Scabies for NEET PG: Burrow Sites, Norwegian Scabies and the Drug of Choice

Scabies is an intensely itchy skin infestation caused by the mite Sarcoptes scabiei var. hominis, and WHO estimates that at least 200 million people have it at any time (WHO fact sheet). Three things organise the topic: where burrows sit in adults versus infants, how crusted (Norwegian) scabies differs from the common form, and which drug is first line.

Sources differ on a few timings and dosing intervals. Where they do, this post gives the range and names the source. For wider skin revision, see our dermatology revision overview.

Key Takeaways

• Cause: Sarcoptes scabiei var. hominis. The female burrows in the stratum corneum, and the night-time itch is a delayed hypersensitivity to mite antigens.

• Adult burrows favour finger webs, wrists, elbows, axillae, waistline, genitals and buttocks, and usually spare the face and scalp. Infants also get palms, soles, scalp and face.

• Crusted (Norwegian) scabies has thousands to millions of mites, itches little, and occurs with immunosuppression or reduced sensation.

• IACS 2020 levels: A confirmed (mite, eggs or faecal pellets seen), B clinical, C suspected.

• First line is permethrin 5% cream from the neck down. Ivermectin is 200 mcg/kg in two doses, not for pregnant women or children under 15 kg.

• Crusted scabies needs ivermectin on days 1, 2, 8, 9 and 15 plus a topical scabicide and a keratolytic.

• Treat contacts together, launder above 50 C for 10 minutes or seal items for 72 hours, and expect itch to last weeks after cure.

What Is Scabies? Cause and Life Cycle

Scabies is caused by Sarcoptes scabiei var. hominis, an obligate human parasite. Its life cycle has four stages: egg, larva, nymph and adult, with three pairs of legs in the larva and four in the nymph. An adult female measures 0.30 to 0.45 mm and an adult male 0.20 to 0.24 mm (CDC DPDx).

The female mates once, stays fertile for life and lives about one to two months (CDC DPDx). She tunnels into the stratum corneum, lays two to three eggs a day and advances about 0.5 to 5 mm daily, while the male dies after mating (DermNet; Sunderkötter et al., 2021).

Timings differ by source: eggs hatch in 3 to 4 days (WHO, CDC), 2 to 3 days (Sunderkötter et al.) or 2 to 5 days (DermNet).

A classic infestation carries only about 10 to 15 mites (WHO). The itch is a delayed, cell-mediated hypersensitivity to mite antigens (DermNet; Sunderkötter et al.), which fits the weeks-long delay before symptoms start.

How Scabies Spreads and When Symptoms Start

Scabies spreads through prolonged skin-to-skin contact, and a person can pass it on before symptoms appear (CDC, About Scabies). Spread through clothing or bedding is rare in common scabies but matters in crusted scabies (Al-Dabbagh, J Trop Med 2026). Off the skin, mites survive for 2 to 3 days (WHO, CDC).

Time to symptoms depends on prior exposure. A first infestation takes about 4 to 6 weeks (WHO, MSD Manual, DermNet), although other sources give 2 to 6 weeks, 4 to 8 weeks or up to 2 months. A re-infestation causes itch within days, given as within 1 to several days (Sunderkötter et al.) and 1 to 4 days by CDC DPDx. Learn the contrast: weeks the first time, days afterwards.

Clinical Features of Scabies: Burrow Sites in Adults and Infants

The cardinal symptom is itch that is worse at night (WHO; CDC). The characteristic lesion is the burrow, a short, serpiginous, grey-white or skin-coloured line. Reported lengths are 3 to 7 mm (Sunderkötter et al.), 5 to 10 mm (DermNet) and 1 cm or more (CDC DPDx).

Burrows can be subtle (DermNet), and failure to find mites is common (MSD Manual). A papular eruption can also appear where female mites are absent, such as the buttocks, scapulae and abdomen (CDC DPDx).

Sources for the adult column: Sunderkötter et al., the MSD Manual, DermNet and the IACS 2020 consensus criteria (face and scalp infrequently involved in older children and adults). Sources for the infant column: the MSD Manual, CDC About Scabies and Sunderkötter et al. for the face; WHO and IACS 2020 for palms, soles, scalp and whole-body involvement.

