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.
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.
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.
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 occur most frequently on the volar wrists, between the fingers, on the elbows, and on the penis. Small papules and vesicles, often accompanied by eczematous plaques, pustules, or nodules, are distributed symmetrically in those sites and in skinfolds under the breasts and around the navel, axillae, belt line, buttocks, upper thighs, and scrotum. Except in infants, the face, scalp, neck, palms, and soles are spared. Crusted scabies resembles psoriasis in its typical widespread erythema, thick keratotic crusts, scaling, and dystrophic nails. Characteristic burrows are not seen in crusted scabies, and patients usually do not itch, although their infestations are highly contagious and have been responsible for outbreaks of classic scabies in hospitals.
Scabies should be considered in patients with pruritus and symmetric polymorphic skin lesions in characteristic locations, particularly if there is a history of household contact with an affected person. Burrows should be sought and unroofed with a sterile needle or scalpel blade, and the scrapings should be examined microscopically for the mite, its eggs, and its fecal pellets. Biopsies (including superficial cyanoacrylate biopsy), scrapings, and dermascopic imaging of papulovesicular lesions as well as microscopic inspection of clear adhesive tape lifted from lesions also may be diagnostic. In the absence of identifiable mites or mite products, the diagnosis is based on clinical presentation and history. Diverse kinds of dermatitis due to other causes frequently are misdiagnosed as scabies.
Permethrin cream (5%) is less toxic than 1% lindane preparations and is effective against lindane-tolerant infestations. Scabicides are applied thinly but thoroughly behind the ears and from the neck down after bathing and are removed 8 h later with soap and water. Successful treatment of crusted scabies requires preapplication of a keratolytic agent such as 6% salicylic acid and then of scabicides to the scalp, face, and ears. Repeated treatments or the sequential use of several agents may be necessary. Ivermectin has not been approved by the U.S. Food and Drug Administration (FDA) for use against any form of scabies, but a single oral dose (200 μg/kg) effectively treats scabies in otherwise healthy persons; patients with crusted scabies may require two doses separated by an interval of 1–2 weeks.
Although effectively treated scabies infestations become noninfectious within a day, itching and rash due to hypersensitivity to the dead mites and their excreted and secreted products frequently persist for weeks or months. Unnecessary re-treatment with topical agents may provoke contact dermatitis. Antihistamines, salicylates, and calamine lotion relieve itching during treatment, and topical glucocorticoids are useful for pruritus that lingers after effective treatment. To prevent reinfestations, bedding and clothing should be washed and/or dried on high heat or heat-pressed, and close contacts, even if asymptomatic, should be treated simultaneously.
Chiggers and Other Biting Mites
Chiggers are the larvae of trombiculid (harvest) mites that normally feed on mice in grassy or brush-covered sites in the tropics and subtropics and less frequently in ...