Pediculicides

Pediculicides treat infestation with head lice (Pediculus humanus capitis), body lice (P. h. humanus) and pubic lice (Pthirus pubis). Treatment depends on proven live lice, not on nits alone. Resistance to pyrethroids is widespread, so physical agents and different classes are often needed.

Mechanism. Permethrin and malathion are neurotoxic. Dimeticone (a silicone oil) coats lice and blocks the spiracles, killing by physical means, and cannot select for resistance. Ivermectin binds glutamate-gated chloride channels. Spinosad causes neuronal excitation and paralysis. Benzyl alcohol blocks spiracles.

Pharmacokinetics. Negligible absorption of topical agents through intact scalp; oral ivermectin is absorbed better with food and has a half-life of about 18 hours.

• Dimeticone 4 percent lotion: physical action; first line where available; no neurotoxicity.
• Permethrin 1 percent crème rinse (5 percent cream for pubic lice): widely available; resistance common.
• Malathion 0.5 percent lotion: second line; flammable.
• Ivermectin 0.5 percent lotion; spinosad 0.9 percent: newer topical agents where available.
• Benzyl alcohol 5 percent lotion: physical action; not for children under 6 months.
• Oral ivermectin: body lice, resistant head lice, pubic lice, outbreaks.
• Wet combing: non-chemical alternative.

Regimens.

• Head lice: dimeticone 4 percent lotion applied to dry hair and scalp, left for 8 hours or overnight, repeated after 7 days; or permethrin 1 percent rinse for 10 minutes on towel-dried hair, repeated after 7 days; or malathion 0.5 percent lotion for 12 hours, repeated after 7 days. Wet combing with a fine-toothed comb every 3 days for at least 2 weeks until no lice are found for 3 sessions.
• Oral ivermectin 200 micrograms/kg, repeated after 7 to 10 days, for treatment failure (ivermectin lotion: single application for 10 minutes).
• Body lice: wash clothing and bedding at 60 degrees Celsius or discard; treat the person only when lice persist (permethrin 1 percent or oral ivermectin). Louse-borne illness (epidemic typhus, relapsing fever, trench fever) is a risk in refugee camps, shelters and conflict zones, including parts of the Horn of Africa. Treat epidemic typhus with doxycycline 100 mg twice daily for 7 days, or 200 mg as a single dose in outbreaks.
• Pubic lice: permethrin 5 percent cream or malathion 0.5 percent, repeated at 7 days; treat sexual partners and screen for other sexually transmitted infections. Eyelash infestation: petrolatum applied twice daily for 10 days.
• Children: treat only when live lice are seen; avoid agents in infants under 6 months without advice; benzyl alcohol is avoided under 6 months.

Selection. Check all close contacts and treat only those infested; do not use pediculicides for prevention. If treatment fails, confirm correct use, then switch class.

Contraindications. Hypersensitivity; malathion near flames or hair dryers (alcohol-based formulations are flammable); lindane is neurotoxic and is avoided.

Effects. Scalp irritation, itching, erythema; dimeticone is greasy; malathion and benzyl alcohol cause irritation; ivermectin lotion causes mild eye irritation.

Interactions. None clinically relevant at topical doses.

Monitoring. Re-examine 7 to 10 days after the second application; document live lice; screen household.

Special groups. Pregnancy: dimeticone and permethrin are acceptable; avoid oral ivermectin. Breastfeeding: dimeticone and permethrin acceptable. Region: crowded living and shared bedding increase transmission in schools and institutions; body louse control (washing clothes, shower, delousing) is the main measure in outbreaks.