Head louse
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| Pediculus humanus Linnaeus, 1758 |
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| Pediculus humanus capitis Charles De Geer, 1767 |
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Pediculus capitis (Charles De Geer, 1767) |
The head louse (Pediculus humanus capitis) is one of the many varieties of sucking lice 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).
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[edit] 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.
[edit] Life cycle
Lice eggs on the hair very close to the scalp are the primary sign of an active infestation. A female can lay up to 50-150 eggs in its life, but it can lay up to 100 eggs in 30 days. She also glues her eggs, sometimes called "nits", which look like tiny white beads, to hair shafts very close to the scalp (usually about a centimeter or two [half an inch to three-quarters of an inch] from the scalp). Eggs are very small, about the size of a period "." (full stop) in normal printing. Eggs may appear white, yellowish, brownish or greyish, but are almost always lighter colored. An egg normally undergoes a 7-9 day incubation before hatching as a baby nymph. It can be passed to adult to child or child to adult
Classically, a louse 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 darker, or raisin-like, as they dry out. "Nits" of this sort are usually found over one centimeter (approximately one-half inch) or more away from the scalp and are not considered a sign of active infestation. Some people refer to lice eggs, live or dead, as "nits." In common usage, an egg that is still incubating may also be called a "nit."
Head lice have no wings and move primarily by crawling. They have no adaptations for jumping.
Whether a louse is male or female is not apparent until it is nearly mature. Fertilization of eggs takes place once the female is mature. The female can then lay 3-7 eggs each day for the next 28 to 30 days, her normal life span. Females may lay multiple eggs on one hair or lay an egg on a hair and then move on.
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. Most head lice at this stage are almost transparent until they have eaten (sucked blood from a human skin). Then they can be seen more easily since it will be possible to see the tiny amount of red or brown blood in their stomach. To the unaided eye, it will appear like a small red dot usually about 0.5 mm or less in diameter. With a magnifying glass or microscope it is possible to see the head and legs, and since a louse is quite transparent at this stage, the digestive system, showing the blood that was ingested.
- Adult: To live, adult lice need to feed on blood. Females are usually larger than males, and can lay several nits in a day. If the louse falls off a person, it usually dies within 1-2 days[citation needed], although experiments have shown them to live longer. A female may continue to lay eggs on other materials similar to the texture of human hair such as stuffed animals with long strands of hair.
[edit] Symptoms
The most common symptom is itching of the scalp, caused when the louse bites through the skin to feed on blood. Another thing to be concerned about is hair loss. The lice themselves are essentially harmless, although excessive scratching may cause bleeding of the scalp, which may increase the risk of microbial infection. Also many have experienced blood as they urinate, which is caused from the lice. They suck your blood and leave droppings in your hair as well as eggs.
[edit] Treatment
[edit] Treatments proven by medical research
[edit] Shaving the Head
Shaving off all the hair is a viable solution.[2] This can be performed alone using mirrors to see all sides of the head, although it is easier to have someone else help with the procedure. A completely shaven head is necessary, a mere buzz cut is not sufficient.
Shaving the head gives the lice little to grasp to stay attached to the head. In addition to head hair, lice may infest facial hair or eyebrows, and these may also need to be removed for effective treatment.
While effective, some patients may find the hair removal aesthetically unappealing.
[edit] Mechanical removal of lice or viable louse eggs by combing
A recent British study [3] suggests that combing wet hair with a special comb, known as "bug busting" or "wet combing," may be significantly more effective than use of insecticides, and can get rid of lice within 1-2 days of infestation.
Regular use of metal headlice combs will prevent major cases of infestation. Suggested use is once every week or two after being in close contact with many people.
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). Plastic combs are effective for very short-term use, but the spacing between the teeth will quickly spread out after repeated usage. Metal combs, being inflexible, are more effective for multiple uses.
Following are instructions for wet combing:
- First, comb through wet hair with an ordinary comb to relieve 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. After every comb-through, examine and rinse the comb. Use an old toothbrush or nailbrush to remove eggs and nits from the comb and flush the eggs down the sink. Comb through the entire head and scalp at least twice, flushing away any lice found. Afterwards, rinse the conditioner out (or shampoo out any oil).
- Repeat the aforementioned steps every two or three days to catch any new lice that have hatched since the previous combing.
