Scabies

Meet the patient

A 7-year-old boy is brought in because he has not slept for two weeks — he scratches all night, his finger webs and wrists are excoriated, and there are faint wavy lines on his wrists. His mother, his father and his grandmother have all started itching too. The family wants "a stronger cream"; what they need is the dose, the day, and everyone treated at once.^[1][6]

Two facts decide everything in scabies: the itch is a delayed hypersensitivity to a handful of mites, so a normal immune host carries only 10 to 15 of them yet itches across the whole body; and treatment fails not because the drug is weak but because the second dose, the contacts and the linen are missed. Hold those two and the management writes itself.^[1]

What scabies is — and why so few mites itch so much

Scabies is a disease of a tiny burden and a loud immune response. The gravid female mite burrows into the stratum corneum, lays two to three eggs a day for a 30 to 60 day life, and the host mounts a type IV hypersensitivity to mite saliva, eggs and faecal pellets (scybala). That hypersensitivity — not the mites — is what itches.^[1][6]

The timing of the itch is the timing of sensitisation. On first exposure the immune system takes 4 to 6 weeks to learn the mite, so the patient may be infested and infectious long before they scratch; on re-exposure, memory T cells fire in 1 to 3 days. This single fact explains the two exam staples — the asymptomatic incubating contact who reinfects everyone, and the patient who itches again within days of a new encounter.^[1]

The host holds the mite to roughly 10 to 15 in classical scabies through a Th1 response of interferon-gamma, interleukin-2 and cytotoxic effectors. When that containment fails — in HIV, HTLV-1, transplant, leukaemia, long-term steroids, Down syndrome or leprosy — the mite runs uncontrolled and crusted scabies follows.^[1][9]

The life cycle in one breath — and why the second dose is mandatory

The mite is an eight-legged arachnid 0.3 to 0.4 mm long. The female burrows at 0.5 to 5 mm a day, eggs hatch in 3 to 4 days, and larvae mature over a further 7 to 10 days to give an egg-to-adult cycle of about 10 to 14 days. The mite survives off the host for only 24 to 72 hours.^[1][4]

The life cycle is why two doses are non-negotiable. Neither permethrin nor ivermectin reliably kills eggs; the first dose clears adults and larvae, the eggs survive and hatch over the next few days, and the second dose at day 7 to 14 catches the hatchlings before they mature and reproduce. Skipping it is one of the commonest causes of "treatment failure" — which is really an unfinished treatment.^[2][4]

Why do we repeat scabies treatment at day 7 to 14?ShowHide

The repeat dose is not a precaution and not a sign the first treatment failed — it is pharmacologically necessary, because permethrin and ivermectin are not reliably ovicidal. Eggs hatch after the first dose; the second dose kills the larvae before they can mature and lay eggs of their own. Skipping the second dose is one of the commonest causes of persistent scabies.^[1][2]

Classical scabies — the distribution and the burrow

The cardinal symptom is itch, worst at night and after a hot bath; the pathognomonic lesion is the burrow. A burrow is a 5 to 10 mm wavy, grey-white or dark thread in the stratum corneum, often ending in a tiny vesicle or papule that is the mite. The distribution is the geography that earns the mark.^[1][4]

The sites to examine are the finger webs (especially between index and middle), the flexor wrists and ulnar styloids, the lateral fingers, the axillae, the periumbilical skin, the waistline, the inframammary folds and areolae, the genitalia and the buttocks. The head and neck are spared in adults because thicker stratum corneum and sebum deter burrowing — but in infants, the elderly and crusted scabies the head, neck, palms and soles are fair game.^[1][12]

The classic trap: a well-washed patient may have subtle burrows, and a patient on topical or systemic steroids has them flattened and atypical — scabies incognito. The diagnosis then rests on the history of nocturnal itch, the distribution, and the affected contacts, with a low threshold to scrape or treat empirically.^[1][4]

Crusted (Norwegian) scabies — the public-health emergency

Crusted scabies is the same mite with the immune handbrake off, and it is contagious on a different scale. Mite counts reach thousands to millions; the skin carries thick, waxy, psoriasiform crusts over the hands, feet, scalp, ears and nails, with nail dystrophy and subungual debris — and itch is often minimal or absent, which is why it is missed and why it seeds outbreaks.^[9][10][11]

