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Causes of head lice

By Dr Ananya Mandal, MD

Head lice infestation is caused by a tiny insect called the Pediculus humanus capitis.

Lice infestation may affect the head, body (by Pediculus humanus variety corporis) and groin (Pthirus pubis).

These insects belong to the phylum Arthropoda, the class Insecta, the order Phthiraptera, and the suborder Anoplura. While all mammals may be hosts for all Anoplura, lice prefer human hosts. (1-5)

What are head lice?

These insects are wingless, white to grey coloured and have 3 pairs of legs.

The average length of the head louse is 1-2 mm.

Each leg ends with a claw to grasp firmly. The body of the insect is flat and covered with tough chitin.

The lice have mouth parts with 6 hooklets. These attach to the human skin during feeding.

Lice feed nearly 5 times a day for approximately 35-45 minutes each time. The female louse is slightly larger than her male counterpart.

Life cycle of head lice

The complete life cycle of the louse is 30-35 days from egg to adult.

The adult female louse lays eggs, called nits. These the female louse glues with a water-insoluble substance to the shaft of the hair.

The lice prefer to attach the eggs close to the scalp. This allows the eggs to stay warm and moist for incubation deriving the warmth from the scalp.

The female head louse lays as many as 10 eggs per 24 hours, usually at night.

The nits are commonly found at the back of the head and behind the ears. They appear yellow or white but may be the same color as the hair at times.

These may be mistaken for dandruff, scabs, or hair spray droplets. However, unlike dandruff and scabs that flake off the nits are firmly glued to the hair.

The eggs take around 7 to 10 days to hatch. Eggs can survive for up to 10 days away from the human host while the adult lice can survive without a human host for only a day or two.

The hatching of the egg produces the nymph that moults three times over 7 to 10 days to form the adult.

An average infestation with head louse involves 10-20 adult lice.

How is head lice spread?

The infestation is spread by close physical contact and by shared objects like combs, helmets, caps, scarves and bedding.

Normally the lice cannot jump or move on smooth surfaces like glass or plastic.

However, they may travel up to 23 cm/min and may get ejected to over 1 m by combing or head gear removal.

Who is at risk of head lice?

Risks of getting infected include overcrowding, closed groups like school children, ill or malnourished patients, poor hygiene etc.

Girls are more at risk due to closer head-to-head contact.

Asians and Whites are more at risk. This may be due to warmer weather.

African Americans are less at risk since their thicker and curlier hair is not optimally suitable for the lice.

Edited by , BA Hons (Cantab)

Further Reading

What is head lice?
Head lice Symptoms
Head lice Diagnosis
Head lice Treatments

Last Updated: Jun 27, 2012

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