- Repeat the aforementioned procedure until lice are not found for at least two treatments in a row.
One may find that a cold shower is best before wet combing. The cold water slows them down making for easier removal.
[edit] 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. Opinion differs as to its safety with pregnant or breastfeeding women. Considered safe by some, other organizations such as La Leche League have disagreed with this since breast milk may contain whatever the mother has eaten or come in contact with.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 odor 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.
[edit] Unverified treatments
[edit] Herbal treatments
Herbal treatments (including tea tree oil) are sometimes used to treat head lice. A small number of studies have been undertaken to evaluate their efficacy. [4], [5]
One trial, involving 143 louse infested children, compared a herbal oil treatment (containing coconut, anise and ylang ylang oils in unspecified concentrations) against a treatment containing a combination of insecticides (0.5% permethrin plus 0.25% malathion, synergised with 2% piperonyl butoxide). It found no significant difference in elimination rates between the two treatments (60/70 with the herbal product v 59/73 with insecticide solution). Although the study found no detectable toxic effects of either solution, certain herbal oils may have a potential for toxic effects. [6]
One herbal product manufacturer states that its products have undergone scientific trials by Insect Research and Development Limited, Cambridge, UK, under the supervision of Dr. Ian Burgess.[7] They suggest that while the action of essential oils, such as lavender, orange, rose, eucalyptus and sandalwood, is not clear, such oils may have an anesthetic effect on certain insects.[8] The herbal product manufacturer states that these trials found that the active ingredients of one of its herbal products (containing lavender, geranium, sandalwood and eucalyptus oils) paralyze lice and prevent eggs from hatching. The lice move away from the scalp upon contact with the product, in an attempt to move away from the source of the irritation. This allows them to be removed with a fine toothed comb, and by washing.[9]. The same manufacturer states that these studies found that another product containing quassia wood, a source of the natural insecticide quassin, was the most effective method of head lice control. [10]
[edit] Aromatherapy
Aromatherapy has been frequently used to treat for lice, but this has not been confirmed as a viable treatment.
[edit] Conclusions and perspectives about head lice - what research is missing
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This article or section may contain original research or unattributed claims. Please help Wikipedia by adding references. See the talk page for details. |
- Currently, there is no evidence or written research to support the pesticidal/chemical cleaning of sheets and clothing, or the treating of earphones, baseball helmets, and furniture (with insecticide sprays). Toxins in the lice control products may have greater side effects on people, animals and the environment than current research shows.
- Infection is spread among people by head to head contact—thus, typically, among 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 may not infect a person. Research is needed to see if lice can be spread by hats, pillows, etc.
- School-based "no-nits" policies (banning children with nits until all nits are removed) do not make sense to everyone — having a child sent home is troublesome and annoying to working parents. Some children sent home for head lice don't have them. Some people, including public health experts, believe that "no-nits" policies should be abandoned, citing reasons such as missed class time, ostracism or teasing by other children, annoyance from a non-health-threatening issue, loss of parents' work time, etc. In countries like England this is not allowed because it is deemed as "discriminatory" and "exclusion". Therefore, the parents of the child will not be informed.
- Supporters of the "no-nits" policy feel that children can work better in school without the infestation; that control is necessary; that children will have better self-esteem if lice infestations are removed, etc.
- Chemical treatment should not be started unless live lice are found. Nits are not a sign of active infestation with headlice. Toxic chemicals may cause unknown side effects.
- Cutting hair or tying it back is not helpful, and may increase the incidence of infestation by making it easier for lice to move on and off 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 not life-threatening. Separated from their host, lice are vulnerable (will die quickly without blood from a host) and have a short life span.
- Headlice do not prefer clean hair; they do not prefer dirty hair; they just like hair.
- It is not scientifically proven that mayonnaise helps get rid of hair lice, but it might be worth a try if other methods do not work. Also try Strawberry Jam. Peanut butter is know to work exceptionally well.
[edit] 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.
[edit] See also
- Pseudonits
- Pediculosis
- Nitpicking
- Delphinium
[edit] 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
- Lice from MayoClinic
- Amazon.com
- The Beauty Brains Cosmetic chemists tell you how to get rid of lice without damaging your hair
- Headlice.org FAQ from the National Pediculosis Association
![[1]](http://commons.wikimedia.org/wiki/Image:Pediculus-egg.jpg){kind=link}