A 35-year-old male presents with severe nocturnal pruritus and burrows on his hands and wrists. Scabies is suspected. What is the most common causative organism of human scabies?

Explanation

## Causative Organism of Human Scabies **Key Point:** *Sarcoptes scabiei var. hominis* is the obligate parasite responsible for virtually all cases of human scabies worldwide. ### Characteristics of *Sarcoptes scabiei var. hominis* | Feature | Details | |---------|----------| | **Classification** | Acarine mite; family Sarcoptidae | | **Size** | 0.3–0.5 mm (barely visible to naked eye) | | **Host specificity** | Highly host-specific; human variety only infests humans | | **Life cycle** | 10–14 days from egg to adult | | **Habitat** | Stratum corneum of epidermis | | **Transmission** | Prolonged skin-to-skin contact (>15 minutes) | | **Prevalence** | Affects 300 million people annually worldwide | **High-Yield:** *Sarcoptes scabiei var. hominis* is the **only** scabies-causing mite in humans. Animal varieties (var. canis, var. suis) cause transient dermatitis in humans but do not establish persistent infection. ### Life Cycle of *Sarcoptes scabiei var. hominis* 1. **Gravid female** burrows into stratum corneum 2. **Lays 2–3 eggs daily** within the burrow 3. **Eggs hatch in 3–4 days** → larvae emerge 4. **Larvae migrate to hair follicles** and molt 5. **Nymphs develop** → second molt → adults 6. **Mating occurs** in follicles or skin surface 7. **Cycle repeats** in 10–14 days **Clinical Pearl:** Only 10–15 mites are present on an infested person at any time, yet they cause intense pruritus due to hypersensitivity reaction to mite antigens and feces (scybala). **Mnemonic:** **SHHS** — *Sarcoptes*, Hominis, Human, Stratum corneum (where it lives). ### Why Other Organisms Are Wrong | Organism | Why Not Correct | |----------|----------------| | *Demodex folliculorum* | Causes rosacea and folliculitis, not scabies; lives in hair follicles | | *Pthirus pubis* | Causes pubic lice (pediculosis pubis); different morphology and transmission | | *Sarcoptes scabiei var. canis* | Causes sarcoptic mange in dogs; causes only transient dermatitis in humans, no persistent infection | [cite:Irvine 5e Ch 14]