Head Lice: the scourge of all schools

Another school year, another year of head lice.
Have no fear. There are many ways to get rid of it!

The best for boys schools: shave heads & use mayo on the head: very natural & cheap.

Read more below. Otherwise there are many powerful options.


Occlusive Agents

There is anecdotal evidence that substances such as mayonnaise, margarine, herbal oils, and olive oil act as occlusive agents and are effective.[2]Dimethicone products have been clinically shown to have ovicidal properties. The product kills eggs by obstructing their aeropyle (similar to obstructing the louse’s respiratory spiracles).[19] Available OTC, LiceMD is formulated with dimethicone.[2]

——–Full Info: for Colin only unless you want to pass on…—-

Abstract and Introduction

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Head lice (Pediculus humanus capitis) are ectoparasites, meaning that they live on or within the skin rather than inside the body. Annually, approximately 6 to 12 million U.S. children aged 3 to 11 years become infested with head lice.[1] Lice infestations are a substantial economic burden on the health care system. Although difficult to assess, the annual direct and indirect costs of infestations, including treatment, lost wages, and school-system expenses, are estimated to be $1 billion.[2,3]
Preschool-age and school-age children are most commonly infested, with household members also at increased risk.[2] Not only is lice infestation a nuisance, but if it is left untreated, it may result in a secondary bacterial infection. Infestation has nothing to do with hygiene practices or hair length, nor do lice transmit disease.[2] Despite these facts, the social stigma, embarrassment, and anxiety resulting from infestation can be considerable for both the child and the parent(s).[2,4]


Louse saliva, which contains vasodilators and anticoagulants, is injected into the host while the louse is feeding on the host’s blood. The host usually develops a sensitivity to the saliva, resulting in pruritus; however, it may take 4 to 6 weeks for the sensitivity and itching to manifest.[2]
The adult head louse is tan to grayish-white and 2 to 3 mm long (about the size of a sesame seed), with six legs. The female louse can live 3 to 4 weeks, and upon maturation she can lay up to 10 eggs per day. The eggs, which are cemented to the base of the hair shaft approximately 4 mm from the scalp, are pigmented to match the hair color of the host, rendering them difficult to see. Eggs farther than 1 cm from the scalp likely are not viable. Once hatched, the eggs are easier to see, as the empty casings appear white. The term nit denotes the empty casing; however, some experts also use the term to describe the viable egg containing the nymph.[2]
The life cycle of head lice consists of three phases: 1) egg without an eyespot (the eyespot signifies a developed nervous system), 2) egg from time laid to time hatched, and 3) nymph (newly hatched louse) to mature egg-laying louse. From the time of laying, an egg takes an average of 4 days to develop an eyespot and 8.5 days to hatch. It takes a nymph an average of 9.7 days to develop into a mature egg-laying adult.[4] Once mature, the female louse can mate and lay eggs in approximately 1.5 days. Eggs cannot hatch at room temperature; they need body heat to incubate.[2] Even under optimal conditions, 10% to 30% of eggs fail to hatch.[5]


Lice usually cannot survive away from the host for 24 hours. Lice do not hop or fly; they can only crawl. However, there have been some reports of lice being ejected from the scalp as a result of static buildup from brushing dry hair. Transmission usually occurs from direct head-to-head contact, and rarely from contact with the infested individual’s brush, comb, or hat.[2]
Since lice avoid light and crawl quickly, it is difficult to detect a live louse on the scalp. A quicker, more efficient way to diagnose head lice infestation is to use a nit comb.[2] Compared with direct visualization, using a nit comb reduces the number of false negatives.[3] Some experts believe that the use of a lubricant, such as oil, conditioner, or water, slows the lice down and facilitates diagnosis.[2]


Usually it is easier to see eggs at the nape of the neck or behind the ears.[2] A diagnosis of lice infestation should not be based on detection of nits alone. When diagnosis is made by the detection of nits alone, up to 66% of children are treated unnecessarily. Treatment should not be administered if living lice cannot be found.


Preventing head lice is difficult, especially in young children. Avoiding head-to-head contact is the surest way to prevent transmission. Children should be taught not to share personal items such as combs, brushes, and hats. Prompt treatment of infested individuals can aid in prevention.[2]


The optimal treatment would be readily available, easy to use, effective and safe, affordable, and non–resistance-promoting.[2]Prophylactic treatment is not recommended.[3]OTC and prescription-only medications are approved for use in the U.S.[2] These products, which are pediculicidal (kills live lice only) or ovicidal (kills lice and eggs), act either on the louse’s nervous system or as occlusive agents.[6]
Many reports exist of resistance to the OTC agents; however, in the case of treatment failure, improper use by the patient should be suspected first. Some experts recommend removing nits with a nit comb after treatment.[2]
Based on life cycle, pediculicidal agents that act on the nervous system ideally should be applied twice. The first application kills all hatched lice. Nine days later (rather than 7 days, as some manufacturers recommend), a second application kills all newly hatched eggs not killed with the first application. If the second application is done on day 7, some eggs laid just before the first application have not yet hatched, based on the average hatching time of 8.5 days.[4]

Indicated Medications

See Table 1 for application instructions.

