Effectiveness of Isopropyl Myristate/Cyclomethicone D5 Solution of Removing Cuticular Hydrocarbons From Human Head Lice (Pediculus Humanus Capitis)

Eric Barnett; Kathleen G Palma; Bert Clayton; Timothy Ballard

Disclosures

BMC Dermatol. 2012;12(15) 

In This Article

Background

Head Lice a Global Issue

Head lice infestation is a fairly common problem globally, with 6–12 million cases in children ages 3–11 reported annually in the US alone,[1] and is typically observed in the school-aged population. Infestation is transmitted most commonly by physical head-to-head contact, but also by sharing hats, hairbrushes, headbands, or clothing.

Insect Exoskeleton-based Cuticular Hydrocarbons are Universal

Head lice, like all insects, have a protective waxy covering on the epicuticle (outer layer of the cuticle on their exoskeletons) that acts as waterproofing for the insect. Therefore, without the wax, the insect is vulnerable to uncontrollable dehydration, and death. These cuticular lipids are composed of various hydrocarbons whose patterns vary by insect.[2]

A Challenge to Cure

Head lice infestations can be difficult to cure completely, and typically require multiple treatments. Some products may act by suffocation of the insects; these may require up to an 8 hour application time. However with additional changes made to the suffocant formulation the 8 hour treatment can be decreased substantially. Eggs (nits) are attached to the hair with protein 'glue' which makes the eggs difficult to remove. Even after treatment, viable eggs may hatch and young nymphs emerge. Isopropyl myristate (IPM) and cyclomethicone D5 do not have an effect on egg development. Only after the operculum (the opening the nymphal louse emerges from) begins to open (4–5 days) can the IPM and cyclomethicone D5 come in contact with the nymph inside. Once the nymph is exposed to the formulation, the nymph will die whether it has emerged from the egg or not.

Pesticide Therapies

Some medical practitioners recommend pesticide treatments containing permethrin, pyrethrin, or malathion. These therapies have been the first line of treatment for head lice since World War II. Today more interest is being placed on treatments that are alternatives to conventional pesticides with different modes of action. These new therapies are unlikely to allow development of resistance in the head louse population.

Increasing Resistance to Pesticides

Of particular concern, is the growing body of evidence suggesting a global emergence in head lice of significant resistance to pesticide products containing permethrins and pyrethroids. Marcoux et al. reported that the widespread use of pyrethrins and pyrethroids have led to significant resistance across various countries including Canada, Argentina, Australia, the United Kingdom, the United States, Japan, Korea, and Israel. This resistance results in prolonged infestations, reinfestation and safety concerns resulting from overdosing with a treatment therapy. Resistance factors associated with insensitivity to DDT, pyrethrins and pyrethroids were found by Marcoux to be present in 97% of head lice collected from several Canadian cities.[3]

Chemistry of Isopropyl Myristate/Cyclomethicone D5 (IPM/D5)

Isopropyl myristate/cyclomethicone D5, is a topical formulation containing 50/50 w/w Isopropyl myristate/decamethylcyclopentasiloxane. Isopropyl myristate, with the chemical formula CH3(CH2)12COOCH (CH3)2,[4] is an ester of myristic acid, an essential fatty acid derived from palm kernel oil and isopropyl alcohol. IPM is used to dissolve lanolin and other oils, and is commonly used as a degreasant or softener in sunscreens, face creams and lipsticks. Due to its probable physical mechanism of action (dissolution of the waxy cuticular covering found on the louse exoskeleton), head louse resistance is not expected to develop against isopropyl myristate.[3] Decamethylcyclopentasiloxane (cyclomethicone D5), with the chemical formula cyclo-(Me2SiO)5[5] is a spreading agent commonly used in cosmetics, that helps the isopropyl myristate to thoroughly coat the hair. Some adulticide activity has been noted in laboratory studies at concentrations higher than 50%. Cyclomethicone D5, which is used extensively as an excipient, can be found in some head lice treatments in concentrations as high as 96%.

About This Study

This study was conducted in an attempt to investigate the mechanism by which IPM/D5 is able to kill head lice after 10 minutes contact. It is well understood that insect dehydration can kill quickly, and it was expected that IPM/D5 was acting via a physical mechanism to disrupt the protective waxy coating on the louse exoskeleton. This study investigates this possible mechanism of action by specifically determining the removal efficiency of head louse cuticular hydrocarbons using a 50/50 w/w mixture of isopropyl myristate/cyclomethicone D5.

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