## Causative Agent of Scabies **Key Point:** Sarcoptes scabiei var. hominis is the obligate human parasite responsible for scabies worldwide. ### Organism Characteristics - **Size:** 0.3–0.4 mm (barely visible to naked eye) - **Morphology:** Eight-legged mite with a rounded body - **Habitat:** Burrows in the stratum corneum of human skin - **Transmission:** Direct skin-to-skin contact; fomites play a minor role - **Incubation period:** 2–6 weeks for first infection; 1–3 days for reinfection ### Why This Mite? Sarcoptes scabiei var. hominis has evolved strict host specificity for humans. The female mite burrows into the epidermis to lay eggs, triggering an immune response that causes the characteristic pruritus and inflammatory lesions. **High-Yield:** The diagnosis is confirmed by identifying the mite, eggs, or fecal pellets on microscopy of skin scrapings from burrows or papules. ### Comparison with Other Mites | Mite | Host | Disease | Lesion Type | | --- | --- | --- | --- | | Sarcoptes scabiei var. hominis | Humans | Scabies | Burrows, papules | | Demodex folliculorum | Humans | Rosacea, folliculitis | Pustules, erythema | | Psoroptes ovis | Sheep, cattle | Mange | Crusted lesions | | Cheyletiella blakei | Cats, dogs | Cheyletiellosis | Transient papules | **Clinical Pearl:** Animal scabies mites (Psoroptes, Cheyletiella) can cause brief pruritic lesions in humans but do not establish persistent infection because they cannot complete their life cycle in human skin. 
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