Permethrin 1% (Nix)

Available OTC, permethrin is the least toxic of the pediculicides. The American Academy of Pediatrics (AAP) considers permethrin the first-line treatment of choice. It causes hyperstimulation of the nervous system of lice, resulting in paralysis and death.[4]Widespread resistance to permethrin has been reported, but the prevalence is unknown.[2] Resistance tends to vary geographically.[4]
Permethrin leaves a residue on the hair that can kill any nymphs remaining after the first application. The use of conditioners and hair products with silicone-based additives may hinder permethrin’s adherence to the hair shaft and impair the residual effect. Adverse effects include pruritus, erythema, and edema.[2]

Pyrethrins With Piperonyl Butoxide (PBO) (RID)

This product is available OTC. Pyrethrin is extracted from the chrysanthemum flower and then formulated with PBO. Individuals who are allergic to ragweed may react to pyrethrins, although this is rare.[2] PBO works synergistically by inhibiting the louse enzymes that catabolize pyrethrins, thus extending pyrethrins’ action.[4]
Pyrethrins with PBO are not completely ovicidal, and 20% to 30% of eggs will remain viable and hatch. This warrants a second treatment, ideally on day 9. Unlike permethrin, pyrethrins with PBO have no residual activity.[2]
Resistance to pyrethrins with PBO has been demonstrated; however, the prevalence is unknown. Resistance seems to differ from community to community and from country to country.[2]

Malathion (Ovide)

Available by prescription only, malathion was reintroduced in 1999 after being removed from the market two different times based on concerns about its long application time, flammability, and odor.[2] Malathion is an organophosphate that inhibits cholinesterase activity, resulting in increased acetylcholine concentrations.[7]The excess cholinergic activity causes hyperstimulation of the louse nervous system and prevents feeding.[4]
Malathion is highly ovicidal, and a single application is sufficient for most patients.[2] Superior activity over 1% permethrin has been demonstrated.[8,9] Because of the increased permeability of infant scalps, malathion is contraindicated in neonates and infants.[7] Malathion has not been studied in children aged less than 6 years, and it is contraindicated in children aged 24 months and younger.[2] The drug is Pregnancy Category B.[7]
Resistance has been reported in the United Kingdom, but the UK formulation differs from that used in the U.S. The U.S. formulation contains terpineol, dipentene, and pine needle oil, all of which are pediculicidal, and the combination of these ingredients with malathion is thought to have delayed resistance in the U.S.[2]
The U.S. formulation contains 78% isopropyl alcohol, which renders it highly flammable. Patients should be counseled to avoid high temperatures and open flames (e.g., hair dryers, cigarettes) while the hair is still wet.[7]
Adverse effects include a slight stinging sensation; however, because inadvertent transdermal absorption has occurred in agricultural use, the patient should be monitored for signs of excessive cholinergic activity, including increased sweating, salivary and gastric secretions, gastrointestinal and uterine motility, and bradycardia.[7]

Benzyl Alcohol (Ulesfia)

Approved by the FDA in April 2009, benzyl alcohol is available only by prescription. Benzyl alcohol is not neurotoxic or ovicidal; it is an occlusive agent and kills lice by asphyxiation.[2,10] Because lice can close their respiratory spiracles when they are submerged in water or other occlusive agents, it is difficult to suffocate them.[11] The benzyl alcohol in Ulesfia prevents the lice from closing the spiracles and allows the vehicle (containing mineral oil and inactive ingredients) to penetrate and obstruct them, resulting in asphyxiation.[10]
Ulesfia is approved for children older than 6 months and is Pregnancy Category B. The most common adverse effects include ocular irritation, application-site irritation, application-site anesthesia, and hypoanesthesia.[10]

Spinosad (Natroba)

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Spinosad, an ovicidal agent, was approved by the FDA in January 2011. Spinosad is indicated for use in patients aged 4 years and older.[12,13] The drug is derived through the fermentation ofSaccharopolyspora spinosa, a soil Actinomycete bacterium.[12,14]The mechanism of action (MOA) is hyperexcitability of the louse nervous system that results in paralysis and death.[12]
In two clinical studies evaluating the efficacy of spinosad and permethrin under actual-use conditions (in the home), spinosad without nit combing was superior to permethrin with nit combing (study 1, 84.6% vs. 44.9%; study 2, 86.7% vs. 42.9%) for complete lice eradication 14 days after the last treatment. Most spinosad patients required only one application (63.8% and 86.2% in studies 1 and 2, respectively); most permethrin patients required two treatments (60.3% and 64.5% in studies 1 and 2, respectively).[13] These data suggest that spinosad is more effective and convenient than permethrin.
This product contains benzyl alcohol and is classified as Pregnancy Category B. The most common adverse reactions (≥1%) are application-site erythema, ocular irritation, and application-site irritation.[12]

