How do lice cause itching? - briefly
Lice bite the scalp and inject saliva containing proteins that provoke an allergic reaction, producing the itch. The insects’ movement and the resulting inflammation of the skin further amplify the sensation.
How do lice cause itching? - in detail
Lice are obligate blood‑sucking ectoparasites that attach to the scalp and other hair‑covered areas. Their presence initiates a cascade of physiological events that result in a persistent pruritic sensation.
The primary source of irritation derives from the insect’s saliva. During each bite, saliva containing anticoagulant proteins, proteolytic enzymes, and other bioactive molecules is injected into the epidermis. These substances act as allergens, provoking a localized hypersensitivity reaction. The immune system responds by releasing histamine and other mediators, which stimulate nerve endings and generate the characteristic itch.
Mechanical factors also contribute. The mandibles used to pierce the skin create micro‑abrasions that expose nerve fibers. Repeated feeding intensifies skin trauma, leading to heightened sensitivity and a continuous cycle of scratching.
The combined effect of allergic and mechanical stimuli can be summarized as follows:
- Salivary allergens → IgE‑mediated response → histamine release.
- Direct tissue damage → exposure of nociceptors → enhanced nerve activation.
- Accumulation of debris (lice feces, eggs) → irritant load on the skin surface.
Prolonged scratching compromises the epidermal barrier, allowing opportunistic bacteria to colonize the affected area. Secondary bacterial infection further amplifies inflammation and intensifies itching, creating a feedback loop that sustains the discomfort.