DermatologyTier 1Disease (DEADMAN)

Parasitic infections, such as head lice and scabies

Description

Scabies - Sarcoptes scabiei var. hominis mite burrows into stratum corneum
- Crusted (Norwegian) scabies - hyperinfestation (thousands-millions of mites) in immunosuppressed/elderly/disabled patients - highly contagious, easily missed
Pediculosis - lice infestation, three forms: head lice (Pediculus humanus capitis), body lice (P. humanus corporis - associated with poor hygiene/homelessness, disease vector), pubic lice (Phthirus pubis - STI-associated)

Scabies - endemic in overcrowded/low-resource settings; in Australia, disproportionately affects remote Aboriginal and Torres Strait Islander communities (prevalence far above national average)
Head lice - very common in primary-school-age children, spreads by direct head-to-head contact (not a hygiene marker)
Outbreaks common in institutional settings - residential aged care, correctional facilities, refugee settings (scabies); schools/households (lice)

Scabies

Female mite burrows into stratum corneum, lays eggs - delayed type IV hypersensitivity to mite/egg/faeces drives the intense pruritus, not the burrow itself
First exposure: 4-6 week delay before symptoms (sensitisation lag) - important for contact tracing/timing
Re-infestation: symptoms within 1-2 days (already sensitised)
Transmission - prolonged skin-to-skin contact (classically 15-20 minutes); crusted scabies transmits via brief contact or fomites given mite burden

Lice

Obligate human ectoparasite, survives briefly off host; eggs (nits) cemented to hair shaft near scalp
Bites -> localised pruritus/hypersensitivity

Scabies - clinical, confirmed by microscopy where needed

Intense pruritus, worse at night, widespread excoriated papules
Burrows - thin grey/red linear tracks, classically finger webs, wrists, axillae, genitals (adults - spares face/scalp; infants - can involve face/scalp/palms/soles)
Skin scraping for microscopy (mite/eggs/scybala) confirms but clinical diagnosis is standard and sufficient to treat
Crusted scabies: thick hyperkeratotic crusted plaques, may be minimally itchy, widespread including face/scalp - suspect in any immunosuppressed patient with an atypical/crusted rash

Pediculosis - clinical

Direct visualisation of live lice and/or nits attached to hair shaft (nits alone, especially >1cm from scalp, may just be old/hatched)
Pubic lice - consider STI screening (marker of sexual contact)

A. Scabies - standard

Topical permethrin 5% cream - whole body neck-down (face/scalp in infants), left on 8-12h (overnight), repeat after 7 days
Oral ivermectin (single dose, repeated at day 7-14) - alternative/adjunct, useful for outbreaks, crusted scabies, or where topical application impractical
Treat all household/close contacts simultaneously, even if asymptomatic (sensitisation lag means contacts may be pre-symptomatic)
Wash bedding/clothing in hot water or seal in a bag for 72h; mite cannot survive long off-host
Pruritus can persist for 2-4 weeks post-successful treatment - do not mistake for treatment failure and re-treat unnecessarily (crotamiton/antihistamines for symptomatic relief)

B. Crusted scabies

Combination therapy: topical permethrin (more frequent application) plus multiple doses of oral ivermectin (e.g. days 1, 2, 8, 9, 15, and further per severity)
Contact/droplet precautions, isolation if admitted - highly contagious given mite burden
Keratolytics to remove crust and improve topical penetration

C. Pediculosis

Topical permethrin or pyrethrin, repeat at day 7-10 to catch newly hatched lice
Wet-combing with conditioner - adjunct/alternative, mechanical removal
Oral ivermectin for treatment-resistant head lice
Treat household contacts with active infestation; exclude from school until first treatment complete (local policy varies)

Overcrowding, low socioeconomic status, remote/institutional living
Crusted scabies - HIV, HTLV-1, immunosuppressive therapy, intellectual disability, elderly/frail
Secondary bacterial infection (Staphylococcus aureus, Streptococcus pyogenes) from excoriation - can progress to impetigo, cellulitis, and in endemic settings post-streptococcal glomerulonephritis/acute rheumatic fever risk (a major driver of scabies control programs in northern Australia)
Pubic lice - other STIs

Standard scabies - resolves with appropriate treatment and contact management; reinfestation common without treating contacts/environment
Crusted scabies - can be fatal if untreated due to secondary sepsis; source of community outbreaks
Chronic untreated scabies in endemic communities - cumulative skin infection burden contributes to rheumatic heart disease and renal disease risk via recurrent streptococcal infection
Lice - self-limiting with treatment; no long-term sequelae beyond secondary infection from scratching

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