The discriminator line: minimal itch with hyperkeratotic crusts in an immunocompromised or institutionalised host is crusted scabies until proven otherwise — isolate first, then confirm. One undiagnosed crusted case can ignite a whole care-home outbreak.^[5][9]

The variants — nodular, bullous, incognito, animal

Nodular scabies gives firm, intensely itchy reddish-brown nodules on the scrotum, penile shaft, groin, axillae, buttocks and areolae — a prolonged hypersensitivity to retained antigen that persists for months after the mite is gone, and is not active infestation unless new burrows appear. Bullous scabies in older adults gives tense blisters on hands and feet that mimic bullous pemphigoid, with eosinophilic spongiosis but a negative direct immunofluorescence.^[1][12]

Scabies incognito is classical scabies flattened and disguised by topical or systemic corticosteroids — a frequent cause of prolonged diagnostic delay in patients "treated for eczema". Animal-transmitted scabies (sarcoptic mange from dogs, cats or pigs) gives a transient pruritic papular eruption without true burrows, because the animal mite cannot complete its cycle on human skin — treat the animal, and the human eruption settles.^[1][12]

Differential diagnosis — and the mimics of crusted disease

A burrow in a typical site with nocturnal itch and affected contacts is pathognomonic; without the burrow, the differential broadens. Atopic dermatitis is flexural in children and lacks burrows; papular urticaria gives grouped bites on exposed areas; pediculosis affects hair-bearing sites with nits on the shaft; folliculitis is folliculocentric; prurigo nodularis is not in the scabetic pattern.^[1][12]

Crusted scabies has its own mimics — psoriasis (especially pustular and plaque), hyperkeratotic eczema of hands and feet, hyperkeratotic tinea, Darier disease and cutaneous T-cell lymphoma. The clue is hyperkeratotic plaques plus a background of contact itch plus immunosuppression or institutional residence; a skin scraping for mites settles it fastest.^[1][9]

Diagnosis — the 2020 IACS criteria, and the delta-wing jet

The 2020 International Alliance for the Control of Scabies (IACS) criteria stratify the diagnosis into three levels, and a candidate should reproduce them. Confirmed scabies (level A) needs direct visualisation of mite, eggs or scybala by microscopy, dermoscopy or histology; clinical scabies (level B) needs a burrow, or typical lesions at typical sites with a contact history, or typical lesions plus a dermoscopic second-tier sign; suspected scabies (level C) covers atypical lesions with a contact history.^[3]

Dermoscopy is the most useful bedside tool, and its sign is worth a name. The delta-wing jet sign is a dark brown triangle at the end of a serpiginous burrow — the mite's head and anterior legs viewed end-on. The burrow ink test (felt-tip over a burrow, wipe with alcohol, ink stays trapped in the track) and the adhesive-tape test are quick clinic tricks; skin scraping in mineral oil is definitive but only about 50 percent sensitive in classical scabies (far higher in crusted).^[1][4]

PCR, reflectance confocal microscopy and AI-assisted video-dermoscopy are emerging but not yet standard, and they matter most for large-scale screening in low-resource settings.^[7]

Management — the two-dose rule, treat everyone, decontaminate

Three rules, broken in that order, are why scabies "comes back": give two doses, treat every contact on the same day, and decontaminate the linen. The drugs are excellent; adherence and public health are where it fails.^[2][4]

^{[1] [2]}

Permethrin is safe in children over two months and in pregnancy; ivermectin is generally avoided in pregnancy and in children under 15 kilograms on limited safety data. When neither is available or suitable, benzyl benzoate 25 percent (irritant, diluted in children), precipitated sulphur 6 to 10 percent (safe in pregnancy, lactation and infants but messy and odorous), malathion 0.5 percent and crotamiton 10 percent (less effective, but helps residual itch) are the alternatives; lindane is no longer recommended for neurotoxicity.^[2][13]