Lindane (Kwell)

This product is no longer indicated for the treatment of head lice. Lindane causes neurotoxicity in humans, and seizures have been reported in children. The AAP does not recommend the use of lindane, and the drug has been banned in California.[2]
Table 1.  Medication Application Instructions
  1. Shampoo hair with nonconditioning shampoo, then towel it dry.
  2. Apply to damp hair and leave on for 10 minutes.
  3. Rinse off with warm water.
  4. Repeat if live lice are seen after 7–10 days.
Pyrethrins With PBO
  1. Apply to dry hair.
  2. Rinse out after 10 minutes.
  3. Repeat after 9 days.
  1. Apply an amount sufficient to entirely cover dry hair and scalp (keep eyes tightly shut).
  2. Allow hair to air dry.
  3. Hair should remain uncovered after application.
  4. Shampoo hair after 8–12 hours.
  5. Use a nit comb after shampooing to remove lice and eggs.
  6. Repeat if live lice are seen after 7–9 days.
Benzyl Alcohol
  1. Apply to dry hair, using enough to completely saturate hair and scalp.a
  2. Rinse off with water after 10 minutes.
  3. Repeat in 7 days (although waiting until day 9 may be more effective).
  1. Apply first to dry scalp, then to dry hair in amount sufficient to cover entirely.
  2. Apply up to 120 mL (depending upon hair length).
  3. Leave on for 10 minutes.
  4. Using warm water, thoroughly rinse hair and scalp.
  5. Repeat if lice are seen after 7 days.
  1. Apply to dry hair in amount sufficient to coat hair and scalp (up to 1 tube).
  2. Leave on for 10 minutes.
  3. Rinse off with water.
aFor hair ≤4″ long, 1 bottle will suffice; hair up to 16″ long may require up to 3 bottles; hair longer than 22″ may require up to 6 bottles.

PBO: piperonyl butoxide.

Source: References 2, 7, 10, 12, 17.

Off-label Medications

Ivermectin (Stromectol)

Ivermectin is an anthelmintic agent structurally similar to macrolide antibiotics, but with no antibacterial activity. A single dose of 200 mcg/kg given orally and repeated 10 days later has been shown to be effective against head lice.[2] A recent study found a dose of 400 mcg/kg given on days 1 and 8 to be more effective than malathion.[15]
A neurotoxic drug, ivermectin causes central nervous system (CNS) hyperstimulation, resulting in paralysis and death.[16] Although ivermectin does not readily cross the blood–brain barrier, young children may be susceptible to CNS adverse effects.[2,16]Children weighing less than 15 kg should not receive ivermectin.[16]
On February 7, 2012, the FDA approved a topical ivermectin lotion for the treatment of lice in patients aged 6 months and older.[17]

Sulfamethoxazole/Trimethoprim (SMZ/TMP) (Bactrim)

This agent is used off-label to treat head lice. Although the MOA is unknown, SMZ/TMP is thought to eradicate symbiotic bacteria in the louse gut.[2] In one study, SMZ/TMP (10 mg/kg/day in 2 divided doses) administered for 10 days with permethrin 1% was superior to both SMZ/TMP and permethrin alone at 2 weeks (95%, 83%, and 79.5%, respectively) and 4 weeks (92.5%, 78%, and 72%, respectively).[18]

Occlusive Agents

There is anecdotal evidence that substances such as mayonnaise, margarine, herbal oils, and olive oil act as occlusive agents and are effective.[2]Dimethicone products have been clinically shown to have ovicidal properties. The product kills eggs by obstructing their aeropyle (similar to obstructing the louse’s respiratory spiracles).[19] Available OTC, LiceMD is formulated with dimethicone.[2]

Shaving the head is an effective means of eradicating lice; however, for cosmetic reasons, it is an undesirable option.[2] Wet combing with nit combs is an attractive alternative to insecticides for some parents. Combing may be facilitated by using shampoo or conditioner as a lubricant; however, evidence is insufficient to conclude that this is an effective method for eradicating lice.[20]

Nonpharmacologic Treatment

Shaving the head is an effective means of eradicating lice; however, for cosmetic reasons, it is an undesirable option.[2] Wet combing with nit combs is an attractive alternative to insecticides for some parents. Combing may be facilitated by using shampoo or conditioner as a lubricant; however, evidence is insufficient to conclude that this is an effective method for eradicating lice.[20]

