Scabies

Scabies is an ectoparasitic skin infestation caused by the mite Sarcoptes scabiei var. hominis, an obligate human parasite that lives in burrowed tunnels in the stratum corneum (see figure Layers of the Skin). Scabies is easily transmitted from person to person through physical contact; animal and fomite transmission probably also occurs. The primary risk factor is interpersonal contact in crowded conditions (as in schools, shelters, barracks, and some households), and caregiving when provided in close contact. Shared bedding and sexual contact may contribute to transmission (1); there is no clear association with poor hygiene.

Crusted scabies, also called Norwegian scabies, usually causes a more severe but often less pruritic rash than common scabies (1). For unknown reasons, crusted scabies is more common among patients who are immunosuppressed (eg, those with HIV infection, hematologic cancer, chronic glucocorticoid or other immunosuppressant use), patients with severe physical disabilities or intellectual disability, and patients with Australian Aboriginal ethnicity. Infestations occur worldwide with exceptionally wide regional variation (2), with countries in the Pacific region including islands (eg, Samoa, Fiji) and Latin America having the highest prevalence. Children are substantially more affected than adolescents and adults. Patients in warm climates develop small erythematous papules with few burrows. The severity of clinical presentation is usually related to the patient’s immune status, not geography.

Scabies

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Photo courtesy of Karen McKoy, MD.

Clinical features of scabies first begin to appear approximately 4 to 6 weeks after initial exposure (1). The primary symptom of scabies is intense pruritus, classically worse at night, although that timing is not specific to scabies. Pruritus represents the formation of delayed hypersensitivity to scabies mite antigens (2).

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 affect any area of the body, including the breasts and penis. The face usually remains uninvolved in adults. Burrows, usually on the wrists, hands, or feet, are pathognomonic for disease, manifesting as fine, wavy, and slightly scaly lines several millimeters to 1 cm long. A tiny erythematous and pruritic papule—the mite—may be visible at one end and is often the first classic sign of a scabies infestation. In some cases where the mite has burrowed further into the skin, further inflammation and the formation of a fluid-filled vesicle may be noted. Such vesicular lesions may be subject to erosion or secondary infection, especially if scratched. In classic scabies, people usually have only 10 to 12 mites.

Classic Scabies

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A pruritic, pustular rash in the webbing between fingers is the hallmark of classic scabies.

DR P. MARAZZI/SCIENCE PHOTO LIBRARY

Signs of classic scabies may be atypical. In Black people 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 older adults, scabies can cause intense pruritus but otherwise subtle skin findings, making it a challenge to diagnose. In patients who are immunosuppressed, there may be widespread nonpruritic scaling (particularly on the palms and soles in adults and on the scalp in children).

Infantile Scabies

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In infantile scabies, pustules on the soles and palms are common.

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Other forms

Crusted scabies (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.

Crusted (Norwegian) Scabies

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This photo shows diffuse scaling and hyperkeratotic plaques in a patient with HIV and crusted scabies.

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Nodular scabies occurs in approximately 7 to 10% of cases, is more common among infants and young children, and may be due to hypersensitivity to retained organisms. Nodules are usually erythematous; measure 5 to 20 mm; and involve the groin, genitals, axillary folds, and buttocks (3). Nodules represent foci of delayed hypersensitivity reactions and may persist for months after eradication of mites.

Nodular Scabies

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Nodular scabies are shown in this photo as multiple brownish red papules on the axilla.

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Bullous scabies occurs more commonly among children. When it occurs in older adults, 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 dermatitis or seborrheic dermatitis.

Scabies incognito looks like a widespread eczema and occurs in patients who use topical glucocorticoids.

• Primarily history and physical examination

• Burrow scrapings

The diagnosis of scabies is suspected by physical findings, especially burrows and itching that is out of proportion to physical findings, along with the presence of similar symptoms among household contacts. The 2020 International Alliance for the Control of Scabies criteria define 3 diagnostic certainty levels (1):

• Confirmed (when there is concrete microscopic or dermoscopic evidence)

• Clinical (when there are typical lesions plus history)

• Suspected (where clinical findings are less typical but the history is supportive)

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.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.

Imaging and magnification of the skin using a hand-held instrument (dermoscopy) or confocal microscopy can be performed to help identify scabies. The presence of a delta-wing jet sign (ie, visualizing dense scabies head parts in a triangular shape and the burrow itself may appear as an S-shaped whitish line trailing behind) on dermoscopy is characteristic (2).

