Why does a lice bite itch? - briefly
Lice saliva contains anticoagulant proteins that provoke a localized immune reaction, causing mast cells to release histamine, which produces the itching sensation.
Why does a lice bite itch? - in detail
Lice feed by piercing the epidermis and injecting saliva that contains anticoagulants, proteases, and other bioactive molecules. These substances prevent blood clotting and facilitate nutrient extraction, but they also act as foreign antigens that trigger the host’s immune system.
The immune response begins within minutes of the bite. Mast cells in the dermis detect the salivary proteins and release histamine, prostaglandins, and leukotrienes. Histamine dilates capillaries, increases vascular permeability, and stimulates nerve endings, producing the characteristic pruritic sensation.
Additional contributors to the irritation include:
- Mechanical disruption of skin cells caused by the mandibles of the insect, which creates a micro‑wound that exposes sensory nerve fibers.
- Secondary colonization by bacteria introduced from the insect’s mouthparts, which can amplify inflammation.
- Individual sensitivity variations; some people generate a stronger IgE‑mediated response, leading to larger wheals and more intense itching.
The duration of the itch depends on the amount of saliva deposited, the density of bites, and the host’s immune reactivity. In most cases, the reaction peaks within 24 hours and subsides as the inflammatory mediators are metabolized. Persistent scratching can damage the epidermis, allowing further bacterial invasion and prolonging the discomfort.
Effective control of the symptom involves reducing histamine activity (e.g., topical antihistamines or corticosteroids) and preventing secondary infection (e.g., antiseptic washes). Eliminating the lice population through appropriate pediculicidal treatment removes the source of salivary exposure, ultimately stopping the cycle of bite‑induced pruritus.