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Ectoparasites are arthropods or helminths that infest the skin or hair of other animals, from which they derive sustenance and shelter. They may penetrate beneath the surface of the host or attach superficially by their mouthparts and specialized claws. These organisms damage their hosts by inflicting direct injury, eliciting a hypersensitivity reaction, inoculating toxins or pathogens, and inciting fear. The main medically important ectoparasites are arachnids (including mites and ticks), insects (including lice, fleas, bedbugs, and flies), pentastomes (tongue worms), and leeches. Arthropods also may harm humans through brief encounters during which they take a blood meal or attempt to defend themselves by biting, stinging, or exuding venoms. Various arachnids (spiders, scorpions), insects (bees, hornets, wasps, ants, flies, bugs, caterpillars, and beetles), millipedes, and centipedes produce ill effects in these manners, as do certain ectoparasites of animals, including ticks, biting mites, and fleas. In the United States, more people die each year from arthropod stings than from the bites of poisonous snakes. Lesions resulting from the bites and stings of arthropods are so diverse and variable that it is rarely possible to identify precisely what kind of insect or tick is involved without a bona fide specimen and entomologic expertise.

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Scabies

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The human itch mite, Sarcoptes scabiei, is a common cause of itching dermatosis, infesting ∼300 million persons worldwide. Gravid female mites that measure ∼0.3 mm in length burrow superficially beneath the stratum corneum, depositing three or fewer eggs per day. Nymphs mature in ∼2 weeks and then emerge as adults to the surface of the skin, where they mate and (re)invade the skin of the same or another host. Transfer of newly fertilized female mites from person to person occurs mainly by intimate contact and is facilitated by crowding, poor hygiene, and multiple sexual partners. Generally, these mites die within a day or so in the absence of host contact. Transmission via sharing of contaminated bedding or clothing therefore occurs infrequently. In the United States, scabies may account for up to 5% of visits to dermatologists. Outbreaks occur in nursing homes, mental institutions, and hospitals.

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The itching and rash associated with scabies derive from a sensitization reaction directed against the excreta that the mite deposits in its burrow. An initial infestation remains asymptomatic for up to 6 weeks, and a reinfestation produces a hypersensitivity reaction without delay. Burrows become surrounded by infiltrates of eosinophils, lymphocytes, and histiocytes, and a generalized hypersensitivity rash later develops in remote sites. Immunity and associated scratching limit most infestations to <15 mites per person. Hyperinfestation with thousands of mites, a condition known as crusted scabies or Norwegian scabies, may result from glucocorticoid use, immunodeficiency, and neurologic and psychiatric illnesses that limit itching and scratching.

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Intense itching worsens at night and after a hot shower. Typical burrows may be difficult to find because they are few in number and may be obscured by excoriations. Burrows appear as dark wavy lines in the epidermis and measure up to 15 mm. Lesions ...

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