Update on treatments for head lice
Head lice infestation is common, and mainly affects children of primary school age.1 Treatments include conventional chemical insecticides; fine tooth louse combs; and fluid preparations that work by a physical rather than chemical mode of action.1 However, each of these fails to eradicate head lice in some patients.1 Other disadvantages include the long contact time required for certain preparations (e.g. 8 hours) and the time commitment for combing regimens. Isopropyl myristate 50% in cyclomethicone solution (Full Marks Solution – SSL International) is a new fluid treatment with a physical mode of action that uses a 10-minute contact time.2 Here, we consider this product in the context of updating advice we gave in 2007 on treatments for head lice.
About head lice infestation
The head louse (Pediculus capitis) is an insect ectoparasite of humans.1 It clings to hair with its claws and feeds by sucking blood from the scalp. Adult females live for up to around 1 month, and lay around 5 eggs per day. The eggs are attached close to the base of hair shafts and take around 7–10 days to hatch. After hatching, the empty eggshells (nits) remain attached to the hair. Juvenile lice (nymphs) take around 6–10 days to become adults. Mature head lice move from one person to another via head-to-head contact between these individuals.
People with head lice only require treatment if active infestation has been confirmed by isolating one or more live lice.1 All affected members of a household should be treated at the same time.
Insecticide-based treatments for head lice infestation are licensed medicinal products that are neurotoxic to lice. Examples include malathion, permethrin and phenothrin. Treatment comprising two 8-hour or overnight applications of such products, 7 days apart (to kill the lice that emerge from eggs after the first treatment), has reported cure rates of around 75% for phenothrin liquid and for malathion lotion.1 In all patients, application of insecticide lotions should be limited to a maximum of three doses at weekly intervals. One problem with such treatments is the development of resistance among lice (reported rates up to 82% for permethrin and 64% for malathion). Also, these products may be unacceptable to some people because of the risk of potential unwanted effects (e.g. skin irritation, exacerbation of asthma) or consequences for the environment.
Fine-tooth combs are designed either to remove head lice or head louse eggs; examples are the combs used in the Bug Buster pack, which are intended for use in wet combing with normal conditioner. Several clinical studies on systematic louse removal using the Bug Buster kit have found cure rates of up to 57% after 14 days.1 A potential advantage is that the kit can be used by other family members or for subsequent episodes of infestation within the family. However, the regimen is time-consuming. For example, a treatment session may take around 20–30 minutes and needs to be repeated every 3 days for a minimum of 2 weeks, extended by 4 days every time an adult louse is found.3 As we have previously concluded, Bug Busting with a comb appears somewhat less effective at first use than insecticides, but may be preferred by people who wish to avoid using chemicals.1
Fluids acting by physical properties
Fluids that were originally designed for use as combing aids can now be marketed for use in their own right, provided that their activity against head lice is due to physical means (e.g. by asphyxiation) rather than by chemical properties.1
Dimeticone 4% lotion (marketed as Hedrin) is an example of a medicinal product that contains no insecticides but has a physical action (in this case, coating the lice and disrupting their ability to manage water).1 It is a colourless and odourless fluid, with a slightly oily texture, that is applied to dry hair. It is left to dry by evaporation, without use of hairdryers or other artificial heat, and washed off after at least 8 hours. It is important that both the scalp and all the hair are wetted with the fluid, including the full length of long hair. The manufacturer recommends two applications of treatment, 7 days apart, in order to kill nymphal lice emerging from eggs, which might not be killed by the first application.4 The product had a reported cure rate of around 70% in a randomised controlled trial comparing it with phenothrin 0.5% (which had a similar cure rate of 75%).1 5 No resistance towards dimeticone has been documented.1 Of note, in January 2007, a year after its launch to public sales, dimeticone 4% lotion was the market leading licensed treatment for head lice in the UK with a share (by value) of 43%.6 In our article in 2007, we concluded that dimeticone could be a first-line alternative to insecticides.1
Recent data on dimeticone 4%
Since publication of our article in 2007, there has been publication of a UK-based assessor-blinded randomised controlled trial comparing dimeticone 4% (supplied in 150mL bottles) with malathion 0.5%, each applied twice by investigators, 7 days apart, in 58 children (aged 1–13 years) and 15 adults (maximum age 48years) with head lice infestation.6 The 73 participants were from 32 families, and randomisation was by individual rather than by family, so individuals in the same family could receive different treatments. In the “worst-case” intention-to-treat analysis (i.e. where the people who dropped out were taken as treatment-failures), more patients on dimeticone had a positive outcome (i.e. cure with or without reinfestation: 69.8% vs. 33.3%, p<0.01; see box for definitions of cure and reinfestation). There were no serious adverse events, and the 2 patients with treatment-related adverse events were in the malathion group (itching or irritation of the scalp or neck during treatment).
