Head Lice Life Cycle: From Nits to Nymphs to Adults, Transmission, and Prevention

Life Cycle (17-35 Days Total)

1. Nit (Egg): Female louse cements egg to hair shaft ~1-2 mm from scalp. Incubation: 7-10 days. Nits <6 mm from scalp = viable.
2. Nymph (3 stages): Hatches; molts 3 times over 9-12 days. Resembles small adult.
3. Adult: 2-3 mm, tan-gray, 6 legs with claws. Lives 30 days on host. Female lays 6-10 eggs/day.

Transmission

• Primary: Direct head-to-head contact (play, sports, sleepovers).
• Secondary (rare): Shared combs, brushes, hats, helmets, bedding — lice survive <24-48 hrs off host.
• NOT transmitted by: Poor hygiene, pets, jumping/flying (lice crawl only).

Diagnosis

• Live nymph/adult on scalp (gold standard).
• Viable nits within 6 mm of scalp.
• Wood's lamp: nits fluoresce pale blue.

Treatment

• Pediculicides: Permethrin 1% (OTC), Pyrethrins+piperonyl butoxide (OTC), Malathion 0.5% (Rx), Spinosad 0.9% (Rx), Ivermectin 0.5% (Rx).
• Wet combing: Conditioner + fine-tooth nit comb every 3-4 days × 2 weeks (essential adjunct).
• Environmental: Wash bedding/clothing in hot water (>130°F), dry high heat; vacuum; seal non-washables 2 weeks.
• Resistance: Widespread to permethrin/pyrethrins; rotate classes.

Prevention

• Avoid head-to-head contact.
• Don't share hair items.
• Regular checks during outbreaks.
• No "preventive" shampoos proven effective.