Scabies is an infestation of the skin with the mite Sarcoptes scabiei
Etiology of Scabies
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 patients who are immunosuppressed (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 (1). Patients in warm climates develop small erythematous papules with few burrows. Severity is related to the patient’s immune status, not geography.
Photo courtesy of Karen McKoy, MD.
Etiology reference
1. Romani L, Steer AC, Whitfeld MJ, et al: Prevalence of scabies and impetigo worldwide: a systematic review. Lancet Infect Dis15(8):960-967, 2015. doi: 10.1016/S1473-3099(15)00132-2
Symptoms and Signs of Scabies
The primary symptom of scabies 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 affect any area of the body, including the breasts and penis. The face 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 dark papule—the mite—is often visible at one end. In classic scabies, people usually have only 10 to 12 mites. Secondary bacterial infection commonly occurs.
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 patients, scabies can cause intense pruritus with 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).
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.
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 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 corticosteroids.
Diagnosis of Scabies
Clinical evaluation
Burrow scrapings
Diagnosis of scabies is suspected by physical findings, especially burrows, and itching that is out of proportion to physical findings and similar symptoms among household contacts.
Imaging and magnification of the skin using a hand-held instrument (dermoscopy) can be done to help identify scabies.
Treatment of Scabies
Primary treatment is topical or oral scabicides (1) (see table Treatment Options for Scabies
2). Treatments should be repeated in 7 days.
3).
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, is safe and effective.
Close contacts should also be treated simultaneously, and personal items (eg, towels, clothing, bedding) should be washed in hot water and dried in a hot dryer or isolated (eg, in a closed plastic bag) for at least 3 days.
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 done periodically to check for persistent scabies.
Treatment references
1. Al-Dabbagh J, Younis R, Ismail N: The current available diagnostic tools and treatments of scabies and scabies variants: An updated narrative review. Medicine 102(21): p e33805. doi: 10.1097/MD.0000000000033805
2. Nolan K, Kamrath J, Levitt JPediatr Dermatol 29(2):141-146, 2012. doi: 10.1111/j.1525-1470.2011.01519.x
3. Seiler JC, Keech RC, Aker JL, et alAm Acad Dermatol Aug 12:S0190-9622(21)02290-8, 2021. doi: 10.1016/j.jaad.2021.07.074
Key Points
Risk factors for scabies include crowded living conditions and immunosuppression; poor hygiene is not a risk factor.
Suggestive findings include burrows in characteristic locations, intense itching (particularly at night), and clustering of cases among household contacts.
Confirm scabies when possible by finding mites, ova, or fecal pellets.