Recent data on dimeticone 92%
A much higher concentration of dimeticone (i.e. 92% rather than 4%) is found in a product marketed over the counter in the UK as NYDA (Pohl Boskamp).7 A published randomised controlled trial compared dimeticone 92% with permethrin 1%, each applied twice, 7 days apart, in 145 children aged 5–15 years with head lice infestation, from a poor urban neighbourhood in Brazil.8 Rates of cure at day 9 (complete absence of viable lice) were higher with dimeticone (97.2% vs. 67.6%, p<0.0001); cure rates at 14 days were not given. Two cases of ocular irritation after dimeticone entered the patients' eyes were reported as adverse events related to treatment.
Isopropyl myristate in cyclomethicone
Isopropyl myristate (an oily fatty acid ester; 50% concentration; Full Marks Solution) is a new physical treatment for head lice infestation that uses a 10-minute contact time for each of two applications, 7 days apart.2 It is produced in a solution of cyclomethicone (a silicone fluid that is also used as the solvent for dimeticone in the formulation of Hedrin).9 The constituents are widely used in cosmetics and do not appear to present any toxicological hazard in repeated use.2
Unlike neurotoxic insecticides, isopropyl myristate does not cross the cuticle of the lice and has no pharmacological activity at the cellular level. Observations of direct application of the product to lice suggest that the product enters and blocks the tracheal breathing system of the insects, which rapidly immobilises them. The mixture also coats the surface of lice with a thin film of fluid that appears to dissolve into the insect's lipid coat. The resultant damage to the coat (which may be enhanced by the process of washing the product out) renders the lice more susceptible to water loss; they shrink, become dehydrated and then die. There are no known mechanisms by which head lice could develop resistance to this mode of action.
Clinical efficacy of isopropyl myristate
Two assessor-blinded randomised controlled trials in the UK have compared the efficacy of two applications, 7 days apart, of isopropyl myristate 50% (Full Marks Solution) versus permethrin 1% (Lyclear crème rinse).2 Permethrin was used on the grounds that it is the only other product in the UK with a 10-minute contact time; however, this product has not been recommended for several years because the contact time is insufficient to kill louse eggs.1 10 The results of the two studies have been analysed together by intention-to-treat methods and published as one report.2
Families with current head lice infestation were enrolled; randomisation was by individual not by family. The two trials involved a total of 168 participants (141 children aged 1–18 years and 27 adults; median age 9 years; range 1–73 years). Sufficient product was applied to dry hair to thoroughly coat the hair and scalp; the hair was then combed through with a normal comb to spread the treatment evenly and ensure thorough coverage to the full hair length.2 The product was washed out after 10 minutes using ordinary shampoo and water.
More patients on isopropyl myristate than on permethrin 1% were treated successfully (91/111 comprising 85 cured and 6 reinfested [82.0%] vs. 11/57 cured and none reinfested [19.3%], p<0.001). Isopropyl myristate was easier to apply to the hair and scalp (p<0.001) and had less odour than permethrin 1% (p<0.001). The mean amount of product used was 116.6g of isopropyl myristate (which equates to 2–3 100mL bottles) and 87.2g of permethrin 1% (2–3 59mL bottles), although larger bottles of product were given to people using isopropyl myristate in the study. The trial authors suggested that people buying a 100mL, 200mL or 300mL bottle of isopropyl myristate (Full Marks Solution) might obtain better results than those using a 59mL bottle of permethrin 1% (Lyclear crème rinse), simply because they have more of the product and are therefore more likely to achieve a more thorough coverage of the hair during treatment.
