How the head lice products work

There are many different types of products for treating head lice infestations and many different types of active ingredients. Because some manufacturers use the same brand name for several completely different products, it is difficult – even for health care professionals – to keep track of what the different products consist of and how they work. This section is designed to give you an overview on how some different types of product classes work.

Conventional insecticides (also known as neurotoxic insecticides)

There are several groups of insecticides that work against the nervous system of the insect by either blocking nerve impulses or by over-stimulating them. A nerve impulse consist of electrical charges that travel along the nerve route. A complex process in which the nerve cell pumps ions in and out through the cell membrane produces this electrical charge.

The nerve ends (junctions) do not touch but have microscopic gaps (synapses) between them. The electrical impulse is not able to cross this gap. Most nerve impulses are able to cross the junction between nerve cells using what is known as a transmitter substance as a bridge. The transmitter-substance is secreted from the end of the nerve cell when it is stimulated by a nerve impulse. One of the most common transmitter-substances is acetylcholine. When a nerve impulse reaches the junction, acetylcholine is released to bridge the gap so the impulse can pass. When the impulse has passed the synaptic gap, acetylcholine is broken down by an enzyme called acetylcholine esterase.

Some insecticides are no longer used for treating head lice and others are considered unsuitable for use on humans. In the Western world, three types of insecticides are currently used to control head lice.

Lindane is a cyclodiene organochlorine insecticide which binds to the receptors of an alternative transmitter-substance chemically called gamma-aminobutyric acid (GABA). GABA is involved in transmission of nerve impulses across nerve junctions (synapses). Lindane then causes repeated stimulations at the receptor site, which results in multiple nerve impulses and seizures. Today, this insecticide is used only in a few countries. In most countries, it has been abandoned as it is readily absorbed through the skin and thus can affect humans and other mammals.

Malathion is an organophosphate insecticide. Unlike many chemicals in this group, it is relatively safe to use in humans. It acts at the nerve synapses but by inhibition of the enzyme acetylcholine esterase. The insecticide binds chemically to one part of the enzyme that links onto the acetylcholine as part of the breakdown process. As a result acetylcholine is not broken down and the nerve impulse continues long after it should have stopped, resulting in spasms and exhaustion of the insect.

Pyrethroids (permethrin, phenothrin, allethrin) are synthetic chemicals derived from the structure of the naturally occurring insecticide pyrethrin. Pyrethroids work in a different way against insect nerves. These chemicals bind onto chemicals on the cell membranes, thus causing a distortion in the membrane structure. As a result, the process by which the cell controls the flow of salts in and out from the cell is disrupted. This in turn results in a false stimulation of an electrical impulse within the nerve cell, which continues as long as the insecticide binds to the cell membrane receptors. The result is exhaustion of the insect nervous system.

All the insecticide chemicals are themselves liable to chemical breakdown by enzymes and other chemicals in the insect’s system. An insect, which is able to break down the insecticide before it is killed, is able to tolerate exposure to that insecticide. One problem with regular use of insecticides is that insects become resistant, meaning they get immune to the insecticides. Furthermore, there may be a selection of insects, which are able to break down the insecticides more easily, or they have differently shaped chemical receptors within the nervous system (mutations) that are not affected by the insecticide. Insects that have these characteristics are referred to as resistant to insecticides. In much of Europe, North and South America, and Australia head lice have developed resistance to one or more of the insecticides used in the treatment.

Natural and plant extracts

There are three main groups of plant extracts used for treating head louse infestation, plant derived insecticides, essential oils and fixed vegetable oils.

Plant derived insecticides are used less frequently nowadays. Most of them act on the insect nervous system, in a similar way to conventional insecticides The mode of action of others is not known. The only widely used plant derived insecticide is natural pyrethrum (see Pyrethroids for mode of action). Other plant derived insecticides used in herbal medicine, homeopathy, and folk medicine, for which the evidence is not particularly good, include quassia extract, from a tropical tree wood, staphysagria, from delphinium plants, and rotenone, from Derris root. Their effects are, most often, poorly documented, thus all these materials should be considered with some care, because they are all toxic to humans at levels often much lower than the medically approved insecticides. There is likewise a risk of sensitization, particularly to rotenone and pyrethrum.

