Head lice (Pediculus humanus capitis) are obligate, ectoparasites that live on the hair of the human scalp. Body lice (Pediculus humanus corporis) are morphologically identical to head lice, but are behaviourally distinct. Body lice, as the name suggests, never go up to the scalp, while the head lice do not deliberately descend on to the body – although they may drop onto the shoulders. They therefore occupy non-overlapping habitats. This territorial separation may have occurred as a form of adaptation to the human innovation of clothing.

The body lice are found hiding in the clothing and attaching their eggs to the textile fibres. These sub-species (Pediculus humanus capitis and Pediculus humanus corporis) are thought to have diverged at least 83,000 years ago, although this could be as early as 170,000 years ago, when humans started wearing clothing.1

Head lice are not known to transmit any disease. Body lice, however, can carry diseases such as louseborne typhus (Rickettsia prowazeki), trench fever (Rochalimaea quintana) and louse-borne relapsing fever (Borrelia recurrentis).These three are acute vector borne diseases that cause serious debilitating fever with muscle and joints pain and exanthems of various types. If untreated, typhus and relapsing fever can be fatal in up to 40% of cases, while trench fever is usually non-fatal, but can last up to a month if left untreated.

The Rickettsia and Borrelia are acquired by the body lice, feeding on infected humans. They are then transmitted to other uninfected humans through the faeces deposited by the infected body lice, on skin wounds during their blood sucking feeds. Transmission of these infectious pathogens can also occur when the body lice are crushed/squashed against the human body and their infected blood meal is released on to breaches in the human skin.

What are human lice?

Human lice are wingless insects, with six legs attached to the thorax. Each leg has a sharp, curved claw, with an opposing thumb, for holding on to the hair shafts (see Fig 1). They have piercing mouth parts for digging into the skin and draining the host’s blood, on which they feed several times a day. Without blood meals, lice will become too dehydrated to feed within 8-12 hours and die within one or two days.

head lice

As the name ectoparasite indicates, lice do not have the ability to burrow under the skin. They do not feed on skin tissue or skin debris. They are exclusively haematophagic (blood sucking). The term pediculosis denotes the condition of being infested with lice. The adult head louse is the size of a sesame seed (3-4mm in length) and greyish-white in colour. The eggs are yellow to white in colour, oval and 0.8mm in length and are glued close to the base of the hair shaft. Viable eggs are usually located within 6mm of the scalp. When empty, the egg capsules are called “nits” and can be mistaken for dandruff or hair spray debris.

Insect micro-anatomy and newer treatments

The largest part of a louse’s body is its abdomen. The abdomen has seven distinct segments. The first six segments have one tiny air hole (spiracle) on each side, through which oxygen can directly enter the body of the insect (see Fig 2). The spiracles lead to tracheae that disperse throughout the body tissues and the haemocoel fluid (blood) to provide a pathway for gaseous exchange directly to the tissues. This anatomical feature is its ‘Achilles heel’, and recently introduced anti-lice drugs (dimeticone derivatives) kill the lice by smothering the spiracles and depriving the insects of their oxygen source.

head lice 2

The spiracles also serve to excrete water by transpiration. Therefore, plugging these vents with silicone-based lotions can also lead to internal water accumulation within the louses body, leading to immobilisation and death. Thus the suffocating drug treatments have a dual mechanism of action and have circumvented the problem of drug resistance to neurotoxic, insecticidal drugs.2,3

Lice transmission and household pets

The humanus capitis species (common head lice) are specific to humans, thus being incapable of transferring to household pets such as cats, dogs, birds etc. Head lice are unable to fly, jump or hop and are only transferable by head-to-head contact, or through combs, hair brushes, or hats. The recent phenomenon of taking ‘selfie’ group photos with ‘heads together’ may facilitate head lice transmission!4

Head lice mainly affect young school children, between the ages of 4 and 11 years, but can be found in any age, sex, race or social class. In the United States, African-Americans are much less likely to be infested with head lice than other ethnic groups. This may be due to the shape and width of the hair shaft, which has an oval cross-section as opposed to a circular cross-section in Caucasians (see Fig 3, for the shape of the claw and the opposing thumb of the louse, which can more easily grasp a circular/cylindrical hair shaft, as compared to an oval shaped hair shaft), or could be down to the use of petroleum jelly on the hair to smooth it in order to reduce brittleness. This petroleum jelly also smothers and helps to eliminate lice.

head lice 3

In the UK, the prevalence of head lice infestation in primary school children is 2%, as ascertained by questionnaire surveys of schools. However, the annual incidence in this target group is 37%.5

Life cycle of the head louse

The life cycle of the head louse has 3 stages: egg, nymph and adult. The entire life cycle from laying the egg to the death of the adult louse is about 30 days. The adult female louse lays about 7-10 eggs daily. Eggs hatch in about 7-10 days and the nymphs that emerge are of a pin-head size, but look like miniature adults. The nymphs mature after three moults and become adults seven days after hatching. The females are larger than the males. As the egg-hatching period is about a week, insecticidal treatments once applied, should be repeated again after a week, for ensuring eradication of any newly hatched nymphs.

