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Scabies is a transmissible ectoparasite skin infection characterized by superficial burrows, intense pruritus (itching) and secondary infection. The word 'scabies' is Latin for 'itch'.
Etiology
Caused by the mite Sarcoptes scabiei, variety hominis, it produces intense, itchy skin rashes when the impregnated female tunnels into the stratum corneum of the skin and deposits eggs in the burrow. The larvae, which hatch in 3-10 days, move about on the skin, molt into a "nympal" stage, and then mature into adult mites. The adult mites live 3-4 weeks in the host's skin.
The motion of the mite in and on the skin produces an intense itch which may resemble an allergic reaction in appearance. The presence of the eggs produces a massive allergic response which, in turn, produces more itching.
Scabies is transmitted readily, often throughout an entire household, by prolonged skin-to-skin contact with an infected person (e.g. bed partners), and thus is sometimes classed as a sexually transmitted disease. Spread by clothing, bedding or towels is a less significant risk, though possible.
Signs, Symptoms, and Diagnosis
A delayed hypersensitivity (allergic) response resulting in a papular eruption (red, elevated area on skin) often occurs 30-40 days after infestation. While there may be hundreds of itching papules, less than 10 burrows are typically found. The burrow appears as a fine, wavy and slightly scaly line a few millimeters to 1cm long. A tiny mite (0.3 - 0.4mm) may sometimes be seen at the end of the burrow. Most burrows occur in the webs of fingers, flexing surfaces of the wrists, around elbows and armpits, the areolae of the breasts in females and on genitals of males, along the belt line, and on the lower buttocks. The face usually does not become involved in adults.
The rash may become secondarily infected; scratching the rash may break the skin and make secondary infection more likely. In persons with severely reduced immunity, such as those with HIV infection, or people being treated with immunosuppressive drugs like steroids, a widespread rash with thick scaling may result. This variety of scabies is called Norwegian scabies.
Diagnosis is made by finding burrows, which often may be difficult because they are scarce, because they are obscured by scratch marks, or by secondary dermatitis (unrelated skin irritation). If burrows are not found in the primary areas known to be affected, the entire skin surface of the body should be examined. When a suspected burrow is found, diagnosis may be confirmed by microscopy of surface scrapings, which are placed on a slide in glycerol, mineral oil or immersion in oil and covered with a coverslip. Avoiding potassium hydroxide is necessary because it may dissolve fecal pellets. Positive diagnosis is made when the mite, ova, or fecal pellets are found.
Treatment
Topical (surface) medications are often effective and must be applied thoroughly to all skin from the face down, especially to areas known to be primarily affected (skin folds, etc.). Medication should remain on for more than 12 hours, and preferably 24, and then washed off.
Although the mites may be rapidly killed, improvement is sometimes slow and residual inflammation may take some time to finally subside. The topical medication of choice is 5% Permethrin because it is safe for all age groups. Lindane (gamma benzene hexachloride) creams or lotions are considered historical treatments, and should be avoided because they have been shown to have neurotoxic effects in children and infants. Similarly, 5–10% sulfur ointments are considered historical.
A single dose of Ivermectin (dosing: 200μg/kg) has been reported to cure, but is an off-label use, and thus considered experimental. Additional topical treatments include 10% crotamiton (except to eyes, nose, mouth), 25% benzyl benzoate cream or lotion; permethrins offer a simpler, one-application treatment which may be applied with in a 5% cream that remains on overnight or for 8-14 hours.
All social contacts and members of the family should be treated similarly. Extensive cleaning or fumigation is unnecessary because the mite does not live long off the host.
Approximately 300 million cases of infestation with scabies occur worldwide annually.
Scabies also occurs in dogs; see Mange.
References
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