Head Lice Lifespan & Life Cycle: Untreated Infestation Timeline
Head lice (Pediculus humanus capitis) are obligate human ectoparasites. Without treatment, an infestation persists indefinitely through continuous reproduction.
Life Cycle (at 30°C / 86°F)
| Stage | Duration | Details |
|---|
| Egg (Nit) | 7–10 days | Cemented to hair shaft ≤1 cm from scalp; resistant to pediculicides |
| Nymph (3 Instars) | 9–12 days total | Require blood meals each molt; size of pinhead → sesame seed |
| Adult | 30 days | Female lays 6–10 eggs/day; lives 3–4 weeks; dies off-host in 24–48h |
Population Dynamics
- One fertilized female → ~100 eggs → exponential growth if untreated.
- Average infestation: 10–20 live lice; can reach hundreds.
- Pruritus from saliva sensitization → 4–6 weeks after first bite (delayed hypersensitivity).
Transmission
- Direct head-to-head contact (primary).
- Fomites (combs, hats, pillows) – minor role; lice die quickly off-host.
- No jumping/flying; crawl 23 cm/min.
Treatment Principles
- Pediculicide + Combing: Permethrin 1% / Pyrethrins + piperonyl butoxide (OTC) or Malathion 0.5% / Spinosad 0.9% / Ivermectin 0.5% (Rx). Repeat Day 9–10.
- Wet Combing (Dimethicone/Conditioner): Every 3 days ×2 weeks; non-chemical, effective if meticulous.
- Environmental: Wash bedding/clothing 60°C; vacuum; bag non-washables 2 weeks. No fumigation needed.
- Check all household contacts; treat simultaneously if infested.
Resistance to permethrin/pyrethrins widespread; use alternative agent if failure after correct application.
Hair Loss - Related Articles