Essential oils are mostly highly fragranced oils obtained by steam distillation of aromatic plants. There are numerous essential oils, mostly with complex chemical composition. However, all essential oils consist of a mixture of chemicals called monoterpenes and their chemical derivatives. All essential oils have the potential to irritate skin and can be toxic in large concentrations. The levels of essential oils used in aromatherapy are designed to be safe for humans, but they are also safe for lice! Most products containing essential oils have levels suitable for aromatherapy, which is designed to keep the product safe. In order to be effective to kill head lice and their eggs the concentration needs to be 8-10% of the essential oil in the product, which is often irritating to the skin, especially if it has small cuts. However, there are some products based on essential oils that include enough essential oil and which seem to be effective against head lice, although they all report stinging or reddening of the scalp as side effects. Essential oils for which there is evidence of effect include oil of anise, oil of eucalyptus, and oil of melaleuca (tea tree oil). All of these are sold in proper formulations in the Western World. There are probably several modes of action of essential oil components, but the most commonly encountered are:

Activity against the insect nervous system. This is similar to the activity of malathion, i.e. they have a mode of action to inhibit the activity of acetylcholine esterase. An example of this is a chemical called terpinen-4-ol found in tea tree oil.

Solvent activity. All monoterpenes are powerful solvents for fats. Insects rely on a coating of waxy material to protect their cuticle and avoid dehydration. Some monoterpenes can dissolve that waxy material so that the head lice or their eggs dehydrate after treatment.

Enzyme disruption. Many monoterpenes can act against the enzymes used in normal body functions, especially those enzymes that cells use to take up food and oxygen. Some of the monoterpenes in the oils that help kill head lice disrupt these enzymes so the insect body systems stop functioning.

It has been shown experimentally that some essential oils used at levels likely to kill head lice and their eggs may cause the glue holding the head louse eggs to the hair to grip more tightly, which would make combing out the eggs and nits more difficult.

Fixed plant oils are heavy oils extracted from fruits and seeds and other parts of plants or by heating. Examples are olive oil, grape seed oil, coconut oil, neem oil. Claims about killing head lice have been made for all of these oils. Nearly all fixed oils are made from a series of saturated and unsaturated fats of different sizes and weights. They all have a good capacity to coat the surface of insects and are not easily washed off. If they are left on an insect for long enough some oil gets into the respiratory system, apart from physically impeding the movement of the head lice. Heavier oils form thicker and more persistent coatings for the insects. However, there is a lot of false information in the public domain about the activity of these oils, for example:

Olive oil and coconut oil are recommended by various consumer groups in certain countries or areas, as being effective on their own with the suggestion that head lice cannot become resistant to them. This is, however, incorrect. In many parts of the world these oils are used, or were formerly used, as hair conditioners after washing hair with soap (where shampoo is too expensive or not available). For example, coconut oil is used throughout Asia and Africa, yet people still get head lice because the head lice have developed resistance. Similarly, olive oil was commonly used in southern Europe for the same purpose but there is no evidence that the Romans or Ancient Greeks were any less prone to head lice than people today.

Mayonnaise and margarine have become modern elements of folklore for treating head lice. Apart from scant evidence in support of the claims, any activity that might be obtained would be wholly due to olive or similar oil components a described above. 

Neem oil is often considered a panacea for a variety of conditions and ailments. Neem does contain a lot of pharmacologically active chemicals but they only have an effect on insects if the insects eat them. As head lice live entirely on blood there is no opportunity for them to ingest the oil. Furthermore, the “active” chemicals are too large molecules they cannot be absorbed through the insect skin. However, neem oil has a chemical effect almost identical to extra virgin olive oil, thus it may have some activity if enough is applied. Products containing neem oil, which claim activity against head lice, are usually made up from powerful detergent chemicals (see Surfactants) 

Enzyme based products claim to be able to kill head lice and help remove head louse eggs from the hair. Most of these products claim to use protease enzymes, which help break down proteins. However, the outside cuticle of the head louse is made from chitin, which is carbohydrate based, thus protease enzymes would have no effect on it. The glue which holds the eggs to the hair strands does contain some protein but it is not susceptible to gentle enzymes and experiments have shown that it is easier to dissolve the hair than to break down the glue that holds the eggs in place! Consequently, it is unlikely that these products are effective other than as lubricants to help in the combing process and in cleaning other materials form the hair surface.

Surface active materials

One group of surface active materials is solvents, of which the components of essential oils described above form part. However, there is a wide range of solvents that have varying action against head lice.

