PHYSICAL INSECTICIDES

How do they act?

Several physical agents are used as pediculocides. They are either silicones or surfactants and their activity depends on interference with the management of water exchange and/or respiration in lice. Compounds used are:

• Dimeticone (generally 4 per cent in lotions and sprays), a long-chain linear silicone, appears to act by coating the insects, disrupting their ability to manage air exchange across their body surface. It also inhibits water excretion, causing physiological stress leading to death.
• A product containing 92 per cent dimeticone has been shown to block the breathing apparatus of lice, leading to suffocation.
• Octane-1,2 diol is an alkyl diol surface-active agent. It appears to act by disrupting cuticular lipid on both head lice and their eggs, resulting in dehydration.
• Cyclometicone/isopropyl myristate (solution and spray). Cyclometicone is a cyclic polydimethylsiloxane and isopropyl myristate is an ester of myristic acid, an essential fatty acid derived from palm kernel oil and isopropyl alcohol. It is used in cosmetics and topical medicinal preparations to aid absorption of constituents through the skin. In treatment against head lice, the product disrupts the exoskeleton’s lipid coating, resulting in dehydration.
• Oxyphthirine is a patented formulation containing dimeticone, caprylic triglyceride (a component of coconut oil), and diisopropyl adipate (a lubricant and emollient widely used in cosmetics). It is marketed as being “gentle to hair and skin”. As with other silicone-based products, it attaches to the respiratory orifices of lice and eggs to suffocate and dehydrate them.

How are they used?

All preparations are applied to dry hair – contact time varies with product from five minutes to eight hours. They are then washed off and the hair combed through to remove lice and eggs. The scalp should be checked after seven days and a second treatment applied if evidence of surviving lice is found.