Scabies is an infestation of the skin by the human itch mite (Sarcoptes scabiei var. hominis). Scabies is found worldwide and affects people of all races and social classes. Scabies can spread rapidly under crowded conditions where close body and skin contact is frequent. Institutions such as nursing homes, extended-care facilities, and prisons are often sites of scabies outbreaks. Child-care facilities also are a common site of scabies infestations. Scabies is highly contagious, and person to person spread occurs via direct contact with the skin. Transfer from clothes and bedding occurs rarely and only if contaminated by infested people immediately beforehand.
Infestation occurs when the pregnant female mite burrows into the skin and lays eggs. After two or three days, the larvae emerge and dig new burrows. They mature, mate, and repeat this cycle every two weeks. The main symptoms of scabies are probably a result of the host immune reaction to the burrowed mites and their products. Scabies presents within two to six weeks of initial infestation, but re-infestation can provoke symptoms within 48 hours. Pruritus is the hallmark of scabies regardless of age. In adults, scabies is characterized by intractable pruritus, worse at night, and with lesions in the web spaces, fingers, flexor surfaces of the wrists, axillae, abdomen (around the umbilicus), lower portions of the buttocks, and genital areas. Very young children often have widespread eczematous erythema, particularly on the trunk, which is sometimes more symptomatic than the lesions on the typical sites. Crusted scabies, also known as Norwegian scabies, is a rare condition caused by the host response to control the mite, resulting in hyperinfestation with millions of mites, severe inflammation and hyperkeratotic reaction, particularly in immunosuppressed individuals.
Skin lesions of may resemble eczema, impetigo, tinea corporis (ringworm) and psoriasis. Scabies is usually diagnosed on history and examination. A history of itching in several family members over the same period is almost pathognomonic. Definitive diagnosis relies on microscopic identification of mites or eggs from skin scrapings of a burrow. However, treatment should be started if scabies is suspected clinically, even if it cannot be confirmed by microscopy.
A variety of treatments are available to treat scabies, including permethrin, benzyl benzoate, malathion, lindane, crotamiton, and ivermectin. The treatment is determined by factors such as local epidemiology of resistance, drug toxicity, and cost and availability. Repeated treatment 7 to 10 days after the initial treatment is sometimes required for complete eradication. If left untreated, scabies can continue for many months. It is important to remember that recurrence of symptoms after attempted treatment does not exclude the diagnosis of scabies because patients may not have treated themselves correctly or may have been re-infested by an untreated contact. For an asymptomatic contact, there may be a delay of three weeks between infestation and the appearance of symptoms, making whole-family or household treatment necessary to prevent further infection and spread. The itch mite may also be eradicated through laundering the affected clothing. Because lice can be present on inanimate fomites, heating infested clothing and bedding with hot water is necessary to destroy adult stage of itch mite.
References:
Banerji A, Canadian Paediatric Society FNI, Métis Health C. Scabies. Paediatr Child Health. 2015;20(7):395-402.
Centers for Disease Control and Prevention. Scabies: Global Health, Division of Parasitic Diseases [updated 24 October 2018]. Available from: https://www.cdc.gov/parasites/scabies/gen_info/faqs.html.
Johnston G, Sladden M. Scabies: diagnosis and treatment. BMJ. 2005;331(7517):619-22.
Johnstone P, Strong M. Scabies. BMJ Clin Evid. 2014;2014:1707.
