Head lice
Exam Tips
- Diagnosis requires a live louse; eggs/nits alone do not confirm active infestation.
- Wet detection combing is more sensitive than visual inspection or dry combing alone.
- Lice spread mainly by direct head-to-head contact; they do not jump, fly, or come from pets.
- Pruritus is a hypersensitivity reaction, so first infestation may be asymptomatic initially.
- See Figure: head louse life cycle (egg -> nymph -> adult) and common rash distribution at nape/post-auricular scalp.
Definition
Head lice infestation (pediculosis capitis) is an ectoparasitic infection of scalp hair by Pediculus humanus capitis, a blood-feeding insect that lives close to the scalp. Active disease is confirmed only when a live louse is identified, because itch or attached eggs (including empty nits) alone do not prove current infestation.
Pathophysiology
Female lice cement eggs to hair shafts near the scalp; eggs hatch in about 7-10 days, nymphs mature over a further 7-10 days, and adults live around 30-40 days while laying multiple eggs. Lice survive by repeated blood meals from the scalp and are transmitted mainly by direct head-to-head contact (they do not jump or fly). Pruritus is largely due to delayed hypersensitivity to louse saliva/faeces, so first infestations may itch late, while reinfestation can cause earlier symptoms.
Risk Factors
- Childhood (especially 4-11 years; peak around 7-8 years)
- Female sex
- Close head-to-head contact in households/schools
- Previous household infestation
- Ethnicity other than black reported as a population-level risk factor in some datasets
- Not associated with poor hygiene or dirty hair
Clinical Features
Symptoms
- Scalp itch (often occipital/post-auricular)
- Tickling or crawling sensation in hair
- Sleep disturbance from nocturnal itching
- Distress, stigma, or school absence
Signs
- Live lice on detection combing (diagnostic)
- Excoriations from scratching
- Papules at nape/hairline
- Pruritic rash behind ears and on posterior neck
- Occipital/cervical lymphadenopathy in inflamed cases
- Secondary bacterial change (erythema, honey-coloured crust suggesting impetigo)
Investigations
Management
Lifestyle Modifications
- Treat only when active infestation is proven by finding live lice
- Use wet combing as treatment (systematic sessions every few days for about 2 weeks to interrupt life cycle)
- Check and treat all close contacts with live lice on the same day to reduce reinfestation
- Provide reassurance: benign condition, not caused by poor hygiene
- Avoid unnecessary environmental decontamination; transmission is mainly head-to-head (clean combs/brushes that contact hair)
Pharmacological Treatment
Topical physical pediculicide (silicone-based)
- Dimeticone 4% lotion: apply enough to saturate scalp and hair, leave for 8 hours or overnight, then wash off; repeat after 7 days
Common first-line option because resistance is less problematic than older neurotoxic insecticides. External use only; avoid eyes/mucosa and broken skin. Product age limits vary by brand; check licence in young infants.
Topical insecticidal pediculicide (organophosphate)
- Malathion 0.5% aqueous liquid: apply to dry hair/scalp until fully wetted, allow to dry naturally, wash out after 12 hours; repeat after 7 days if live lice remain or as product advises
Use when appropriate per local formulary/licensing. Safety: many preparations are flammable (keep away from flames, cigarettes, hairdryers, and heat sources during application and while hair is wet with product). Avoid contact with eyes and inflamed skin.
Topical physical pediculicide (solvent/oil blend)
- Isopropyl myristate 50% with cyclomethicone 50% solution: apply to dry scalp/hair for 10 minutes then wash off; repeat after 7 days
Acts by dissolving louse wax layer (non-neurotoxic mechanism). Usually avoided in very young children unless licensed for age; may irritate skin/scalp.
Complications
- Persistent pruritus and sleep disruption
- Excoriation from scratching
- Secondary bacterial skin infection (for example impetigo, furunculosis)
- Post-auricular/nuchal eczematous rash from hypersensitivity
- Psychological distress, anxiety, and social stigma
Prognosis
Overall prognosis is excellent with correct detection and complete treatment of active cases plus contacts. Without treatment, infestation can persist for prolonged periods with recurrent itching and occasional secondary infection.
Sources & References
💊BNF Drug References(4)
- Dimeticone[management.pharmacological]
- Malathion[management.pharmacological]
- Malathion[cautions]
- Permethrin[management.pharmacological]
✅NICE Guidelines(1)
- Head lice[overview]
📖Textbook References(7)
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1483)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 26)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1833)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1656)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1656)[context]
- David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1483)[context]
- Oxford Handbook of Clinical Diagnosis (Huw Llewelyn, Hock Aun Ang, Keir Lewis etc.) (Z-Library).pdf(pp. 169, 170, 171)[context]