Scabies and ectoparasites

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

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.

Key points

Scabies is caused by Sarcoptes scabiei mites burrowing in the stratum corneum; intense nocturnal itch and similarly affected close contacts are key clues.
Typical burrows and papules involve finger webs, flexor wrists, axillae, waist, buttocks, genital skin and breasts; the adult head is usually spared.
First-line treatment is permethrin 5% cream applied meticulously to the whole required skin surface, left for 8 to 12 hours and repeated after seven days.
Treat all household and close skin contacts at the same time even if asymptomatic because sensitisation and symptoms can be delayed.
Reapply topical treatment to hands after washing and include under nails, between digits, soles, buttock cleft, genital skin and other commonly missed sites.
Crusted scabies needs specialist combination therapy, repeated dosing and strict contact precautions; a single routine permethrin application is inadequate.
Itch can persist for several weeks after successful eradication; diagnose failure from new burrows, live mites, incorrect application or untreated contacts rather than itch alone.
Head lice require detection of live lice before treatment; wet-combing or a licensed physical insecticide pathway differs from scabies treatment and bedding fumigation is unnecessary.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Sarcoptes scabiei infestation

Fertilised female mites transfer mainly through prolonged skin contact and burrow within stratum corneum to lay eggs.

Crusted host susceptibility

Immune suppression, frailty, neurological impairment and reduced scratching permit uncontrolled mite multiplication and massive infectious burden.

Louse species

Pediculus humanus capitis infests scalp hair, Pthirus pubis coarse body hair and body lice primarily inhabit clothing seams.

Environmental arthropods

Bedbugs and fleas feed on human skin but live mainly in rooms, furnishings or animal environments rather than permanently on the person.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Delayed hypersensitivity
Itch and papules reflect immune sensitisation to mite, egg and faecal antigens rather than the small number of classic-scabies mites alone.
2
Burrow reproduction
Female mites tunnel superficially and deposit eggs, with newly hatched larvae maturing on skin and sustaining the life cycle.
3
Hyperinfestation
Failure of cellular control permits exponential mite reproduction within thick keratin, producing crusting and extreme transmissibility.
4
Excoriation barrier failure
Scratching disrupts epidermis and enables staphylococcal and streptococcal invasion, causing impetigo, cellulitis and occasionally systemic infection.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Nocturnal household itch

Generalised itch worse at night with symptomatic household or sexual contacts strongly supports scabies after a typical incubation interval.

Burrows and papules

Fine serpiginous burrows, excoriated papules and nodules in finger webs, wrists, axillae, waist, genital and buttock skin are characteristic.

Genital nodules

Persistent itchy nodules on penis, scrotum or groin can remain after mite eradication and should not automatically trigger repeated scabicide.

Crusted phenotypeRed flag

Thick widespread scale, fissuring and crusting with variable itch in frailty or immune compromise indicates a very high mite burden.

Head lice

Live moving lice on systematic wet combing confirms infestation; nits alone may be empty shells from previous infection.

Pubic or body lice

Lice or nits on coarse hair with itch and blue-grey macules suggests pubic lice, while body lice and excoriations cluster under clothing seams.