Patients with crusted scabies may have extremely high levels of IgE and elevated eosinophils (3).

• Topical permethrin, or other topical scabicidesTopical permethrin, or other topical scabicides

• Sometimes oral ivermectinSometimes oral ivermectin

• Symptomatic management of pruritus (oral antihistamines)

Primary treatment of scabies involves topical or oral scabicides (1) (see table Treatment Options for Scabies). Permethrin is the first-line topical medication (). Permethrin is the first-line topical medication (2).

Topical therapy is primarily with permethrin. Older children and adults should apply permethrin to the entire body from the neck down and wash it off after 8 to 14 hours. Treatments should be repeated in 7 days.is primarily with permethrin. Older children and adults should apply permethrin to the entire body from the neck down and wash it off after 8 to 14 hours. 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. Reapplications on the hands and feet may be necessary (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. Reapplications on the hands and feet may be necessary (3). Mittens on infants can keep permethrin out of the mouth. ). Mittens on infants can keep permethrin out of the mouth.

For adults and children 4 years of age and older, spinosad 0.9% topical suspension should be applied to the entire cutaneous surface from the neck down, including the soles and feet (For adults and children 4 years of age and older, spinosad 0.9% topical suspension should be applied to the entire cutaneous surface from the neck down, including the soles and feet (4). In patients who are bald, the suspension should be applied to the scalp, forehead, hairline, and temples. The suspension should be allowed to dry for 10 minutes before getting dressed and then left on the skin for 6 hours before showering or bathing. Treatment should be repeated in 1 week.

Precipitated sulfur 6 to 10% in petrolatum is an alternative therapy typically reserved for infants < 2 months of age or pregnant individuals. It is applied for 24 hours for 3 consecutive days, and is safe and effective. Lindane is no longer recommended because it can be neurotoxic (Precipitated sulfur 6 to 10% in petrolatum is an alternative therapy typically reserved for infants Lindane is no longer recommended because it can be neurotoxic (5).

Oral therapy is with ivermectin, which 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 in nursing homes. is with ivermectin, which 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 in nursing homes.

Symptomatic management of pruritus is usually with oral antihistamines (eg, hydroxyzine 25 mg orally up to 4 times a day). Secondary infection should be considered in patients with weeping, yellow-crusted lesions (ie, impetigo) and treated with the appropriate systemic or topical antistaphylococcal or antistreptococcal antibiotic. After eradication of mites, pruritus can also be treated symptomatically with topical glucocorticoids.Symptomatic management of pruritus is usually with oral antihistamines (eg, hydroxyzine 25 mg orally up to 4 times a day). Secondary infection should be considered in patients with weeping, yellow-crusted lesions (ie, impetigo) and treated with the appropriate systemic or topical antistaphylococcal or antistreptococcal antibiotic. After eradication of mites, pruritus can also be treated symptomatically with topical glucocorticoids.

Symptoms and lesions take up to 3 weeks 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 performed periodically to check for persistent scabies.

Recommended strategies for the prevention of scabies focus on prompt identification and treatment of cases and their close contacts, environmental decontamination, and, in high-prevalence settings, community-level interventions (1). Avoidance of direct contact with infested individuals and their personal items, especially in institutional or crowded settings is important. Public-health strategies include establishing surveillance programs for early detection, regionally appropriate protocols for treatment and prevention, and procedures for notification, all of which can further streamline preventive efforts (2).

Key prevention measures include empiric treatment of all close contacts (eg, household members, those with direct skin-to-skin contact) even if they are asymptomatic. Asymptomatic carriers can transmit the infestation; clinical symptoms may be delayed or not manifest at all.

Decontamination measures are also recommended; clothing, bedding, and linens should be washed in hot water (≥ 50 to 54° C [≥ 122 to 130° F]) and dried on a high-heat cycle for 10 minutes, or contaminated items should be isolated in a sealed plastic bag for up to 1 week (3). Only items in contact with the patient in the previous 2 to 3 days require cleaning.

Mass medication administration with oral ivermectin or topical permethrin has been shown to be particularly effective in outbreak settings, and substantially reduces scabies transmission in endemic communities (Mass medication administration with oral ivermectin or topical permethrin has been shown to be particularly effective in outbreak settings, and substantially reduces scabies transmission in endemic communities (4, 5).