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.

Scabies

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.

Pediculiasis

Etiology and Epidemiology Nymph and adult forms of human lice—Pediculus capitis (the head louse), P. humanus (the body louse), and Pthirus pubis (the pubic louse)—feed at least once a day and ingest human blood exclusively. The saliva of these lice produces an irritating rash in sensitized persons. Eggs are cemented firmly to hair or clothing, and empty eggs (nits) remain affixed for months after hatching. Lice are generally transmitted from person to person. Head lice are transmitted among schoolchildren and body lice among disaster victims and indigent people; pubic lice are usually transmitted sexually. The body louse is a vector for the transmission of diseases such as louse-borne typhus, relapsing fever, and trench fever.

Diagnosis The diagnosis can be suspected if nits are detected, but confirmatory measures should include the demonstration of a live louse.

Myiasis

In this infestation, maggots invade living or necrotic tissue or body cavities and produce clinical syndromes that vary with the species of fly. Certain flies are attracted to blood and pus, and newly hatched larvae enter wounds or diseased skin. Treatment consists of maggot removal and tissue debridement.

Leech Infestations

Medicinal leeches can reduce venous congestion in surgical flaps or replanted body parts. Pts occasionally develop sepsis from Aeromonas hydrophila, which colonizes the gullets of commercially available leeches.