Red flags requiring action

Hyperkeratotic widespread scale or crust in immune compromise suggests highly contagious crusted scabies.
Hypotension, fever, spreading erythema or rapidly painful skin suggests secondary bacterial infection and sepsis.
Institutional clustering in a care home, ward, prison or shelter requires coordinated outbreak management.
Infants, pregnancy, breastfeeding, frailty and immune compromise change treatment application and product choice.
Eyelash or eyelid lice require careful specialist product selection to avoid ocular toxicity.
Persistent symptoms with no mites after correct treatment may be post-scabetic inflammation rather than ongoing infestation.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Complete skin examinationFirst step
Why
Identify distribution, burrows, nodules, crusting, secondary infection and commonly missed treatment sites.
Interpretation and limitations
Examine finger and toe webs, wrists, axillae, umbilicus, waist, buttocks, genital and breast skin; extend to scalp in infants, older or immunocompromised people.
02
Dermoscopy
Why
Visualise a mite at the end of a burrow and guide targeted sampling.
Interpretation and limitations
The triangular mite and trailing burrow support diagnosis, but a negative examination does not exclude low-burden classic scabies.
03
Skin scraping or adhesive sampling
Why
Demonstrate mite, egg or faecal pellets when diagnosis is uncertain or crusted disease is suspected.
Interpretation and limitations
Positive microscopy confirms infestation; limited sensitivity means negative sampling must be interpreted with distribution and contact history.
04
Bacterial assessment
Why
Detect impetigo, cellulitis or sepsis complicating excoriated or crusted skin.
Interpretation and limitations
Culture purulent or broken lesions when clinically useful and use systemic sepsis assessment for fever, spreading inflammation or organ dysfunction.
05
Systematic wet detection combing
Why
Confirm live head lice and monitor eradication without relying on nits alone.
Interpretation and limitations
Use a fine detection comb through conditioned wet hair from roots to ends; finding living lice, not old eggshells, establishes active infestation.
06
Contact and setting map
Why
Define everyone requiring simultaneous treatment and identify an institutional outbreak.
Interpretation and limitations
List household, sexual, care and close skin contacts and symptom timing; crusted disease expands environmental and contact management substantially.
07
Immune and diagnostic review
Why
Identify why crusted, recurrent or treatment-resistant infestation has occurred.
Interpretation and limitations
Review immune suppression, frailty, neurological impairment, application technique, reinfestation and mimics before ordering broad immune tests.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Atopic or contact dermatitis

Chronic flexural scale, exposure pattern and absence of burrows or affected contacts supports inflammatory dermatitis, though scabies can exacerbate eczema.

Papular urticaria and bites

Grouped exposed-site papules with environmental timing suggests bedbugs, fleas or other arthropod reactions rather than burrowing mites.

Delusional infestation

Fixed conviction despite repeated negative expert examination requires compassionate dermatological and mental-health assessment after genuine infestation is excluded.

Dermatitis herpetiformis

Intensely itchy grouped vesicles on extensor surfaces with coeliac association follows a different distribution and biopsy pattern.

Drug eruption

A temporally related widespread symmetrical rash with systemic or mucosal features suggests medication hypersensitivity and may require urgent withdrawal.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01CLASSICApply permethrin completely twiceFirst stepClinical or microscopic findings support classic scabies without crusted disease.

1Provide written instructions and enough permethrin 5% cream to cover the whole required surface, including folds, genital skin, buttock cleft, soles, interdigital and subungual sites.
2Apply to cool dry skin, leave for 8 to 12 hours, reapply to hands after washing and wash off after the exposure period.
3Repeat the complete application after seven days to kill mites emerging from eggs and treat every household and close skin contact simultaneously.
4Use emollient, topical corticosteroid or antihistamine symptom care as appropriate and review only when new burrows, live mites or persistent unexplained disease suggests failure.

02CRUSTEDContain and intensify eradicationHyperkeratotic widespread disease, very high mite burden or institutional transmission suggests crusted scabies.

1Use single-room and contact precautions, protective clothing and infection-prevention notification and minimise transfer of contaminated linen and equipment.
2Obtain dermatology or infection-specialist confirmation and use repeated permethrin plus weight-based oral ivermectin on the specialist schedule, with keratolytic treatment when advised.
3Treat exposed contacts simultaneously and use hot laundering or sealed isolation of clothing, bedding and towels according to current UKHSA guidance.
4Continue precautions and repeat clinical or microscopic assessment until the specialist clearance criteria are met; one negative scraping alone may be insufficient.

03HEAD LICEConfirm live lice before treatmentScalp itch, visible nits or school or household exposure raises concern for head lice.

1Use systematic wet detection combing to find live lice and examine close household contacts without excluding children solely for old nits.
2Choose wet combing on days 1, 5, 9 and 13 with a check on day 17, or use a licensed physical insecticide exactly according to its product instructions.
3Treat confirmed household cases on the same day and repeat or switch only according to the product or detection schedule.
4Avoid environmental insecticide sprays and excessive laundering because head lice survive poorly away from the scalp.

04OTHER ECTOPARASITESMatch control to the organismLice are found on pubic or body hair, or bites occur without an organism living on the patient.

