Head louse

Head lice (Pediculus humanus capitis) are one of the many varieties of sucking lice (singular "louse") specialized to live on different areas of various animals.

As the name implies, head lice are specialized to live among the hair present on the human head and are exquisitely adapted to living mainly on the scalp and neck hairs of their human host. Lice present on other body parts covered by hair are not head lice but are either Pubic lice (Pthirus pubis) or Body lice (Pediculus humanus humanus).

Description

The adult head louse resembles a miniature ant that appears flat when viewed from the side through a strong magnifying glass. Head lice have a head, thorax and abdomen with six legs, but their two front legs are very large in order to grab onto the hair shafts. Head lice are tan to greyish-white in color.

Life cycle

Lice eggs on the hair very close to the scalp are the primary sign of an active infestation. The female louse glues her eggs, sometimes called "nits", which look like tiny white beads, to hair shafts very close to the scalp. Eggs are very small, about the size of a period (full stop) in normal printing. Eggs may appear yellowish, brownish or greyish, but almost always lighter colored. Eggs normally undergo a 7-9 day incubation before hatching as a baby nymph.

Classically, a lice egg does not become a "nit" until after it has completed its incubation stage, thus leaving a "nit". A "nit" is either the empty shell remaining after the nymph has departed or the dead egg that remains if incubation was not successful. Dead eggs will appear dark, or raisin-like, as they dry out. "Nits" of this sort are usually found one-half inch or more away from the scalp and are not considered a sign of an active infestation. In common usage, an egg that is still incubating may also be called a "nit".

There are three nymph instar stages as the baby louse matures, with the louse shedding its exoskeleton at the end of each stage as it gets larger. The nymph stage typically lasts 10 to 12 days.

Whether a lice is male or female is not apparent until they are nearly mature. Fertilization of eggs takes place once the female reaches the mature stage. The female can then lay 3-7 eggs each day for the next 28 to 30 days, her normal life span.

There are three main stages in the life of a head louse: the nit, the nymph, and the adult.

Nit: [1]Nits are head lice eggs. They are hard to see and are found firmly attached to the hair shaft. They are oval and usually yellow to white. Nits take about 1 week to hatch.
Nymph: The nit hatches into a baby louse called a nymph. It looks like an adult head louse, but is smaller. Nymphs mature into adults about 7 days after hatching. To live, the nymph must feed on blood. It metamorphoses 3 times before it reaches the adult stage.
Adult. Females lay nits (a few hundreds of eggs); they are usually larger than males. To live, adult lice need to feed on blood. If the louse falls off a person, it dies within 1-2 days.

Symptoms

The most common symptom is itching of the scalp. As the louse feeds on human blood, its bites into the skin cause itching. However, lice are essentially harmless. Rarely, bites can become secondarily infected; scratching may break the skin and help cause this secondary infection.

Treatment

Treatments that are proven by medical research to work

Mechanical removal of lice or viable louse eggs by combing

There is good evidence to support combing wet hair with a special comb, known as "bug busting" or "wet combing". A recent British study [2] suggests that such fine combing may be significantly more likely to be effective than use of insecticides.

A special finetooth comb that can pick out lice is used. The space between the teeth of the comb should be no more than 0.3 mm (0.01 inch). The following are instructions for wet combing:

First comb through wet hair with an ordinary comb to get rid of knots and tangles.
Apply conditioner (or olive oil) to make it easier to comb the hair with the finetoothed comb.
Comb through every bit of hair, pulling the comb from the scalp to the hair ends. If you find lice, rinse them off the comb and down the sink. Work through the hair until you’ve gone through it at least twice, flushing away any lice you find. Afterwards, rinse the conditioner out (or shampoo out any oil).
Do this every three or four days to make sure that you catch any new lice that have hatched since you last combed the hair.
Keep doing the combing until you no longer find any lice for at least two treatments in a row.

Malathion

Malathion works as well as other agents used to kill lice. Malathion must be left on for at least eight hours for it to work. It is applied to dry hair until the scalp and hair are wet and thoroughly coated. If lice are found 7-10 days after treatment, treat again with the same or different medication. The alcohol in malathion lotion and the terpenoids may cause stinging. Malathion is considered safe in pregnancy or breastfeeding.

Lindane

Although lindane may work as well as other insecticide based agents, its use is limited by its potential neurotoxicity. It is applied, for only four minutes, to wet the hair and skin or scalp of the affected area and surrounding hairy areas. Treatment may be repeated after seven days if necessary. Lindane should be avoided in pregnancy and breastfeeding. If it must be used in pregnancy, it should only be used once.