A memory aid of our own, not a standard mnemonic: WEB-WAG for the main adult sites. Web spaces, Elbows, Belt line, Wrists, Axillae, Genitals. Add buttocks, thighs, feet and, in women, the areolae. In adults the face and scalp are usually spared; in infants they can be involved.

Nodular scabies

Nodular scabies produces nodules of about 5 to 20 mm (Sunderkötter et al.; MSD Manual), or 3 to 15 mm in DermNet, on the genitals, groin, axillae and buttocks. They can persist for months after cure and can be mistaken for active disease (Al-Dabbagh).

Crusted (Norwegian) Scabies: Why It Is Different

Crusted scabies is a hyperinfestation. It carries thousands to millions of mites rather than 10 to 15 (WHO; DermNet), and CDC gives a figure of up to two million mites and eggs (CDC clinical overview). Itch is mild or absent, so the diagnosis is easy to miss, and eosinophilia and raised IgE are common (Sunderkötter et al.; MSD Manual).

The risk groups share impaired immunity or impaired scratching:

• HIV infection (WHO; CDC clinical overview; MSD Manual) and HTLV-1 infection (systematic review).
• Immunosuppressive drugs, including glucocorticoids (MSD Manual; Sunderkötter et al.).
• Older or institutionalised people, and conditions that blunt scratching or sensation, such as spinal cord injury (CDC).
• Leprosy and Down syndrome (DermNet).

Our own memory aid: crusted scabies has few itches, many mites.

Complications of Scabies

The main complication is secondary bacterial infection with Staphylococcus aureus and Streptococcus pyogenes (DermNet). It causes impetigo, deeper infection such as abscesses and cellulitis, and sometimes sepsis (WHO; Al-Dabbagh). The streptococcal link carries the serious sequelae:

• Post-streptococcal glomerulonephritis. WHO reports evidence of acute renal damage in up to 10% of affected children in resource-poor settings.
• Rheumatic fever and rheumatic heart disease, linked through S. pyogenes impetigo (WHO; DermNet).
• Variant presentations. Bullous scabies occurs mainly in the elderly, and in scabies incognito steroid misuse delays the diagnosis (DermNet).

Diagnosing Scabies: IACS 2020 Criteria, Dermoscopy and Scraping

The IACS 2020 consensus criteria grade a diagnosis as confirmed, clinical or suspected (Engelman et al., Br J Dermatol 2020).

The history features are H1, itch, and H2, contact with someone with crusted scabies, or close contact with someone with scabies or unexplained itch or typical scabies lesions. Levels B and C apply only when other diagnoses are less likely. They do not apply to crusted, bullous or other variant presentations, and they are not for monitoring cure.

A burrow on its own is B1, because A3 needs the mite itself to be seen (dermoscopy review, J Clin Med 2026). A memory aid of our own: A means the mite is actually seen, B means the picture is convincing, C means it could be scabies.

Dermoscopy: the delta-wing jet sign

On dermoscopy, the mite's head and front end appear as a dark triangle with the burrow trailing behind it like a contrail. This is the delta-wing jet with contrail sign, also called the kite sign (J Clin Med 2026 review) or the hang glider sign (Indian Dermatol Online J 2021). Crusted scabies shows the noodle sign (Indian Dermatol Online J 2021). Remember it as jet equals mite, contrail equals burrow.

Skin scraping and the burrow ink test

CDC's scraping method uses a scalpel blade and mineral oil on a non-excoriated burrow, then a coverslip, viewing at 40 to 100x and confirming at 200 to 400x (CDC DPDx). A negative scraping does not exclude scabies, and the MSD Manual advises against KOH because it dissolves faecal pellets. In the burrow ink test, ink applied to a suspected burrow enters it and leaves a zig-zag line (DermNet).

Treatment of Scabies: Permethrin 5% Is the Drug of Choice

CDC treats permethrin 5% cream as first line (CDC clinical care; CDC STI Guidelines 2021). The table sets it beside the other agents.

Using permethrin and ivermectin correctly

Cover the web spaces and genitals too, because incomplete application was strongly associated with treatment failure (Al-Dabbagh). Infants and young children also need the head and neck treated, avoiding periorbital and perioral skin (MSD Manual).

