How does permethrin act on lice? - briefly
Permethrin, a synthetic pyrethroid, binds to voltage‑gated sodium channels in the louse nervous system, preventing channel closure and causing sustained nerve depolarization. This hyperexcitation induces paralysis and rapid death of the parasite.
How does permethrin act on lice? - in detail
Permethrin is a synthetic pyrethroid that targets the nervous system of head‑lice (Pediculus humanus capitis). The compound binds to voltage‑gated sodium channels on neuronal membranes, delaying channel closure and prolonging depolarization. Continuous influx of sodium ions produces repetitive firing of nerve impulses, leading to hyperexcitation, loss of coordinated movement, and eventual paralysis.
The insect’s cuticle permits rapid absorption of the topical formulation. Once inside the hemolymph, permethrin distributes throughout the body, reaching the central nervous system within minutes. The neurotoxic effect is dose‑dependent; concentrations of 1 % permethrin lotion applied for ten minutes are sufficient to achieve >99 % mortality in susceptible lice populations.
Metabolic detoxification in lice involves cytochrome P450 enzymes that can oxidize permethrin to less active metabolites. Repeated exposure selects for over‑expression of these enzymes, reducing susceptibility and giving rise to resistant strains. Resistance monitoring typically includes bioassays measuring knock‑down time and survival rates after standard exposure.
Key pharmacodynamic characteristics:
- Target site: neuronal sodium channels (site 2)
- Action: prolongation of channel opening → sustained depolarization
- Outcome: hyperexcitation → paralysis → death
- Absorption: rapid through cuticle, systemic distribution in hemolymph
- Metabolism: oxidative degradation by P450 enzymes, potential resistance mechanism
Formulation considerations affect efficacy. Emulsifiable concentrates, lotions, and shampoos differ in solvent composition, which influences skin penetration and residue on hair shafts. Proper application—covering the entire scalp and hair, leaving the product on for the recommended duration, and thorough rinsing—ensures maximal contact with lice and nits.
In summary, permethrin eliminates head‑lice by disrupting neuronal sodium channel function, causing uncontrolled nerve activity that incapacitates the parasite. Its rapid cuticular uptake, potent neurotoxicity, and formulation‑dependent delivery underpin its high effectiveness, while enzymatic detoxification in the insect can diminish activity and necessitate resistance surveillance.