School Screening and the “No Nits” Policy

The No Nits policy is based on the premise that all nits are viable eggs. Based on this logic, all children with live lice or nits are immediately dismissed from school or other institutions (e.g., day care). This can cause considerable distress for both the child and the parent(s).[3] The economic burden resulting from the No Nits policy can be hefty, including loss of wages and/or the cost of a babysitter.[6]
Children at higher risk for developing an active infestation have five or more nits within one-quarter inch of the scalp; however, only about one-third of these cases actually convert to an active infestation.[2,5] One study reported that only 31% of children with visible nits had active lice infestation, and only 18% of those with nits alone went on to develop an active infestation within 14 days.[5]
Because of the low conversion to active infestation, as well as the low risk of transmitting lice in the school setting, the AAP recommends abandoning the No Nits policy.[2,5] The AAP advises that children with signs or symptoms be checked by the school nurse. Since an active infestation has probably existed for at least a month before the child experiences signs or symptoms, the AAP recommends that the child remain in class but be discouraged from coming in close contact with others. The child’s parents should be notified that day so that appropriate treatment may be administered promptly. Sending home a child who has a serious infestation should be left to the discretion of the school nurse. Any children likely to have had head-to-head contact with the afflicted individual should be checked for lice.[2] Infested children should be allowed to return to school immediately after the initial treatment.[3]
Lice infestation is common, and the patient or caregiver may seek help from the pharmacist first. The pharmacist is poised to recommend products and to counsel regarding their proper use. See Table 2 for counseling points.
Table 2.  Treatment Counseling Points
  • Shake bottle before using and wash hands after applying.
  • Use warm (not hot) water to rinse off products over a sink (rather than in a bathtub or shower). This limits systemic exposure.
  • Be sure to treat all of the hair and scalp. Pay close attention to the back of the neck and behind the ears.
  • Check all household members, and treat those with live lice or nits occurring ≤1 cm from the scalp. Family members who share a bed with the patient should be treated.
  • Pediculicidal agents ideally involve 2 treatments administered 9 days apart.
  • Using a nit comb after treatment may increase the likelihood of complete eradication.
  • Itching/burning sensations do not necessarily mean that treatment has failed. Topical corticosteroids and/or antihistamines may alleviate these adverse effects.
  • Although indirect transmission is rare, personal items may be washed at 130°F. Items that cannot be washed (e.g., furniture, car seats, carpet) should be thoroughly vacuumed.
  • Since nits need warmth to hatch, pediculicide spray is not recommended for personal items. Although probably unnecessary, large items (e.g., pillows, comforters) may be bagged in plastic for 2 weeks.
Recent Articles: 
BMC Dermatol. 2013 Apr 1;13:5. doi: 10.1186/1471-5945-13-5.

Single application of 4% dimeticone liquid gel versus two applications of 1% permethrin creme rinse for treatmentof head louse infestation: a randomised controlled trial.


Medical Entomology Centre, Insect Research & Development Limited, 6 Quy Court, Colliers Lane, Stow-cum-Quy, Cambridge CB25 9AU, UK. ian@insectresearch.com



A previous study indicated that a single application of 4% dimeticone liquid gel was effective in treating head louse infestation. This study was designed to confirm this in comparison with two applications of 1% permethrin.


We have performed a single centre parallel group, randomised, controlled, open label, community based trial, with domiciliary visits, inCambridgeshire, UK. Treatments were allocated through sealed instructions derived from a computer generated list. We enrolled 90 children and adults with confirmed head louse infestation analysed by intention to treat (80 per-protocol after 4 drop outs and 6 non-compliant). The comparison was between 4% dimeticone liquid gel applied once for 15 minutes and 1% permethrin creme rinse applied for 10 minutes, repeated after 7 days as per manufacturer’s directions. Evaluated by elimination of louse infestation after completion of treatment application regimen.


Intention to treat comparison of a single dimeticone liquid gel treatment with two of permethrin gave success for 30/43 (69.8%) of the dimeticone liquid gel group and 7/47 (14.9%) of the permethrin creme rinse group (OR 13.19, 95% CI 4.69 to 37.07) (p < 0.001). Per protocol results were similar with 27/35 (77.1%) success for dimeticone versus 7/45 (15.6%) for permethrin. Analyses by household gave essentially similar outcomes.


The study showed one 15 minute application of 4% dimeticone liquid gel was superior to two applications of 1% permethrin creme rinse (p < 0.001). The low efficacy of permethrin suggests it should be withdrawn.
BMC Dermatol. 2013; 13: 5.
Published online 2013 April 1. doi:  10.1186/1471-5945-13-5
PMCID: PMC3620921

Single application of 4% dimeticone liquid gel versus two applications of 1% permethrin creme rinse for treatment of head louse infestation: a randomised controlled trial



A previous study indicated that a single application of 4% dimeticone liquid gel was effective in treating head louse infestation. This study was designed to confirm this in comparison with two applications of 1% permethrin.


We have performed a single centre parallel group, randomised, controlled, open label, community based trial, with domiciliary visits, in Cambridgeshire, UK. Treatments were allocated through sealed instructions derived from a computer generated list. We enrolled 90 children and adults with confirmed head louse infestation analysed by intention to treat (80 per-protocol after 4 drop outs and 6 non-compliant). The comparison was between 4% dimeticone liquid gel applied once for 15 minutes and 1% permethrin creme rinse applied for 10 minutes, repeated after 7 days as per manufacturer’s directions. Evaluated by elimination of louse infestation after completion of treatment application regimen.


