ECTOPARASITES is a topic covered in the Harrison's Manual of Medicine.

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Ectoparasites are arthropods or helminths that infest the skin or hair of animals, from which they derive sustenance and shelter. These organisms can inflict direct injury, elicit hypersensitivity, or inoculate toxins or pathogens.


Etiology and Epidemiology Scabies is caused by the human itch mite Sarcoptes scabiei var. hominis, which infests ~300 million people worldwide.

  • Gravid female mites burrow within the stratum corneum, deposit eggs that mature in ~8 days, and emerge as adults to reinvade the same or another host.
  • Scabies transmission is facilitated by direct skin-to-skin contact with an infested person and by crowding, uncleanliness, or contact with multiple sexual partners.

Clinical Manifestations Itching, which is due to a sensitization reaction against excreta of the mite, is worst at night and after a hot shower. Burrows appear as dark wavy lines (3–15 mm in length), with most lesions located along the digital web spaces or on the volar wrists, elbows, scrotum, and penis. Crusted scabies (formerly termed Norwegian scabies)—hyperinfestation with thousands of mites—is associated with glucocorticoid use and immunodeficiency diseases.

Diagnosis Scrapings from unroofed burrows reveal the mite, its eggs, or fecal pellets.

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