1For pubic lice, examine all coarse hair, use a licensed treatment under sexual-health guidance, manage recent sexual contacts and offer STI testing.
2For body lice, prioritise access to washing, clean clothing and hot laundering or replacement of infested garments, with public-health support for vulnerable settings.
3For bedbugs, explain that insects live in the environment, arrange professional pest control and treat only bite reactions or secondary skin infection on the person.
4Check eyelash involvement before any product application and seek ophthalmology or specialist advice to prevent ocular exposure.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

First-line topical scabicide for classic scabies when application is complete and all close contacts are treated simultaneously.

Permethrin 5% cream

Apply thoroughly to the whole required skin surface, leave for 8 to 12 hours, wash off and repeat the complete treatment after seven days.

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.

Provides an alternative topical scabies treatment when permethrin is unsuitable or local guidance recommends another product.

Malathion 0.5% aqueous liquid

Apply to the whole required skin surface, leave for 24 hours, wash off and repeat after seven days when selected as an alternative.

Use the aqueous preparation, follow product age and pregnancy advice, avoid eyes and flames or heat according to formulation and ensure simultaneous contact treatment.

Adds systemic mite killing when enormous burden, crusted scale or practical application failure makes topical treatment alone inadequate.

Oral ivermectin for crusted scabies

Give 200 micrograms/kg orally on the repeated specialist schedule for crusted or otherwise selected scabies, combined with intensive topical 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.

Kills live head lice through a physical mechanism when wet-combing alone is not chosen or has been unsuccessful.

Head-lice physical insecticide

Apply a licensed dimeticone or other nationally recommended physical product exactly at the quantity, exposure time and repeat interval stated for that formulation.

Confirm living lice first, match repeats to the exact product, keep flammable formulations away from fire and heat and avoid unlicensed environmental sprays.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Secondary bacterial infection

Repeatedly excoriated scabetic skin can develop impetigo, cellulitis, abscess or invasive streptococcal and staphylococcal bacterial disease.

Institutional outbreak

Delayed clinical recognition, staff movement and untreated close contacts permit rapid care-home, hospital ward, shelter or prison scabies transmission.

Post-scabetic dermatitis

Persistent immune inflammation causes itch and nodules for weeks after mite death and can lead to unnecessary toxic retreatment.

Sleep and mental-health harm

Nocturnal itch disrupts sleep, concentration and wellbeing and carries stigma that may delay disclosure and contact treatment.

Reinfestation

Untreated contacts, incomplete skin coverage or failure to repeat treatment restores infestation despite initial symptom improvement.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Ask about new burrows and nocturnal itch in the patient and all contacts two to four weeks after simultaneous treatment.
Distinguish persistent hypersensitivity from failure by reviewing application coverage, repeat dose, handwashing, untreated contacts and evidence of live mites.
In crusted scabies document distribution, crust thickness, serial microscopy when used, contact list and infection-prevention clearance.
Monitor excoriations for impetigo, cellulitis, fever and spreading pain and treat bacterial complications on their own severity pathway.
For head lice repeat wet detection after the chosen schedule and do not use remaining nits alone as evidence of live infestation.
Record sexual-health testing and partner management for pubic lice and social or housing support for body-lice infestation.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Itch is an immune signal

First infestation may be asymptomatic for weeks, allowing transmission before hypersensitivity develops; reinfestation becomes symptomatic sooner.

Hands are a common failure site

Permethrin washed off during the overnight exposure loses activity unless reapplied immediately to hands and interdigital skin.

Crust can contain millions

Crusted scabies has vastly greater mite burden than classic disease and can spread through brief contact and contaminated fomites.

Treat contacts without waiting for itch

Asymptomatic incubation is common, so simultaneous contact treatment prevents the treated patient returning to an infested network.

Nits do not prove active head lice

Empty eggshells remain attached after successful treatment; only a living louse confirms current infestation.

Bedbugs do not live on skin

Patient-applied insecticide cannot eradicate an environmental harbourage in furniture and rooms; professional pest control is the causal intervention.

11Common pitfallsFrequent interpretation and management errors.

01
Do not apply permethrin only to visibly itchy areas or forget genital, interdigital, subungual and buttock-cleft skin.
02
Do not treat one household member while leaving asymptomatic close contacts untreated.
03
Do not diagnose treatment failure from itch alone during the expected post-scabetic inflammatory period.
04
Do not manage crusted scabies with a single routine topical application and no infection-prevention response.
05
Do not treat head lice based only on old nits or use household insecticide sprays.
06
Do not apply a scabicide near eyes for eyelash lice without specialist product advice.