## Causative Organism of Scabies **Key Point:** Sarcoptes scabiei var. hominis is the sole causative agent of human scabies worldwide. This is the human-specific variety of the mite. **High-Yield:** The organism is a microscopic arachnid (8-legged) that burrows into the stratum corneum, creating characteristic linear or S-shaped burrows. The female mite lays 2–3 eggs daily within the burrow. ### Identification Features | Feature | Details | | --- | --- | | **Size** | 0.3–0.4 mm (barely visible to naked eye) | | **Morphology** | Oval body with 4 pairs of legs; anterior pair has suckers | | **Habitat** | Stratum corneum of skin | | **Life cycle** | 10–14 days from egg to adult | | **Transmission** | Prolonged skin-to-skin contact; fomites (rare) | **Clinical Pearl:** The 'delta wing' sign (triangular anterior end) seen on dermoscopy is pathognomonic for Sarcoptes scabiei and aids rapid diagnosis without formal microscopy. **Mnemonic:** **ITCH** = *Interdigital spaces, Trunk flexures, Creases (wrists, elbows, axillae), Hands and genitalia* — classic distribution sites where burrows are found. ### Why Only var. hominis Causes Human Scabies Although Sarcoptes scabiei infests multiple animal species (var. canis in dogs, var. bovis in cattle), only the human variety (var. hominis) can sustain long-term infection in humans. Animal varieties may cause brief, self-limited dermatitis in humans (zoonotic scabies) but do not establish persistent infection. [cite:Irvine Textbook of Dermatology Ch 22]
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