Head lice: causes, symptoms and treatments

Head lice: causes, symptoms and treatments

In light of the latest news on the widespread resistance of head lice to common treatments, we look at what causes head lice, how to spot them and possible treatments that can be used to get rid of them.

Head lice (Pediculus humanus capitis) are tiny, wingless parasitic insects that live in human hair. Unlike body lice, head lice are not a health hazard, a sign of uncleanliness or a cause of disease. They are particularly common among preschool and elementary school-age children, members of their households, and their parents, guardians and caretakers.1

It is estimated that 6-12 million infestations occur in the US each year among children 3-11 years of age. Head lice are a very common problem, are highly contagious, annoying and sometimes tough to eradicate.2

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Here are some key points about head lice. More detail and supporting information is in the main article.

  • Head lice are tiny insects that live on the scalp, usually on children aged 10 and under
  • Head lice have six legs, each with a curved claw that can tightly grasp hair
  • They can move easily between hairs (at a rate of 23 cm per minute), but cannot fly or jump
  • Human blood is the food of choice for a head louse, with one drawing a tiny drop at each meal time
  • Pets and other animals do not play a role in the spread of head lice
  • Head lice are almost always caught directly from another person through direct head-to-head contact
  • Head lice are less common in black children and more common in girls than boys
  • If a head lice falls off a person, they quickly starve within a few hours
  • An adult female louse will lay six eggs each day
  • Head lice live for 3-4 weeks
  • Conventional treatment of head lice involves over-the-counter or prescription medication
  • Head lice are increasingly resistant to some conventional medications.

What are head lice?

A louse egg (a nit) is about the size of a flake of dandruff. They are yellow or white, are oval in shape and are attached to hair close to the scalp - an environment that provides an ideal temperature for incubating the egg.

Head lice are tiny grey/brown insects. They are about the size of a sesame seed.

The female louse produces a sticky substance which adheres each egg to the hair shaft. If a nit is located further than 1/4-1/2 inch from the scalp, they are almost always either already hatched, non-viable nits, empty nits or casings.5

Once hatched, the immature head louse (nymph) is whitish to grey-brown in color, has six hook-like claws, and is smaller than a pinhead. The louse egg hatches after 6-9 days, and the nymph becomes an adult louse about 7 days after hatching.

Adult head lice are about 1/8 inch - the size of a sesame seed - and live for around 3-4 weeks (30 days). Without a blood meal, head lice can live around 1-2 days off the host.3,5,17

Adult female head lice are larger than males and can lay around 8 eggs each day. Lice in darker hair will appear darker.17

Head lice feed on blood. They inject saliva into their hosts while they feed to prevent blood from clotting, which can result in an allergic itching sensation. Secondary bacterial infection of the skin can result if the skin is scratched too vigorously. While the presence of head lice can be frustrating, they do not transmit disease and are not dangerous.4

Head lice or head nits are often found near the neckline at the back of the head and behind the ears. They are sometimes located on the eyelashes or eyebrows, but this is uncommon.

Causes of head lice

A head lice infestation, called pediculosis capitis, results from the direct transfer of lice from the hair of one person to the hair of another through head-to-head contact. Head lice cannot fly, jump or swim.6 Most infestations in healthy children involve fewer than 10 live lice.18

Lice infestations are spread most commonly by close head-to-head contact.

It is uncommon for lice to be transmitted through contact with personal items or fabric items used by a person infested with head lice. However, it is sensible to avoid sharing brushes, combs, headbands, headphones, towels, clothing or hats with anyone who has an active infestation of head lice.7

To survive, an adult head louse must feed on blood. They can live for approximately 30 days on a person's head, however, if they fall off, they will die within 1-2 days.

An infestation of head lice is not the result of dirty hair or poor hygiene - all hair types can be affected regardless of length and condition. Head lice can only affect humans - they cannot be passed on to animals or caught from them - cats, dogs and other non-human animals do not play a role in the spread of head lice.

Data show that head lice can survive under water for several hours. However, they are unlikely to be spread by the water in a swimming pool. Head lice tend to hold tightly to hair when submerged in water and chlorine levels found in swimming pool water do not kill them.

It is possible for female head lice to begin laying eggs from 9 days after she has hatched. To break the cycle and stop the lice spreading, the lice need to be removed within 9 days of hatching.

Symptoms of head lice

Itching (pruritus) is the most common symptom of head lice infestation. The itching is caused by an allergic reaction to louse saliva, with sensitization typically taking 4-6 weeks after the first bite.18

Not all people are allergic to head lice, however, and the infestation can be asymptomatic and go unnoticed for some time.8,9

Itching, the most common symptom of all types of lice infestation, is caused by an allergic reaction to louse saliva.

Some people become extremely sensitive to lice bites and have severe itching. Others build up a tolerance to the bites and have little or no itching, even with repeated infestations.

Other symptoms of a head lice infestation may include:9

  • Tickling or a sensation of something moving in the hair
  • Irritability and difficulty sleeping
  • Sores on the head from scratching.

On the next page we look at tests and diagnosis of head lice and the available treatment options for the condition.

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Head Lice - Causes, Symptoms and Treatment Options (Video Medical And Professional 2020).

Section Issues On Medicine: Medical practice