CDC advises taking ivermectin with food, although food advice differs between references. WHO advises against ivermectin in pregnant women and children under 15 kg, and CDC says its safety is not established in either group (CDC clinical care). In pregnancy and lactation, permethrin is preferred (CDC STI Guidelines), with sulfur as the alternative (DermNet; MSD Manual).

WHO recommends two ivermectin doses of 200 mcg/kg for community-wide treatment, with permethrin 5% where ivermectin is contraindicated. In a nonrandomised before-after study of four villages in Tehri-Garhwal, Uttarakhand (1,211 people), two doses 7 days apart cut scabies prevalence from 7.5% to 0.9% at 12 months (Engelman et al., 2024).

Treatment of crusted scabies

Crusted scabies needs a topical scabicide, repeated oral ivermectin and a keratolytic. The CDC STI guideline gives ivermectin 200 mcg/kg on days 1, 2, 8, 9 and 15, adding days 22 and 29 in severe cases. It pairs this with topical permethrin daily for 7 days and then twice weekly until cure, or with 25% benzyl benzoate.

CDC's clinical care page lists schedules of 3, 5 or 7 doses and states there is no consensus on the best. Sunderkötter et al. agree on days 1, 2, 8, 9 and 15, and add mechanical removal of crusts and isolation precautions for staff. A memory aid of our own: 1-2, 8-9, 15, two doses, two more a week later, then one a week after that.

Contacts, Linen and Itch After Scabies Treatment

Treat all household members and close contacts at the same time (CDC, About Scabies), even without symptoms (WHO). Look-back periods differ: two months of skin contact on CDC's public page, one month for household and close contacts in the CDC STI guideline. Treating only the index patient is a major cause of apparent failure (Al-Dabbagh).

Wash and dry clothing and bedding above 50 C (122 F) for 10 minutes, or seal items in a bag for at least 72 hours (CDC, About Scabies; CDC clinical overview). CDC says spraying and fogging are unnecessary, and Al-Dabbagh stresses laundering for crusted scabies rather than for every common case.

Post-scabetic itch and apparent treatment failure

Itch can persist after the mites are gone, because the skin keeps reacting to residual mite antigens. It was traditionally thought to last 2 to 4 weeks, but cohorts report mean or median durations of about 52.5 to 56 days (Al-Dabbagh). WHO notes that itch often worsens for 1 to 2 weeks after treatment starts, and CDC says retreatment may be necessary if itch lasts 2 to 4 weeks or new burrows appear.

Persistent itch alone does not prove failure, and topical steroids for 3 to 4 weeks may be used (DermNet). Most apparent failures are pseudoresistance: poor application, untreated contacts or reinfestation (Al-Dabbagh). Look for these before blaming drug resistance.

Differential Diagnosis of Scabies

Scabies mimics many itchy dermatoses. The sources list dermatitis and eczema, papular urticaria and arthropod bites, folliculitis, impetigo, subacute prurigo, psoriasis, bullous disorders and infantile acropustulosis (DermNet; IACS 2020; Al-Dabbagh). For the scaly differential, see our notes on psoriasis.

Common Mistakes When Revising Scabies

• Treating only the patient. Contacts are treated at the same time.
• Skipping web spaces and genitals when applying permethrin.
• Calling every persistent itch a failure. Post-scabetic itch can last weeks.
• Giving ivermectin to a pregnant woman or a child under 15 kg. Permethrin is preferred there, and sulfur is the alternative in young infants.
• Expecting crusted scabies to itch. Itch is mild or absent.
• Expecting nodules to vanish at cure. They can last for months.

To revise, redraw the adult-versus-infant sites table and the classic-versus-crusted table from memory, then recite one limit for each drug. Then practise with dermatology fungal infections and infestations questions.

Conclusion

Scabies follows the burrow: web spaces and wrists in adults, palms, soles and head in infants, and thick crusts carrying thousands of mites in crusted disease. Diagnose with the IACS 2020 levels, treat with permethrin 5% first, keep ivermectin for patients outside the pregnancy and 15 kg limits, and treat contacts and linen together. Expect the itch to outlast the mites.