Intention to treat comparison of a single dimeticone liquid gel treatment with two of permethrin gave success for 30/43 (69.8%) of the dimeticone liquid gel group and 7/47 (14.9%) of the permethrin creme rinse group (OR 13.19, 95% CI 4.69 to 37.07) (p < 0.001). Per protocol results were similar with 27/35 (77.1%) success for dimeticone versus 7/45 (15.6%) for permethrin. Analyses by household gave essentially similar outcomes.


The study showed one 15 minute application of 4% dimeticone liquid gel was superior to two applications of 1% permethrin creme rinse (p < 0.001). The low efficacy of permethrin suggests it should be withdrawn.

Trial registration

Current Controlled Trials ISRCTN88144046.
Keywords: Dimeticone, Head lice, Insecticides, Pediculosis, Permethrin, Physical mode of action, Treatment regimen


In Europe control of head louse infestation is now mainly achieved by use of physically acting preparations []. The majority of these are based on the polydimethysiloxane, known as dimeticone. This compound exists in a variety of forms from the volatile hexamethyldisiloxane, with a viscosity of just 0.65 centistokes (cSt) through to high molecular weight gums with a viscosity of several million cSt. The level of polymerisation during manufacture results in a range of materials with such variable physical characteristics that selection of the right molecular weight and viscosity is potentially critical for gaining maximum effectiveness in this application.
The first dimeticone based product was Hedrin 4% lotion, approved as a medicine for sale in the UK in 2005, which used dimeticone of 100K cSt viscosity in a decamethylcyclopentasiloxane (cyclomethicone D5) volatile fluid vehicle. This product is also widely used in Europe, mostly as a class I medical device, and has been shown efficacious in several clinical field studies []. Since that time variant formulations have been developed in order to improve handling and application of the product and also the cosmetic characteristics.
The original 4% dimeticone lotion is applied for 8 hours or overnight on two occasions a week apart. Subsequent investigations both in the laboratory and in clinical trials have looked at shorter application times for this product and for the variants. One recently described study outcome found that no lice of any development stage were present following the first application of product in a two treatment regimen using a 15 minutes exposure time for a spray gel variant []. This report describes the first study investigating the efficacy of a single application regimen for Hedrin 4% dimeticone liquid gel, tested in a comparison trial against two applications of 1% permethrin creme rinse.



We recruited participants mostly through contact with families who had expressed a wish to be involved in research and advertising on local radio and in parish magazines. An information booklet was sent to each family and an appointment arranged for an investigator to visit. All household members were offered screening for head lice using a standard detection comb (“PDC”, KSL Consulting, Denmark). The intensity of infestation was graded as heavy, medium, or light using criteria employed in previous studies [].
We also used eligibility criteria from previous studies [], excluding those with known sensitivity to treatment components or long terms scalp conditions other than lice, those treated for lice within 2 weeks previously, and who had used hair bleach, dyes, or permanent waves, or had been treated with trimethoprim containing preparations within the last month. Pregnancy, breast feeding, participation in another clinical study within 4 weeks or prior participation in the current study, were also grounds for exclusion. All eligible family members could be enrolled and on agreement were conducted through a standard informed consent and assent procedure. The practical lower age limit was 2 years although approval for the products was as young as 6 months. There was no upper age limit.
Demographic data were collected after consent was taken, including gender, age, hair characteristics, and appointments made for subsequent assessment and treatment visits. No payment was offered for participation. Ineligible household members with lice were offered standard of care treatment using 4% dimeticone liquid gel to minimise reinfestation of study participants.


Ethical approval was granted by Central London Research Ethics Committee 2 (EudraCT 2011-000257-23).
The study was conducted in compliance with Good Clinical Practices, and conformity with the principles of the Declaration of Helsinki and of European Union Directive 2001/20/EC. All participants received study information at least 24 hours before enrolment and all stated that they understood the purpose and requirements of the investigation before giving consent. Parents or guardians gave written consent for children younger than 16 years and children also provided written assent, where able to do so, witnessed by the parent or guardian.


The products used for this randomised controlled trial were different in both appearance and method of application so blinding at the time of treatment was not possible.
4% dimeticone liquid gel (Hedrin Once liquid gel, Thornton & Ross Ltd, Huddersfield, UK) contains 4% high molecular weight dimeticone, 1,6,10-dodecatrien-3-ol,3,7,11-trimethyl, PEG/PPG dimeticone co-polymer, and silica silylate. It was supplied in 150mL polyethylene bottles and applied systematically to dry hair over the whole scalp using the inbuilt dropper cap. In each case the investigators spread the fluid through the hair using their fingers to ensure thorough coverage. The product was applied once for 15 minutes and then washed out using shampoo and water.
1% permethrin creme rinse (Lyclear creme rinse, Omega Pharma UK, London, UK) contains 1% permethrin in a conditioner base with 20% isopropanol. It was supplied in 59mL HDPE bottles with a flip cap. It was applied liberally to shampoo washed and towel dried hair. Investigators ensured that all parts of the hair and scalp were thoroughly coated, which in most cases required more than one bottle. This product was applied for 10 minutes on two occasions 7 days apart followed by rinsing with water only.
In each case parent/guardians were advised of the time to wash the treatments off. Use of nit combs or other pediculicides during the course of the studies was not permitted.

