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Scabies is an infestation of the skin with the mite Sarcoptes scabiei. Scabies causes intensely pruritic lesions with erythematous papules and burrows in web spaces, wrists, waistline, and genitals. Diagnosis is based on examination and scrapings. Treatment is with topical scabicides or, sometimes, oral ivermectin.
Etiology
Scabies is caused by the mite Sarcoptes scabiei var. hominis, an obligate human parasite that lives in burrowed tunnels in the stratum corneum. Scabies is easily transmitted from person to person through physical contact; animal and fomite transmission probably also occurs. The primary risk factor is crowded conditions (as in schools, shelters, barracks, and some households); there is no clear association with poor hygiene. For unknown reasons, crusted scabies is more common among immunosuppressed patients (eg, those with HIV infection, hematologic cancer, chronic corticosteroid or other immunosuppressant use), patients with severe physical disabilities or intellectual disability, and Australian Aborigines. Infestations occur worldwide. Patients in warm climates develop small erythematous papules with few burrows. Severity is related to the patient's immune status, not geography.
Symptoms and Signs
The primary symptom is intense pruritus, classically worse at night, although that timing is not specific to scabies.
Classic scabies:
Erythematous papules initially appear in finger web spaces, flexor surfaces of the wrist and elbow, axillary folds, along the belt line, or on the lower buttocks. Papules can spread to any area of the body, including the breasts and penis. The face remains uninvolved in adults. Burrows are pathognomonic for disease, manifesting as fine, wavy, and slightly scaly lines several mm to 1 cm long. A tiny dark papule—the mite—is often visible at one end. In classic scabies, people usually have < 100 mites.
Signs of classic scabies may be atypical. In blacks and other people with dark skin, scabies can manifest as granulomatous nodules. In infants, the palms, soles, face, and scalp may be involved, especially in the posterior auricular folds. In elderly patients, scabies can cause intense pruritus with subtle skin findings, making it a challenge to diagnose. In immunocompromised patients, there may be widespread nonpruritic scaling (particularly on the palms and soles in adults and on the scalp in children).
Other forms:
Crusted (Norwegian) scabies is due to an impaired host immune response, allowing mites to proliferate and number in the millions; scaling erythematous patches often involve the hands, feet, and scalp and can become widespread. Nodular scabies is more common among infants and young children and may be due to hypersensitivity to retained organisms; nodules are usually erythematous, 5 to 6 mm, and involve the groin, genitals, axillary folds, and buttocks. Nodules are hypersensitivity reactions and may persist for months after eradication of mites. Bullous scabies occurs more commonly among children. When it occurs in the elderly, it can mimic bullous pemphigoid, resulting in a delay in diagnosis. Scalp scabies occurs in infants and immunocompromised people and can mimic dermatitis, particularly atopic or seborrheic dermatitis. Scabies incognito is a widespread atypical form resulting from application of topical corticosteroids.
Diagnosis
Diagnosis is suspected by physical findings, especially burrows, and itching that is out of proportion to physical findings and similar symptoms among household contacts. Confirmation is by finding mites, ova, or fecal pellets on microscopic examination of burrow scrapings; failure to find mites is common and does not exclude scabies. Scrapings should be obtained by placing glycerol, mineral oil, or immersion oil over a burrow or papule (to prevent dispersion of mites and material during scraping), which is then unroofed with the edge of a scalpel. The material is then placed on a slide and covered with a coverslip; potassium hydroxide should be avoided because it dissolves fecal pellets.
Treatment
Primary treatment is topical or oral scabicides (see Table 2: Parasitic Skin Infections: Treatment Options for Scabies ). Permethrin is the 1st-line topical drug.
Older children and adults should apply permethrin or lindane to the entire body from the neck down and wash it off after 8 to 14 h. Permethrin is often preferred because lindane can be neurotoxic. Treatments should be repeated in 7 days.
For infants and young children, permethrin should be applied to the head and neck, avoiding periorbital and perioral regions. Special attention should be given to intertriginous areas, fingernails, toenails, and the umbilicus. Mittens on infants can keep permethrin out of the mouth. Lindane is not recommended in children < 2 yr and in patients with a seizure disorder because of potential neurotoxicity.
Precipitated sulfur 6 to 10% in petrolatum, applied for 24 h for 3 consecutive days, is safe and effective and usually used in infants < 2 mo of age.
Ivermectin is indicated for patients who do not respond to topical treatment, are unable to adhere to topical regimens, or are immunocompromised with Norwegian scabies. Ivermectin has been used with success in epidemics involving close contacts, such as nursing homes.
Close contacts should also be treated, and personal items (eg, towels, clothing, bedding) should be washed in hot water and dried in a hot drier or isolated (eg, in a closed plastic bag) for at least 3 days.
Pruritus can be treated with corticosteroid ointments and/or oral antihistamines (eg, hydroxyzine 25 mg po qid). Secondary infection should be considered in patients with weeping, yellow-crusted lesions and treated with the appropriate systemic or topical antistaphylococcal or antistreptococcal antibiotic.
Symptoms and lesions take up to 3 wk to resolve despite killing of the mites, making failed treatment due to resistance, poor penetration, incompletely applied therapy, reinfection, or nodular scabies difficult to recognize. Skin scrapings can be done periodically to check for persistent scabies.
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Table 2
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| Treatment Options for Scabies |
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Therapy
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Instructions
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Comments
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Permethrin* 5% (60 g) cream
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Apply to whole body; wash off after 8–14 h
Repeat in 1 wk
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1st-line treatment
Can cause stinging and itching
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Lindane 1% (60 mL) lotion
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Apply to whole body; wash off after 8–12 h in adults and 6 h in children
Repeat in 1 wk
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Not recommended for children < 2 yr, pregnant or lactating women, people with extensive dermatitis, people with an uncontrolled seizure disorder, and those with severe skin conditions involving skin barrier compromise
Potentially neurotoxic
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Ivermectin
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200 mcg/kg po for 1 dose
Repeat in 7–10 days
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Indicated as a 2nd-line treatment to permethrin
For use in institutional epidemics and immunocompromised patients
Caution required when given to elderly patients with hepatic, renal, or cardiac disorders
Not recommended for pregnant or lactating women; unproven safety in children < 15 kg or < 5 yr
May cause tachycardia
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Crotamiton 10% cream/lotion
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Apply after bath to whole body, apply 2nd dose after 24 h, and bathe 48 h after 2nd dose
Repeat both doses in 7–10 days
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Sulfur ointment 6–10%
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Apply to whole body at bedtime for 3 nights and leave each application on for 24 h
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Very effective and safe
May be limited by its malodor
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*Pyrethrins are natural components of chrysanthemum flowers, with strong insecticidal activity, pyrethroids are synthetic and natural relatives of pyrethrin, and permethrin is a commonly used synthetic pyrethroid. Pyrethrins are combined with a piperic acid derivative (piperonyl butoxide) to enhance efficacy.
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Key Points
Last full review/revision March 2013 by James G. H. Dinulos, MD
Content last modified April 2013
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