MALATHION

How does it act?

Malathion is an organophosphorus compound. It is a potent cholinesterase inhibitor, preventing the breakdown of acetylcholine and interfering with neuromuscular transmission in the head louse, paralysing it and preventing it from feeding. It is oil-soluble and is absorbed by a process of passive diffusion through the lipid coat of both insect and egg. Malathion is poorly absorbed through human skin, and it is also much more efficiently detoxified by human metabolic processes than by those of insects. It is therefore safe for occasional or intermittent use at low concentrations as a pediculocide. The single product available is formulated as a creamy aqueous solution containing 0.5 per cent malathion, which is also used as a treatment for scabies and pubic lice.

There are no contraindications to the use of malathion, apart from known sensitivity. It is not contraindicated in pregnant or breastfeeding women, although manufacturers recommend caution. The only reported side effect is very rare skin irritation.

How is it used?

The solution is rubbed gently into dry hair until all the hair and scalp is thoroughly moistened. Application should extend to the neck area and behind the ears, as treatment failure can occur if insecticide does not reach the entire scalp. The hair is then allowed to dry naturally and the solution is left on for 12 hours, usually overnight.

The hair is then shampooed in the normal way and combed with a fine-toothed comb while still wet, to remove dead and dying lice from the scalp and empty egg cases attached to the hair shafts. A second application after seven days is recommended to kill any lice emerging from eggs that may have survived the initial treatment.