Crusted scabies — combined, multi-dose, isolated

Crusted scabies needs combined therapy and strict isolation, never a single dose. Apply topical permethrin daily or every 2 to 3 days for 7 days, then twice weekly until cured, and give oral ivermectin 200 micrograms per kilogram on days 1, 2, 8, 9, 15, 22 and 29 — seven doses in the first month. Add keratolytics (salicylic acid 5 to 10 percent or urea 10 to 40 percent) to the crusts so the scabicide can penetrate, isolate the patient under contact precautions, and treat all contacts and carers simultaneously. Relapse is common, so follow up for months.^[5][9][10]

Why treatment "fails" — the recurring pitfalls

A 2024 systematic review and meta-analysis pinned treatment failure to a small set of modifiable factors rather than drug weakness: poor adherence, incorrect application, untreated contacts, fomite re-infestation, and — increasingly — permethrin resistance from voltage-gated sodium-channel mutations.^[8]

^{[1] [8]}

When failure is real, the structured response is: re-check adherence, confirm all contacts were treated, decontaminate the environment, repeat the full two-dose regimen, and rotate to oral ivermectin or combination therapy if resistance is suspected. Moxidectin, with its long skin half-life, may one day allow true single-dose treatment and is under investigation.^[7][8]

Complications — the group A streptococcus chain

Scratching breaches the barrier and lets Staphylococcus aureus and group A Streptococcus in. Impetigo, folliculitis, cellulitis, ecthyma and furunculosis follow, and in endemic paediatric populations scabies-associated pyoderma is a major driver of acute post-streptococcal glomerulonephritis one to three weeks later — haematuria, oedema, hypertension, renal impairment — and contributes to the burden of rheumatic heart disease. Swab infected skin and treat with anti-staphylococcal and anti-streptococcal antibiotics alongside the scabicide.^[6][14]

In crusted scabies, heavy bacterial colonisation of crusts can progress to bacteraemia, sepsis and rarely necrotising soft-tissue infection — the reason early recognition matters beyond the itch.^[9][10]

Prognosis and disposition

Classical scabies is cured with two correct doses and simultaneous contact treatment, and the prognosis is excellent. The single counselling point that saves the most phone calls is that post-scabetic itch persists 2 to 6 weeks after eradication — it is residual hypersensitivity, not failure, unless new burrows appear. Antihistamines, emollients and a mild topical steroid settle it.^[1][2]

Crusted scabies has a guarded prognosis — relapses are common and demand repeated combined therapy and prolonged skin care. Follow up at 2 to 4 weeks, and again at 6 weeks for crusted disease. Notify public health for institutional outbreaks; schools, care homes, hospitals and prisons need an outbreak plan of early identification, coordinated mass treatment and environmental decontamination.^[5][14]

Special populations

Evidence, guidelines and regional differences

The European guideline (EADV 2017) recommends permethrin 5 percent cream first-line (neck-to-toe, repeated at 7 days) with oral ivermectin as an alternative first-line for crusted disease, outbreaks and impractical topical therapy; the 2024 International Journal of Dermatology guideline grades crusted severity and mandates combination therapy; the 2020 IACS criteria standardise diagnosis for practice and surveillance.^[2][3][5]

Mass drug administration is the headline public-health advance: the Skin Health Intervention Fiji Trial (SHIFT) showed community-wide ivermectin cuts scabies and impetigo prevalence, and the WHO and IACS now set thresholds for starting and stopping MDA in high-burden, remote or closed settings.^[1][6][14]

The mantra, and the mnemonics

^{[1] [2]}

The mantra: two doses, everyone on the same night, hot-wash the linen — and itch for six weeks is not failure unless a new burrow appears.^[1][2]

Ward-round test — three stems, thirty seconds each

Stem 1 — the itchy family from the top of the topic (answer)ShowHide

A 7-year-old with two weeks of nocturnal itch, excoriated finger webs and wrists, and faint burrows; mother, father and grandmother are itching too. What is the diagnosis and the complete prescription? Model: Classical scabies, transmitted by prolonged skin-to-skin contact within the household. The whole family gets permethrin 5 percent cream neck-to-soles (including webs, under nails, genitalia), left on overnight and repeated at 7 to 14 days — or oral ivermectin 200 micrograms per kilogram, two doses 7 to 14 days apart where topical is impractical. Treat every contact on the same day, hot-wash and hot-dry linen and clothing from the last 72 hours (or seal for 7 days), and warn that itch may persist 2 to 6 weeks without signalling failure. The second dose is mandatory because the drugs are not reliably ovicidal.^[1][2]