Outcome measures

The primary outcome measure was elimination of infestation after completion of the treatment regimen. Following treatment on Day 0 we performed follow-ups on Days 1, 6, 9, and 14 by dry detection combing using the “PDC” comb to assess the efficacy of treatment. If any lice were found they were fixed in the case record and the development stage subsequently recorded.
Outcomes of treatment were classified as cure, reinfestation following cure, or treatment failure (with a sub-category for ovicidal failure). Because one of the treatments in this study used one application only we found it necessary to redraft the algorithm used in previous studies for determining outcome criteria.

Sample size

We anticipated that there was likely to be a disparity in efficacy between dimeticone liquid gel and permethrin creme rinse, based on previous data. Consequently, it was possible that, if different members of a family were randomly allocated different treatments, those receiving the more effective preparation could be reinfested from those receiving the less effective product. Although the protocol allowed for identification of reinfestation at a low level it could not address problems resulting from heavy reinfestation. Therefore a different randomisation model was employed in which all members of a household received the same treatment allocation, with randomisation by family rather than by individual.
An analysis of the variance of the cure rates in relation to household sizes was made using data from previous studies and from this we estimated that the number of participants required using the randomise-by-family approach was higher than the number required using the randomise-by-individual approach (assuming independence and ignoring dropout) by a factor of 1.46.
For a randomise-by-individual approach, assuming a confidence level of 95% and a power of 95%, and conservatively taking the expected cure rates to be 90% for dimeticone liquid gel and 50% for permethrin creme rinse, we estimated the required sample size as 29 participants per treatment. Under the same assumptions, the required number of participants per treatment under the randomise-by-family approach was 42, equivalent to an estimated 22 families per treatment.

Randomisation and blinding

The randomised treatment allocation sequence was derived using a free to access online computer generated list from http://www.randomization.com (seed 25270, 22nd June 2011). Allocation at the point of delivery was made from instruction sheets enclosed in opaque, sealed, sequentially numbered envelopes distributed to investigators in balanced blocks of eight. Participant families were allocated the next available numbered envelope held by the investigator. Investigators conducting follow up visits were separate from those involved in treatment and thus remained blind of treatment allocation.

Statistical analysis

We conducted analyses based on both the “intention-to-treat” (ITT) and the “per-protocol” (PP) populations. Differences between groups in baseline characteristics, safety, acceptability, and efficacy were tested using Fisher’s exact test for yes/no variables and the Mann–Whitney U test for ranked variables. Where analyses showed important differences in baseline characteristics between the groups, chi-squared and rank tests stratified for these characteristics were conducted and 95% confidence limits presented for the difference between groups in the primary endpoint.



Ninety participants from 44 households were enrolled between 1st July and 3rd November 2011, of whom four participants in two households later dropped out or were lost to follow up (Figure 1). A further six participants completed the study but were non-compliant, either combing out lice (five participants) or out of time on one assessment visit (one participant). Therefore the ITT population analysed was 90, but as a result of the protocol violations the per-protocol population was 80 across 40 households broken down as follows: dimeticone liquid gel = 35 participants from 20 households; permethrin creme rinse = 45 from 20 households.
Figure 1

Flowchart of participant progress through the study.
Baseline characteristics were recorded for all participants at Day 0 of whom 73 (81.1%) were female and ranged from 2 to 45 years in age, mean age 11.7 years. The 44 households, varied in size from 2 to 9 occupants (mean 4.39). Numbers of participants per household were: 1 (17 households), 2 (16 households), 3 (6 households), 4 (3 households), 5 or 6 participants (1 household each). Of the 90 participants, the initial infestation was classified as “light” in 52, “moderate” in 17 and “heavy” in 21. There was no difference between treatments in this characteristic.
Consistent with the large proportion of females in the study, the proportion with hair “ears to shoulders” or “below shoulders” was very high (89.3% in the dimeticone group and 84.9% in the permethrin group). There were similarly high numbers of participants with “thick” hair (61.7% and 53.8% respectively). Both groups had approximately 37% of participants with “wavy” or “slightly curly” hair, which is a recently observed increased trend toward “wavy” hair compared with older studies. Only a small proportion of participants had hair that was dry (7.4%) or oily (8.1%).