In these trials, there were 26 adverse events in 18 participants using isopropyl myristate and 15 adverse events in 14 participants using permethrin (18/111 [16.2%] vs. 14/57 [24.6%]; not significantly different).2 Most adverse events were common childhood ailments (i.e. not related to treatment). All adverse events considered possibly or probably related to, or with an unknown relationship to, the product were mild and occurred in the isopropyl myristate group (one occurrence each of rash, nausea, dry skin, eye pain when the solution accidentally dripped into the eye during application, eczema and headache).
Cure was defined as no evidence of head lice after the second treatment at days 9 and 14
Reinfestation after cure was defined as no adult lice or stage 3 nymphs after the first treatment; on days 9 or 14, no more than 2 adult lice or stage 3 nymphs and no younger nymphs
One further published assessor-blinded randomised controlled trial, conducted in North America, involving 60 participants, compared isopropyl myristate 50% in cyclomethicone (up to three applications, on days 0, 7 and 14) with a pyrethrin 0.33% plus piperonyl butoxide 4% combination product (two treatments, on days 0 and 7).11 Patients were assessed on days 0, 7, 14 and 21. The study defined treatment successes as patients returning on day 21 and no longer infested with live lice. However, comparison at day 21 could bias the results against the pyrethrin comparator, since patients in this group could have received a maximum of only two treatments while those in the isopropyl myristate group could have received three treatments. Excluding the 3 patients who were free of lice at day 14 but positive on day 21, more patients in the isopropyl myristate group were free of lice at day 14 compared with the comparator group, although both cure rates were low (40% vs. 17%, p≤0.05). Adverse events were reported as mild, similar between the treatment groups, and consistent with those observed for other insecticides.
With lotion preparations, the scalp and the full length of the hair should be coated; the quantities required depend on hair length and are around 50–100mL per application.10 Two applications are required, 7 days apart. Taking these issues into account, the table shows the cost to the NHS of a course of treatment for head lice.
Treatments for head lice include conventional insecticides (e.g. malathion, phenothrin); medical devices (e.g. fine tooth louse combs); and topical preparations working by a physical rather than chemical mode of action (e.g. dimeticone). None of these treatments eradicate head lice in all users. Potential disadvantages of conventional insecticides include unwanted effects and the development of resistance by lice, and combing regimens are time-consuming. Recent data on dimeticone 4% (Hedrin) showed similar efficacy to that in previously published studies (around 70%), so it remains a reasonable first-line alternative to chemical insecticides.
Isopropyl myristate 50% in cyclomethicone solution (Full Marks solution) is a new treatment option for head lice infestation with a physical mode of action, which requires a 10-minute treatment time. This is much shorter than the standard contact times of several hours for conventional insecticides or dimeticone. The product has a physical rather than a chemical mode of action and therefore lice are less likely to develop resistance to it than to conventional neurotoxic insecticides. Published studies of the treatment have suggested a cure rate of up to 82%. However, it has not been directly compared with other insecticides generally used for the treatment of patients with head lice in the UK, with dimeticone or with wet combing methods such as Bug Busting. Until such comparative data are available, it cannot be recommended in preference to established treatments for patients with head lice.
|Preparation/method||Product name||Cost to the NHS|
|Dimeticone 4%||Hedrin lotion||£6.83 for 150mL bottle|
|Isopropyl myristate 50% in cyclomethicone||Full Marks solution||£6.27 for 200mL|
|Malathion aqueous formulation||Derbac M liquid Quellada-M liquid||£5.70 for 200mL|
|£4.62 for 200mL|
|Permethrin||Lyclear Crème Rinse||£4.32 for 2 × 59mL|
|Phenothrin aqueous||Full Marks liquid||£5.70 for 200mL|
|Wet combing||Bug Buster Kit||£4.55 for one kit|
↵* Costs based on prices in the Drug Tariff.
To find out more about DTB, or to subscribe (or to have a 30-day free trial, please go to dtb.bmj.com).