Infectious Disease and Microbiology – Scabies

Scabies is a contagious skin infestation caused by the human itch mite, Sarcoptes scabiei var. hominis. The condition is characterized primarily by intense itching and a characteristic skin eruption caused by infestation and the host immune response to the mite.

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Scabies occurs worldwide and affects people of all ages, sexes, ethnicities, and socioeconomic groups. Historically, hundreds of millions of cases have been estimated globally each year. The disease is particularly common in areas with overcrowding, limited access to healthcare, and resource-constrained living conditions.

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Individuals living in institutions, crowded households, shelters, or other settings involving prolonged close contact have an increased risk of transmission. Immunocompromised individuals and frail or institutionalized older adults are particularly susceptible to severe forms of infestation.

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Crusted scabies, previously called Norwegian scabies, is a severe form characterized by an extremely high mite burden. It is particularly associated with impaired cellular immunity and other conditions that reduce the ability to scratch or mount an effective immune response. Reported risk factors include immunosuppression, corticosteroid therapy, HTLV-1 infection, malnutrition, Down syndrome, and certain neurologic or cognitive conditions.

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Prevention requires prompt identification and treatment of affected individuals and their close contacts. Household members and other individuals with prolonged skin-to-skin contact should generally be managed simultaneously to prevent reinfestation.

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Clothing, towels, and bedding recently used by an affected person should be appropriately cleaned, usually by hot washing and hot drying. Items that cannot be washed can be isolated according to appropriate infection-control recommendations. Avoiding prolonged direct skin contact with an untreated infected person also reduces transmission.

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Scabies is primarily transmitted through prolonged direct skin-to-skin contact. Transmission through contaminated clothing, bedding, or other fomites is less common in ordinary scabies but becomes considerably more important in crusted scabies because of the very large number of mites present.

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After reaching the skin, fertilized female mites burrow into the superficial epidermis and deposit eggs. The eggs hatch into larvae, which subsequently develop into nymphs and adult mites.

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During a person’s first infestation, symptoms commonly take several weeks to develop because manifestations depend largely on sensitization to the mites and their products. In previously infected individuals, symptoms may develop within only a few days after reinfestation.

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The characteristic symptom is intense generalized pruritus, which is typically worse at night. Other household members or close contacts experiencing similar nocturnal itching can provide an important diagnostic clue.

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Typical examination findings include erythematous papules, excoriations from scratching, and thin linear or serpiginous burrows. Common locations include the interdigital spaces of the fingers, flexor surfaces of the wrists, elbows, axillae, periumbilical region, waistline, buttocks, genital region, breasts, ankles, and feet.

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In healthy adults, the head, face, and neck are usually spared. However, infants, young children, older adults, and immunocompromised patients may develop involvement of the scalp, face, neck, palms, or soles.

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Nodular scabies presents with persistent, intensely pruritic reddish-brown or violaceous nodules, particularly around the genital region, groin, and axillae. These lesions may persist even after the mites have been successfully eradicated because of continued hypersensitivity.

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Crusted scabies has a different appearance from classic scabies. Patients develop extensive hyperkeratotic, crusted, or psoriasiform plaques that may involve the hands, feet, nails, scalp, face, or other large areas of the body. Because these patients can harbor enormous numbers of mites, crusted scabies is extremely contagious.

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Diagnosis is often made clinically from the combination of characteristic itching, distribution of lesions, burrows, and an appropriate exposure history. Confirmation can be obtained by demonstrating mites, eggs, or fecal material from a skin scraping examined microscopically.

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Dermoscopy can assist diagnosis by allowing direct visualization of characteristic structures associated with the mite and its burrow. Videodermoscopy may provide greater magnification. Skin biopsy is rarely required but can be useful when the presentation is atypical and the diagnosis remains uncertain.

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The differential diagnosis includes eczema, contact or atopic dermatitis, drug eruptions, pediculosis corporis, dermatitis herpetiformis, lichen planus, pityriasis rosea, papular urticaria, and other pruritic dermatoses.

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Permethrin 5% cream is a standard first-line treatment for classic scabies. It is generally applied thoroughly over the recommended body surface, commonly from the neck downward in adults, and left in place for approximately 8–14 hours before being washed off. Treatment is commonly repeated after about one week because the initial application may not eliminate all newly developing mites.

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Application must include easily overlooked areas such as between the fingers and toes, underneath the fingernails, around the umbilicus, buttocks, groin, and external genital region. In infants, older adults, and some immunocompromised patients, treatment may also need to include the scalp and other areas above the neck while avoiding the eyes and mouth.

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Oral ivermectin is another important treatment option and is especially useful for outbreaks, treatment failure, difficulty applying topical therapy, or crusted scabies. A commonly used regimen is 200 micrograms/kg orally, with a second dose approximately 7–14 days later. Multiple doses combined with topical therapy are generally required for crusted scabies.

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Other topical treatments include sulfur preparations and, depending on local availability, benzyl benzoate or crotamiton. Sulfur preparations have historically been useful when treatment options are limited, including in certain very young infants or during pregnancy. Treatment selection should take age, pregnancy, comorbidities, availability, and local recommendations into account.

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Lindane has historically been used as an alternative scabicide but is now generally avoided or restricted because systemic absorption can cause serious neurotoxicity, including seizures. Safer alternatives are preferred whenever available.

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Close contacts should generally be treated at the same time as the affected patient, even when they do not yet have symptoms, because the incubation period can be several weeks. Failure to treat contacts is an important cause of apparent treatment failure and recurrent infestation.

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Recently used clothing, towels, and bedding should be cleaned at the time treatment begins. Environmental cleaning is particularly important in crusted scabies. Routine excessive cleaning or pesticide treatment of the entire home is generally unnecessary for ordinary scabies.

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Antihistamines and other symptomatic treatments may help control itching. Importantly, pruritus can continue for several weeks after successful eradication because the inflammatory and hypersensitivity response does not disappear immediately.

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Persistent itching shortly after treatment therefore does not necessarily indicate treatment failure. Reassessment is appropriate when symptoms continue, worsen, new burrows or characteristic lesions appear, treatment was applied incorrectly, or untreated contacts could have caused reinfestation.

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Patients with crusted scabies require particularly careful management because of their high mite burden, infectiousness, and association with underlying immunosuppression or debilitating disease. Dermatology or infectious-disease consultation may be appropriate, and infection-control precautions are especially important in institutional outbreaks.

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The prognosis of uncomplicated scabies is excellent with appropriate therapy. Treatment failures are most often related to incorrect application, inadequate treatment of contacts, reinfestation, or failure to recognize crusted scabies rather than true resistance.

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The most important complication is secondary bacterial infection of excoriated skin, particularly with Staphylococcus aureus or Streptococcus pyogenes. Scabies can also exacerbate preexisting eczema and, in heavily affected populations, secondary streptococcal skin infection can contribute to more serious postinfectious complications.