The Day 1 analyses included all 90 participants; Day 6 analyses related to 89 participants following a drop out, with a subsequent drop out of 2 other participants from the same household so that 87 participants remained on Day 9. The Day 14 analyses included 86 participants due to one lost to follow up, with exclusion of a further 6 participants from the PP analyses for protocol violations.
For the intention to treat outcomes by individual, we conducted an endpoint analysis of rate of cure (or exceptionally cure followed by reinfestation as defined by algorithm) in the 90 participants in the ITT population. According to these criteria, success was achieved overall by 30/43 (69.8%) for dimeticone liquid gel group (26 cure, 4 reinfestation) and by 7/47 (14.9%) for the permethrin creme rinse group (6 cure, 1 reinfestation). The difference in rate of success between the dimeticone and permethrin groups was estimated as 54.9% (95% CI of 35% to 75%) (OR 13.19, 95% CI 4.69 to 37.07) which meant that there was a highly significant (p < 0.001) superiority of dimeticone liquid gel compared with permethrin creme rinse in the population tested. Dimeticone liquid gel not only showed a high ovicidal effect, as judged by not finding young nymphs following the single application of the product (32 participants), but also a high proportion were entirely louse free throughout (the “cure” group of 26 participants) (Table 1). In contrast, nearly all participants in the permethrin group were found to have newly hatched nymphs at some point in the study, showing the product has a low capacity to inhibit eggs from hatching in addition to any impact on efficacy resulting from insecticide resistance (Table 2).
Table 1

Comparison of outcomes by participant – 4% dimeticone versus 1% permethrin
Table 2

Numbers of lice recovered – 4% dimeticone versus 1% permethrin
We obtained per-protocol outcomes by individual by elimination of protocol violators from the analysis to give PP success rates of 27/35 (77.1%) for the dimeticone liquid gel group (25 cure, 2 reinfestation) and 7/45 (15.6%) for the permethrin creme rinse group (6 cure, 1 reinfestation), giving an advantage of 61.6% (95% CI, 40% to 80%) (OR 18.32, 95% CI 5.93 to 56.6) for the dimeticone treatment over permethrin, which was also highly significant (p < 0.001).
Using the same approach in the analysis of outcome by household, the advantage to dimeticone liquid gel was similar (p < 0.01 or p < 0.001) for all the outcomes, equivalent to a relative total success rate of 6.50 (95% confidence interval (CI) 1.66 to 25.5), or an odds ratio (OR) of 14.4 (95% CI 2.68 to 77.8), reflecting the difference of risk for a whole household compared with that for an individual.
All participants received the required number of treatments. Dimeticone liquid gel was given once, and the mean total product used was 63.91g (equivalent approximate cost = €6.20-€7.40). Permethrin creme rinse was given twice, and each treatment often involved two bottles and occasionally three. Over the two treatments combined, the mean number of bottles of creme rinse used per family member was 3.31, and the mean total product used, 141.58g (equivalent approximate cost = €8.30-€16.60), which was significantly (p < 0.001) more than for dimeticone.

Adverse events

Members of seven families experienced one or more adverse events, nine people treated with permethrin and eight with dimeticone. Only two people had an adverse event that was considered possibly related to treatment. One was a rash on the back of the neck following each treatment with permethrin creme rinse and the other, dry skin, following dimeticone liquid gel. There were no serious adverse events.


