Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Recognise classic and crusted scabies, treat the entire exposure network correctly and distinguish lice and environmental ectoparasites requiring different examination, product and control pathways.
Classic scabies follows prolonged direct skin contact and produces a delayed hypersensitivity reaction to mites, eggs and faeces. The number of mites is usually small despite severe itch.
Crusted scabies occurs when host immunity or scratching response fails to control reproduction. Millions of mites can inhabit thick scale, enabling transmission through brief contact, clothing, bedding and the environment.
Eradication fails most often because treatment misses skin, is washed from hands, is not repeated, or close contacts are untreated. Written application instructions and simultaneous network treatment are therefore core management.
Ectoparasites are not interchangeable. Head lice live and lay nits on scalp hair, pubic lice attach to coarse hair and can be sexually transmitted, body lice live mainly in clothing seams, and bedbugs require environmental rather than patient-applied eradication.
Fertilised female mites transfer mainly through prolonged skin contact and burrow within stratum corneum to lay eggs.
Immune suppression, frailty, neurological impairment and reduced scratching permit uncontrolled mite multiplication and massive infectious burden.
Pediculus humanus capitis infests scalp hair, Pthirus pubis coarse body hair and body lice primarily inhabit clothing seams.
Bedbugs and fleas feed on human skin but live mainly in rooms, furnishings or animal environments rather than permanently on the person.
Itch and papules reflect immune sensitisation to mite, egg and faecal antigens rather than the small number of classic-scabies mites alone.
Female mites tunnel superficially and deposit eggs, with newly hatched larvae maturing on skin and sustaining the life cycle.
Failure of cellular control permits exponential mite reproduction within thick keratin, producing crusting and extreme transmissibility.
Scratching disrupts epidermis and enables staphylococcal and streptococcal invasion, causing impetigo, cellulitis and occasionally systemic infection.
Generalised itch worse at night with symptomatic household or sexual contacts strongly supports scabies after a typical incubation interval.
Fine serpiginous burrows, excoriated papules and nodules in finger webs, wrists, axillae, waist, genital and buttock skin are characteristic.
Persistent itchy nodules on penis, scrotum or groin can remain after mite eradication and should not automatically trigger repeated scabicide.
Thick widespread scale, fissuring and crusting with variable itch in frailty or immune compromise indicates a very high mite burden.
Live moving lice on systematic wet combing confirms infestation; nits alone may be empty shells from previous infection.
Lice or nits on coarse hair with itch and blue-grey macules suggests pubic lice, while body lice and excoriations cluster under clothing seams.
Chronic flexural scale, exposure pattern and absence of burrows or affected contacts supports inflammatory dermatitis, though scabies can exacerbate eczema.
Grouped exposed-site papules with environmental timing suggests bedbugs, fleas or other arthropod reactions rather than burrowing mites.
Fixed conviction despite repeated negative expert examination requires compassionate dermatological and mental-health assessment after genuine infestation is excluded.
Intensely itchy grouped vesicles on extensor surfaces with coeliac association follows a different distribution and biopsy pattern.
A temporally related widespread symmetrical rash with systemic or mucosal features suggests medication hypersensitivity and may require urgent withdrawal.
Repeatedly excoriated scabetic skin can develop impetigo, cellulitis, abscess or invasive streptococcal and staphylococcal bacterial disease.
Delayed clinical recognition, staff movement and untreated close contacts permit rapid care-home, hospital ward, shelter or prison scabies transmission.
Persistent immune inflammation causes itch and nodules for weeks after mite death and can lead to unnecessary toxic retreatment.
Nocturnal itch disrupts sleep, concentration and wellbeing and carries stigma that may delay disclosure and contact treatment.
Untreated contacts, incomplete skin coverage or failure to repeat treatment restores infestation despite initial symptom improvement.
First infestation may be asymptomatic for weeks, allowing transmission before hypersensitivity develops; reinfestation becomes symptomatic sooner.
Permethrin washed off during the overnight exposure loses activity unless reapplied immediately to hands and interdigital skin.
Crusted scabies has vastly greater mite burden than classic disease and can spread through brief contact and contaminated fomites.
Asymptomatic incubation is common, so simultaneous contact treatment prevents the treated patient returning to an infested network.
Empty eggshells remain attached after successful treatment; only a living louse confirms current infestation.
Patient-applied insecticide cannot eradicate an environmental harbourage in furniture and rooms; professional pest control is the causal intervention.
Do not apply permethrin only to visibly itchy areas or forget genital, interdigital, subungual and buttock-cleft skin.
Do not treat one household member while leaving asymptomatic close contacts untreated.
Do not diagnose treatment failure from itch alone during the expected post-scabetic inflammatory period.
Do not manage crusted scabies with a single routine topical application and no infection-prevention response.
Do not treat head lice based only on old nits or use household insecticide sprays.
Do not apply a scabicide near eyes for eyelash lice without specialist product advice.
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An adult has typical nocturnal itch, burrows and affected household contacts without crusted disease. Which treatment plan is most likely to eradicate the infestation?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
Follow age- and host-specific scalp and face instructions, avoid eyes and mucosa, reapply after handwashing and investigate technique or reinfestation before repeating multiple courses.
Use the aqueous preparation, follow product age and pregnancy advice, avoid eyes and flames or heat according to formulation and ensure simultaneous contact treatment.
Use specialist and current product guidance; avoid or seek expert advice in pregnancy, breastfeeding and low body weight, review interactions and do not use a single dose for crusted disease.
Confirm living lice first, match repeats to the exact product, keep flammable formulations away from fire and heat and avoid unlicensed environmental sprays.