Why does the head itch if there are lice? - briefly
Lice feed by biting the scalp and releasing saliva that contains proteins which trigger an allergic or irritant reaction, leading to itching. Their constant movement and the presence of shed eggs and feces further stimulate nerve endings, intensifying the sensation.
Why does the head itch if there are lice? - in detail
Lice attach to the scalp with claws that pierce the skin and feed on blood. Each bite introduces saliva that contains anticoagulants and proteins foreign to the human body. The immune system recognizes these substances as antigens, triggering a localized inflammatory response. Histamine release from mast cells causes vasodilation and nerve irritation, producing the characteristic itching sensation.
The irritation is amplified by several factors:
- Mechanical trauma: Repeated crawling and chewing damage hair follicles and surrounding epidermis, creating micro‑abrasions that stimulate sensory nerves.
- Allergic reaction to saliva: Individuals vary in sensitivity; some develop a pronounced hypersensitivity, leading to intense pruritus.
- Secondary bacterial colonization: Scratching breaks the skin barrier, allowing bacteria to enter and provoke further inflammation.
- Accumulation of debris: Lice excrement (fecal pellets) and dead insects settle on the scalp, acting as irritants that sustain the itch cycle.
The nervous system transmits the itch signal via C‑fibers to the spinal cord and brain, where it is perceived as an urge to scratch. Scratching provides temporary relief by temporarily overriding the itch signal but also worsens the condition by spreading lice, increasing skin damage, and perpetuating the inflammatory loop.
Effective treatment must eliminate the parasites, reduce the allergic component, and protect the skin from infection. Pediculicidal agents remove the insects, antihistamines or topical corticosteroids diminish the immune reaction, and proper hygiene prevents reinfestation.