Permethrin

Permethrin may work better than lindane, though most trials were done before permethrin was widely used and resistance to the drug developed. It is used as 1% creme rinse left on for 10 minutes. If the first application fails to kill the eggs (that is, if young hatching lice are found using a suitable detection comb) a second treatment is indicated, usually seven days after the first treatment. Undertreatment in this situation could contribute to the development of resistance to the drug. There is no role for a third application, as this will contribute to resistance and is not likely to be effective. Permethrin has not been studied in pregnancy.

Pyrethrins

Pyrethrins are used as a 0.33% shampoo or mousse, by applying enough to thoroughly wet the hair and leaving it on for 10 minutes. All pyrethrum products that conform to the US Food and Drug Administration criteria for use in humans require a second application after 7-10 days, to ensure treatment of lice emerging from eggs that have not been killed by the first application. Pyrethrins work as well as permethrin.

Insecticide resistance

In recent years, resistance to the above-mentioned insecticides has become an important factor in treatment failure. In other words, the lice have adapted through a process of evolution, to survive treatments with insecticides.

Resistance has been confirmed as affecting permethrin treatments. Similar resistance is suspected for pyrethrins but is not yet confirmed. Resistance to lindane has been identified in the Netherlands, Israel, Malaysia, and Denmark but has not yet been confirmed in the United States. Malathion is apparently resistance-free, except in the United Kingdom and France.

If a patient has adequately used over the counter products but still has lice, the most reliable prescription treatment is malathion. This product is less easy to apply than a shampoo because it is a freeflowing liquid and must be left on for at least eight hours. It has a pungent odour due to a high level of monoterpene components in the essential oils that were included to mask the odour of malathion. The monoterpenes contribute much of the pediculicidal and ovicidal activity of the product. Evidence for activity of malathion is good. About 2-3% of malathion applied to the scalp is absorbed through the skin. In some cases, lindane may be prescribed instead of malathion. Evidence for lindane’s effectiveness is relatively poor, and lindane is about 4-10 times more toxic than malathion. A high proportion of lindane applied to the skin is absorbed through the skin.

Treatments that may work but need further research

Herbal treatments and aromatherapy

Herbal treatments (including tea tree oil) are sometimes used to treat head lice. No studies have evaluated their efficacy or possible toxicity.

Aromatherapy

Aromatherapy has been used but this is not supported by research evidence.

Misconceptions about head lice - what the research actually shows

There is no evidence to support the cleaning of sheets and clothing, or the treating of earphones, baseball helmets, and furniture with insecticide sprays.
Infection is spread between people only by relatively prolonged head to head contact; thus it is typically spread between people who know each other well. Lice seen on chairs, pillows, and hats are dead, sick, or elderly or are cast skins of lice—these cannot infect a person.
School based "no-nits" policies (banning children with nits until all nits are removed) do not make sense — less than 20% of school children with nits will go on to develop infestation within 14 days. About half of children sent home for head lice don't have them. Many public health experts believe that "no-nits" policies should be abandoned.
Treatment should not be started unless live lice are found. Nits are not a sign of active infestation with head lice.
Cutting hair, or tying it back, is not helpful and may increase the incidence of infestation by making it easier for lice to move off of and on to the scalp.
Head lice are probably more common in girls because girls are more likely to have close contacts during play—not because they have longer hair.
Head lice are harmless. If detached from their host they are vulnerable and effectively dead.
Head lice do not prefer clean hair, they do not prefer dirty hair, they just like hair.

Use in Archaeogenetics

Lice are also important in the field of Archaeogenetics. Because most "modern" human diseases have in fact recently jumped species from animals into humans through close agricultural contact, and also given fact that neolithic human populations were too scattered to support contagious "crowd" diseases, lice (along with such parasites as intestinal tapeworms) are considered to be one of the few ancestral disease infestations of humans and other hominids. As such, analysis of mitochondrial lice DNA has been used to map early human and archaic human migrations and living conditions. Because lice can only survive for a few hours or days without a human host, and because lice species are so specific to certain species or areas of the body, the evolutionary history of lice reveals much about human history. It has been demonstrated, for example, that some varieties of human lice went through a population bottleneck about 100,000 years ago (supporting the Single origin hypothesis), and also that hominid lice lineages diverged around 1.18 million years ago (probably infesting Homo erectus) before re-uniting around 100,000 years ago. This recent merging seems to argue against the Multi-regional origin of modern human evolution and argues instead for a close proximity replacement of archaic humans by a migration of anatomically modern humans, either through sexual contact, fighting, and/or cannibalism.

External links

Treating head lice - an article from the British Medical Journal
What to do if your child has head lice from Seattle Children's Hospital
Extinct humans left louse legacy
Facts of lice
Home Remedy for Head Louse
Head Lice Information @ lice-head.com
Lice from MayoClinic.com