It has long been considered that longer application times when using head louse treatment lotions are more effective than shorter applications. This was demonstrated in vitro using insecticide based products in which the formulation vehicle evaporates and concentrates the active material on the louse surface. The same approach was initially taken with physically acting preparations such as 4% dimeticone lotion and preliminary results from a first small study did indicate that an 8 hour or overnight treatment was more effective than one of only 20 minutes []. However, in a later study we found that, contrary to expectation, reducing the application time for 4% dimeticone spray gel to 15 minutes not only increased the effectiveness but appeared to be wholly effective with a single application [].
Although the majority action of 4% dimeticone lotion is derived from the dimeticone being deposited in the insect spiracular and tracheal systems, resulting in blocking of water excretion [], it is necessary for it to be carried there by the solvent component of the product. If the solvent does not deliver sufficient dimeticone to the target site the activity may be reduced.
The perceived advantage of the 4% dimeticone gel over the original 4% lotion is that the gel seems to adhere to the lice more effectively and, in the spray form, is easier to direct and control ensuring all lice are killed and those eggs close to hatching are inhibited from doing so, as was demonstrated in our earlier study in which the product gave 100% efficacy after the first application in that two application trial []. In this study, most lice found by investigators during follow up assessments could be attributed to eggs that hatched after treatment, presumably because they had been missed as a result of inadequate spreading of the liquid gel using the dropper bottle. Some of the children had also been observed to present with hair dampened by sweat at the time of the treatment application, which may have acted as a barrier to the silicones forming a complete coating on some louse eggs, with the possible result that silicone was not able to reach the vulnerable surfaces of the eggs due to the presence of a water film. The four infestations in the ITT group without nymphs were all attributable to reinfestation from contacts either within the family or the local community.
This randomised controlled trial has clearly demonstrated a difference in overall activity between a physically acting preparation using a single application and two applications of an insecticide based product affected by insecticide resistance. Hitherto, the majority of claims that different products containing silicones, and other physically acting preparations, are effective with a single application have not been based on published clinical data but have been extrapolations either from in vitro data or else obtained in studies where the products were applied twice. This study has not only confirmed efficacy for a single application of Hedrin 4% dimeticone liquid gel, and demonstrated a difference in overall activity between a physically acting preparation and an insecticide based product affected by insecticide resistance, but has also demonstrated that extrapolations about treatment outcomes may be misleading, in the absence of clinical confirmation.
This trial has also provided the first clear evidence that louse eggs do not all hatch within 7 days (Table 2). We and many other investigators in the past had assumed this hatching period primarily because previously we had no other experimental evidence upon which to base a conclusion. We found from our earlier double application study [] that this does not necessarily constitute a significant problem for treatment because any lice hatching from eggs after a first treatment appear to be killed by a second treatment at 7 days. A treatment given at that time delivers more of the active material to any unhatched eggs, which further inhibits their chances of hatching. However, this information that eggs can take longer than one week to hatch emphasises the need for vigilance in confirming whether a treatment has been effective, by detection combing after completion of the treatment regimen, and also confirms the need for adequate application technique []. So, just because more than 90% of people appear louse free 2 days after a second treatment it does not mean that further checks are not necessary as some viable eggs may remain.
Although numerous lice were eliminated using the 1% permethrin creme rinse at each of the treatment applications, more than 60% of participants received no respite from infestation and, of those who appeared louse free one or two days after treatment, fewer than half were cured of their infestation, even though we used more than twice the supposedly adequate application dose on each participant. Why then did we choose 1% permethrin as the comparator product rather than some alternative preparation? The reason 1% permethrin was used in this study was to provide data in comparison with a known medicinal product for those territories where such data are required by decision makers before issuing approvals. On this basis the number of possible comparators is extremely limited because most products have a limited geographic distribution. The only other insecticide active used widely is 0.5% malathion but this was not selected because the range of formulations used in different territories are not comparable even within themselves (shampoos, alcohol based lotions with or without terpenoids added, and water based emulsions). Additionally, the one product still listed in the UK was not available at the time the study was conducted.
The outcome using permethrin mirrors closely the findings of a previous investigation comparing treatments using this product with and without combing [], and also reflects the outcome found in other studies. Given the number of investigations in which permethrin and other insecticide products have delivered an unacceptably low efficacy [,], a review of the licensing of these preparations appears long overdue. It is understandable that sponsors may wish to use insecticide based products as comparators in clinical studies because they have a long history of use and are familiar to the decision makers within the various health services where the profile of new products needs to be raised. However, we have found that ethics review boards are increasingly reluctant to authorise their use in clinical investigations primarily because their efficacy is not much greater than using a placebo. Therefore, as long as these products retain a marketing authorisation, and the validity of this is not reviewed by the various competent authorities, consumers and prescribers will continue to use ineffective products. Consequently, now is probably the time for such a review to take place and for insecticide products to be removed from the market simply because so many treatments fail.


This study has shown that a single application of 4% dimeticone liquid gel is effective to eliminate head louse infestation and that the higher viscosity of this product allows this to be achieved using a treatment time of 15 minutes. However, as with all treatments, it is possible to miss some louse eggs during treatment, requiring post treatment vigilance for emerging nymphs more than 7 days after treatment.

Competing interests

IFB has been a consultant to Thornton & Ross Ltd, the sponsor of this study and manuscript, and to various other makers of pharmaceutical products, medical devices, and combs for treating infestations of head lice and their eggs. ERB and NAB declare no competing interests in this work.

Authors’ contributions

IFB, ERB, NAB collectively conceived, designed, and carried out the investigation. IFB performed some analyses of the data and wrote the draft manuscript. ERB managed the documentation of the study. NAB collected data. All authors read and approved the final manuscript.

Pre-publication history

The pre-publication history for this paper can be accessed here:


This study was sponsored by Thornton & Ross Ltd, Huddersfield, UK. The sponsor played no role in the study design, its execution, interpretation of the data, or the preparation of the manuscript. Statistical analyses were performed on behalf of the sponsor by Peter Lee of PN Lee Statistics and Computing Ltd, an independent statistical consultancy. Some treatments and assessments were performed by Audrey Pepperman, and assessments by Benedict Hall and Rebecca French. Product use and storage conditions for investigational product were monitored by Mark N Burgess. Dr Paul Silverston evaluated adverse events and acted as the medical contact throughout the course of the study. Katharine Coombs and Yvonne Cooper entered the data for statistical analysis and Dr. John Fry provided statistical assistance and review. The documents and conduct of Good Clinical Practice for this clinical trial were monitored by Janet Selby-Sievewright of Harrison Clinical Research Ltd.


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