Alcohols are often used as solvents for other chemicals and form a major component of active ingredients in some lotions for use against head lice. However, several of the many alcohols have some activity against head lice. Over the years, the widespread use of volatile alcohols (ethyl and isopropyl alcohols) has resulted in head lice adopting resistance towards them. However, there are alcohols with larger molecular size that also show activity. Some of them are not suitable to apply to human skin because they may be irritant or cause sensitization but others are used frequently:
Waxy alcohols such as cetyl alcohol and stearyl alcohol are used in conditioners. In the early 1980s laboratory tests showed that this type of alcohols had potential to kill head lice over the years, as more people used conditioning rinses routinely in their hair and many people used conditioners as lubricant for combing out head lice, the head lice became less susceptible. Currently, most head lice in most Western countries are not as easily killed by conditioners based on these waxy alcohols.

Medium chain alcohols are fluids that are used as solvents or spreading agents in cosmetics and toiletries. Examples are lauryl alcohol and myristyl alcohol, which have been shown to have activity against head lice and have been added to head louse products to assist the activity of other components. These alcohols are believed to dissolve and disperse the waxy protective layer covering of the head louse, which either causes them to dehydrate or allows other chemicals to interact with the head louse cuticle. There are some other alcohols which have a similar effect, for example benzyl alcohol. However, this alcohol can cause sensitization.

Diols are alcohols that have two alcohol groups. Several compounds in this group are wetting and spreading agents used in cosmetics. They are all derived from plants such as coconut and a 1,2-octanediol, which has been developed for use against head lice. This is also believed to dissolve the waxy coating of head lice causing them to dehydrate.

Detergents are chemicals that readily dissolve fats and are present in many shampoos and hair conditioning products. A variety of detergent materials have been used against head lice such as sodium lauryl sulphate and cocamide diethanolamine (cocamide DEA). These materials are too diluted in most cosmetic shampoos but when used in more concentrated mixtures, especially with other components such as coconut oil combined, they can prove effective to killing head lice but not louse eggs.
Occlusive agents

Materials in this group are mostly oily fluids that can coat the surface of insects.  Some people suggest that they can enter the respiratory system of the louse to work by suffocation.  However, the spiracles of lice have a complex honeycomb structure to stop or minimise the risk of fluids entering them.  Also the neck of the tracheae has a valve that remains closed most of the time because small insects like lice have a low oxygen requirement, which is why they can survive immersion in water for up to 12 hours.  So if an occlusive fluid has a low enough surface tension it can enter the spiracles and block the openings but it does not suffocate them.  Instead there is some evidence that they can prevent the lice from eliminating the water they take in with the blood they feed on, which disrupts their normal excretion processes, and may lead to rupture of the gut.

Mineral oils are purified petroleum derivatives. Several of these materials, such as light liquid paraffin, have been used medicinally for decades as lubricants. However, because they are oily they readily coat the waxy surface of insects and cause some blockage of the spiracle holes of the respiratory system. More viscous materials like petroleum jelly (petrolatum) also stick to the outside of the head louse and can produce a thick coating over the whole body surface of the insect. The disadvantage of these materials is that they have no activity against head louse eggs and are not readily emulsified by shampoos so are difficult to remove from the hair.

Silicones (siloxanes) are synthetic oil like materials that have a low surface tension and come with a wide range of viscosity. Different products contain different types and different viscosity siloxanes. All the products work in a similar manner. Because siloxanes have a low surface tension they are able to flow over waxy surfaces forming extremely thin coating film. They are also able to enter the breathing spiracles of head lice and can accumulate in the outer part of the respiratory system. Head lice immersed in silicones stop all activity in the same way as when immersed in water and other liquids. Even though the silicone fluid may completely block (occlude) the tubes of the tracheae it cannot suffocate the insect because oxygen can pass through silicones. However, silicones are good water barriers and prevent the head lice from excreting excess water taken in after a blood meal. This causes considerable stress to the mechanism of excretion and can cause a well fed head louse to rupture the gut and it dies.

Fatty acid esters are oily materials used extensively in cosmetics and toiletries as moisturizers and water barriers. Many of these materials have some activity against insects in a similar way to mineral oils and fixed vegetable oils, from which many are derived. Fatty acids esters are mostly used to help a preparation work by creating an interface between the fluid and the surface of the head louse. These chemicals can block spiracle holes when mixed with low surface tension solvents, like certain light silicones. The fatty acid esters can also mix with, or dissolve, the surface wax of the head louse so that the mixture can then be washed off leaving the head louse unable to maintain its water balance and it dies.