Stem 2 — the care-home resident with crusts and no itch (answer)ShowHide

An 82-year-old care-home resident with dementia has thick waxy crusts over the hands, feet and scalp and minimal itch; two carers and a roommate have just started scratching. What is this, and what is the first action? Model: Crusted (Norwegian) scabies — hyperkeratotic crusts, minimal itch, an enormous mite load, in an institutionalised host, now seeding an outbreak. The first action is contact-precaution isolation of the index case, then combined daily or alternate-day permethrin plus multi-dose oral ivermectin (days 1, 2, 8, 9, 15, 22 and 29) with keratolytics, and simultaneous treatment of all residents, carers and contacts plus environmental decontamination and public-health notification. Offer HIV testing if no risk factor is apparent. One missed crusted case can close a ward.^[5][9][10]

Stem 3 — the patient still itching at four weeks (answer)ShowHide

A patient treated with a single application of permethrin four weeks ago is still itching; no new burrows, no new contacts symptomatic. Is this treatment failure, and what do you do? Model: No — this is most likely post-scabetic itch, a residual hypersensitivity that persists 2 to 6 weeks after successful eradication. The key discriminator is the absence of new burrows: if none are appearing, the mite is gone. Settle the itch with antihistamines, emollients and a mild topical steroid, and reassure. Only confirm new burrows (or a new symptomatic contact) before retreating — and if true failure, audit adherence, contacts, linen and the second dose, then rotate to ivermectin or combination therapy, considering permethrin resistance.^[1][8]

References14ShowHide

1. [1]Fernando DD, Mounsey KE, Bernigaud C, et al. Scabies Nat Rev Dis Primers, 2024.PMID 39362885
2. [2]Salavastru CM, Chosidow O, Boffa MJ, et al. European guideline for the management of scabies J Eur Acad Dermatol Venereol, 2017.PMID 28639722
3. [3]Engelman D, Yoshizumi J, Hay RJ, et al. The 2020 International Alliance for the Control of Scabies Consensus Criteria for the Diagnosis of Scabies Br J Dermatol, 2020.PMID 32034956
4. [4]Sunderkötter C, Wohlrab J, Hamm H. Scabies: Epidemiology, Diagnosis, and Treatment Dtsch Arztebl Int, 2021.PMID 34615594
5. [5]Uzun S, Durdu M, Yürekli A, et al. Clinical practice guidelines for the diagnosis and treatment of scabies Int J Dermatol, 2024.PMID 38922701
6. [6]Heukelbach J, Feldmeier H. Scabies Lancet, 2006.PMID 16731272
7. [7]Tavoletti G, Avallone G, Sechi A, et al. Scabies: An updated review from epidemiology to current controversies and future perspectives Travel Med Infect Dis, 2025.PMID 40754226
8. [8]Mbuagbaw L, Sadeghirad B, Morgan RL, et al. Failure of scabies treatment: a systematic review and meta-analysis Br J Dermatol, 2024.PMID 37625798
9. [9]Sun YJ, Gong HB. Crusted Scabies N Engl J Med, 2025.PMID 40768718
10. [10]Selvaraj R, Purushothaman S, Indra Couppoussamy K. Crusted scabies Indian J Dermatol Venereol Leprol, 2023.PMID 37317720
11. [11]Kolar KA, Rapini RP. Crusted (Norwegian) scabies Am Fam Physician, 1991.PMID 1718155
12. [12]Thomas C, Coates SJ, Engelman D, et al. Ectoparasites: Scabies J Am Acad Dermatol, 2020.PMID 31310840
13. [13]Ertugrul G, Aktas H. Comparison of sulfur ointment and permethrin treatments in scabies Dermatol Ther, 2022.PMID 36193643
14. [14]Widaty S, Miranda E, Cornain EF, et al. Scabies: update on treatment and efforts for prevention and control in highly endemic settings J Infect Dev Ctries, 2022.PMID 35298417