Why does the head itch after having lice? - briefly
The itching is caused by an allergic response to lice saliva combined with the mechanical irritation of the insects moving on the scalp, which provokes inflammation and histamine release.
Why does the head itch after having lice? - in detail
The itching results from a combination of physiological and immunological reactions triggered by the presence of the parasites.
Adult insects and nymphs feed on blood by inserting their mouthparts into the scalp. Their saliva contains anticoagulants, enzymes, and proteins that prevent clotting and facilitate feeding. These substances are recognized as foreign by the body, provoking a localized inflammatory response. Histamine released from mast cells irritates nerve endings, creating the characteristic pruritus.
The eggs (nits) attached to hair shafts also contribute. As they develop, they produce metabolic waste that accumulates on the scalp surface. This waste, together with the dead bodies of insects that fall off after feeding, serves as an additional source of irritation.
Repeated bites can compromise the integrity of the skin barrier. Minor abrasions allow opportunistic bacteria or fungi to colonize the area, leading to secondary infections that intensify discomfort and prolong the sensation of itch.
The nervous system amplifies the signal through a feedback loop: scratching damages the epidermis, releasing more inflammatory mediators, which in turn heighten the urge to scratch. This cycle often persists until the infestation is eradicated and the scalp heals.
Key factors behind the pruritic sensation:
- Salivary proteins acting as allergens
- Histamine and other inflammatory mediators
- Accumulated waste from eggs and dead insects
- Skin microtrauma from repeated scratching
- Possible secondary microbial infection
Effective treatment must eliminate the parasites, cleanse the scalp of residual debris, and reduce inflammation, thereby breaking the itch–scratch cycle and allowing tissue recovery.