Step by Step
Ss
Sarcoptes scabiei — burrows in the stratum corneum
The scabies mite, Sarcoptes scabiei, burrows into the stratum corneum (the outermost skin layer), with a strong preference for the webbing between fingers, the wrists, and the genital region.
Itch
Intense pruritus — a delayed hypersensitivity reaction
The intense itching associated with scabies — classically worse at night — isn't a direct reaction to the mite itself, but a Type IV (delayed-type) hypersensitivity reaction to mite proteins. This is why itching can take weeks to develop after the initial infestation, and why it can persist even after treatment has killed the mites, since the immune reaction takes time to settle.
Dx
Diagnosis — skin scraping under the microscope
Diagnosis is confirmed by taking a skin scraping, applying mineral oil, and examining it under the microscope for mites, eggs, or fecal pellets.
Tx
Treatment — the whole household, all at once
Treatment is topical permethrin applied to the entire body from the neck down. Critically, all household contacts must be treated simultaneously — even those without symptoms yet — since the delayed hypersensitivity reaction means someone can be infested and contagious well before they start itching. All bedding and clothing should also be washed in hot water.
A patient presents with intense nighttime itching and visible burrows between the fingers; other household members report mild itching too, though less severe — this is scabies, and every person in the household needs simultaneous treatment with topical permethrin, regardless of symptom severity, to prevent reinfestation from an asymptomatic carrier.
Applied Walkthrough
1
A patient presents with intense itching, worse at night, along with visible burrow tracks between the fingers.
2
The patient's partner reports only mild itching, and their children report none at all.
3
Ask: should treatment be limited to the patient with the worst symptoms? No — because the itching is a delayed hypersensitivity reaction, someone can be actively infested and contagious well before symptoms appear. Treating only the most symptomatic person leaves mites circulating in the household, guaranteeing reinfestation.
4
The correct approach is treating every household member simultaneously with topical permethrin, regardless of who currently has symptoms, along with washing all bedding and clothing — this breaks the cycle of transmission that symptom-based treatment alone would miss.
Exam Application
Exams test the mechanism behind scabies itching (Type IV delayed hypersensitivity to mite proteins, not a direct reaction to the mite), the diagnostic approach (skin scraping with mineral oil under microscopy), and — most importantly — the requirement to treat the entire household simultaneously regardless of individual symptom status, since delayed hypersensitivity means asymptomatic household members can still be infested and contagious.
⚠ Common Trap
The most common trap is treating only the symptomatic individual and assuming household members without itching are unaffected. Because the itching reflects a delayed immune reaction rather than the infestation itself, someone can be actively carrying and transmitting mites for weeks before symptoms ever appear — which is exactly why whole-household treatment is the standard of care, not an overly cautious extra step.
✓ Quick Self-Check
1. Where does the scabies mite characteristically burrow?
The stratum corneum, especially between the fingers, the wrists, and the genital region.
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2. What causes the intense itching in scabies, and why can it take time to appear?
A Type IV (delayed-type) hypersensitivity reaction to mite proteins — since it's an immune reaction rather than a direct response to the mite, it can take weeks to develop after initial infestation.
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3. How is scabies diagnosed?
Skin scraping combined with mineral oil, examined under the microscope for mites, eggs, or fecal pellets.
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4. What is the standard treatment for scabies?
Topical permethrin applied to the entire body from the neck down.
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5. Why must all household contacts be treated simultaneously, even if asymptomatic?
Because the delayed hypersensitivity reaction means a person can be infested and contagious well before symptoms appear — treating only symptomatic individuals leaves an active source of reinfestation in the household.
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