Head lice treatments

The treatment choice would depend on the patient/ the parent and the treatment history. However, the availability and accessibility of medications may be significant factors in the treatment choice. Three types of treatment are available.

Wet combing

This is a process of detection combing that involves combing wet hair with a fine toothed comb to remove live lice and nymphs as they hatch. Wet combing should be done several times over a two-week period. In clinical studies, cure rates of 38-57% have been described after 14 days treatment.6,7

Chemical insecticides

1971: Malathion was launched in the UK for the treatment of head lice. It was an organo-phosphorus compound that inhibits acetyl-cholinesterase, leading to an excess of acetylcholine, causing spastic paralysis and death of the lice. A 0.5% aqueous solution (Derbac-M) or a 0.5% lotion (Quellada-M) are marketed in the UK. The therapeutic contact time recommended is 12 hours and a second treatment is always recommended after a week to kill any newly hatched nymphs. Unfortunately, drug resistance and a failure rate of up to 64% has been reported.8

1992: Pyrethrins were launched in the UK market.9 They are extracted from the flowering heads of Chrysanthemum cinerariaefolium. The derivatives of pyrethrins, such as permethrin and phenothrin, have a ‘knock down’ effect on the head lice due to being neurotoxic and causing death through exhaustion of energy. Permethrins are safe with very low mammalian toxicity. They also have a short contact time of only 10 minutes. A second treatment is advised after a week.10

2009: Phenothrin was a synthetic pyrethroid (Full Marks liquid), but was withdrawn from the UK market in 2009. Unfortunately, resistance to permethrin has emerged throughout the UK, with a reported treatment failure rate ranging from 81-87%.11

Physical insecticides

Silicone oils: Dimeticones are silicone oils (linear polydimethylsiloxanes of varying chain lengths) with a low surface tension and special rheological properties. The term rheology in physics denotes the study of the flow properties of fluids and soft solids, dealing with aspects such as viscosity, elasticity and plasticity. These polydimethylsiloxanes show a creeping/spreading flow. They are colourless, odourless, and come with a slightly oily texture. They have a physical mode of action, as they occlude the spiracles, thus stopping oxygen intake and also blocking water transpiration, leading to internal water accumulation and death of the lice. The silicone oils are effective against the adult lice as well at the eggs.

2006: Hedrin was marketed as 4% dimeticone in a silicone solvent. NYDA is a 92% dimeticone formulation, also launched in 2006, containing two different dimeticones with different viscosities, presumably aiding rapid penetration into the spiracles. Both these liquids are applied to dry hair. When the silicone liquids are applied, the solvent evaporates and the lice are left coated and are virtually embalmed. The creeping mobility of the dimeticone quickly leads to internal penetration into the spiracles. There is no evidence of resistance and the future development of drug resistance is unlikely due to the unique physical mode of action. The therapeutic contact time of Hedrin and NYDA is 8 hours, after which the hair is rinsed with water. A second treatment after a week is essential to eliminate any newly hatched nymphs. The cure rate for Hedrin is 70% and 97% for NYDA.12,13 In a small clinical trial, it was found that 4% dimeticone liquid gel – in both 1 hour and 15 minute preperations – presented improved outcomes compared to any previous study of a siloxane-based product.14

Fatty acid ester and silicone

2009: Isopropyl myristate/cyclomethicone was marketed as Full Marks solution (this is different from the previously discontinued, phenothrin-based Full Marks liquid). IPM/C is a combination of an oily fatty acid ester and cyclomethicone at a 50% concentration. It is applied to dry hair and rinsed out with water and shampoo after 10 minutes. A second treatment after a week is recommended. The cure rate is 82%.11

Warning: Hedrin comes with a special warning: “hair should be kept away from naked flames, cigarettes and other sources of ignition, while treatment with Hedrin is underway”. This product warning was added following a report that a patient who was using Hedrin, set fire to his hair, while playing with a cigarette lighter.15 For all silicone/dimeticone based products, the clear instruction to the patient after applying the dimeticone solution should be to allow the hair to dry by natural evaporation, refraining from using hair dryers and avoiding sources of ignition.


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15. http://news.bbc.co.uk/1/hi/england/dorset/6577485.stm