Why and how do lice appear? - briefly
Lice infestations begin when adult females deposit eggs (nits) on hair shafts, and the emerging nymphs feed on human blood. Transmission is driven by close head-to-head contact, sharing of personal items, and environments where hygiene is insufficient.
Why and how do lice appear? - in detail
Lice are obligate ectoparasites that feed exclusively on the blood of mammals. Three species commonly infest humans: Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Pthirus pubis (pubic louse). Each species completes its life cycle on the host, progressing from egg (nit) to nymph to adult in a matter of weeks.
The reproductive cycle begins when a female deposits 5–10 eggs per day onto hair shafts or clothing fibers. Eggs adhere firmly with a cement-like substance, hatching after 7–10 days. Emerging nymphs undergo three molts over 9–12 days before reaching reproductive maturity. Adults live 30–40 days on the host, during which they mate and lay additional eggs, sustaining the infestation.
Factors that promote colonization include:
- Direct skin‑to‑skin contact, especially prolonged head‑to‑head interaction among children.
- Sharing personal items such as combs, hats, scarves, or bedding.
- Overcrowded living conditions that increase the likelihood of contact.
- Warm, humid environments that accelerate egg development and nymph maturation.
- Inadequate grooming or infrequent hair washing, which reduces mechanical removal of lice and nits.
Transmission routes differ among species. Head lice spread primarily through head contact, whereas body lice move via contaminated clothing and linens; pubic lice transfer through sexual contact or close intimate contact. Once established, a population can expand rapidly because each adult female produces dozens of eggs during its lifespan.
Population dynamics are sensitive to temperature (optimal range 28–30 °C) and relative humidity (70–80 %). Deviations from these conditions slow development or increase mortality. Host immune response does not eradicate lice; instead, it may cause itching and secondary skin irritation, which can lead to scratching and secondary infection.
Effective control requires eliminating all life stages. Measures include:
- Mechanical removal of nits with fine‑tooth combs.
- Application of pediculicidal agents that target both adults and developing eggs.
- Washing and drying clothing, bedding, and personal items at temperatures ≥ 60 °C.
- Isolating infested individuals from close contact until treatment is complete.
Understanding the biological cycle, environmental preferences, and transmission pathways explains why lice appear and persist in human populations.