## Causative Agent of Scabies **Key Point:** Sarcoptes scabiei var. hominis is the obligate human parasite responsible for scabies worldwide. ### Organism Characteristics - **Size:** 0.3–0.5 mm (barely visible to naked eye) - **Morphology:** Oval-shaped, eight-legged mite (acarine) - **Habitat:** Burrows in the stratum corneum of human skin - **Transmission:** Direct skin-to-skin contact; fomites play minimal role - **Lifecycle:** Complete lifecycle (egg → larva → nymph → adult) occurs in 10–14 days within the burrow ### Why Sarcoptes scabiei var. hominis? - Host-specific; cannot survive long off human skin - Females burrow into skin to lay eggs; males remain on surface - Causes intense pruritus due to hypersensitivity reaction to mite antigens and feces (scybala) **High-Yield:** The mite itself is not easily visible clinically; diagnosis relies on clinical signs (burrows, distribution) and dermoscopy or microscopy of burrow scrapings. [cite:Irvine Dermatology 5e Ch 12] 
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