the lice in any portion of the life cycle. Usually there are many more nits than adults or nymphs. Finding nits within a quarter inch from the scalp confirms an infestation that needs to be treated. Nits can be distinguished from dandruff flakes as they are extremely adherent to the hair shaft, whereas dandruff can be easily moved on along the hair shaft.
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