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.
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Scabies
- Permethrin cream (5%) should be applied thinly from the jawline down after bathing and removed 8-14 h later with soap and water. A dose of ivermectin (200 µg/kg) is also effective but has not yet been approved by the FDA for scabies treatment.
- For crusted scabies, first a keratolytic agent (e.g., 6% salicylic acid) and then scabicides are applied to the scalp, face, and ears in addition to the rest of the body. Two doses of ivermectin, separated by an interval of 1-2 weeks, may be required for pts with crusted scabies.
- Itching and hypersensitivity may persist for weeks or months in scabies and should be managed with symptom-based treatment. Bedding and clothing should be washed in hot water and dried in a heated dryer, and close contacts (regardless of symptoms) should be treated to prevent reinfestations.
- Scabies infestations become noncommunicable within 1 day of effective treatment.
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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 or years 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.
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Pediculiasis
- If live lice are found, treatment with 1% permethrin (two 10-min applications 10 days apart) is usually adequate. If this course fails, treatment for ≤12 h with 0.5% malathion may be indicated. Eyelid infestations should be treated with petrolatum applied for 3-4 days.
- Body lice usually are eliminated by bathing and by changing to laundered clothes.
- Pediculicides applied from head to foot may be needed in hirsute pts to remove body lice.
- Clothes and bedding should be deloused by placement in a hot dryer for 30 min or by heat pressing.
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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.