Scabies is a skin infestation caused by an almost microscopic mite, Sarcoptes scabei. Although distribution is worldwide, scabies occurs more frequently in tropical resource poor countries. The life cycle is completed entirely on the human host. The gravid female mite burrows into the top layer of skin to lay her eggs. The larvae hatch, reach the skin surface, mate and complete the cycle. Transmission is by skin-to-skin contact, and is most likely within families, in institutions or schools, or by sexual contact. The initial infestation is asymptomatic for approximately one month. Then an intense inflammatory immune mediated reaction occurs, associated with intense persistent itching, which is the major hallmark of the condition. Re-infection produces a more immediate reaction within 48-72 hours. Other characteristic findings are the presence of a linear burrow (which is often obscured by excoriation) and a distribution pattern of lesions on the webs of fingers, the wrists, the axillae, and on the genitalia in the case of sexual contact. Infants are more likely to have a generalized body rash. If infection occurs in an immunocompromized or elderly person, a particular pattern of generalized rash occurs with thick scaling. Called Norwegian scabies, this variety involves thousands of mites in contrast to the 10-50 found in the usual adult infestation, and is highly contagious. The diagnosis of scabies can be established by microscopic examination of skin scrapings for eggs or mites, but is more commonly made by the characteristic pattern of lesions, the presence in other family members or close contacts, and the presence of intense itching. Institutional epidemics can occur.
A number of treatment options are available. Thorough cleaning and heat drying of bed clothing is indicated, as the mite may survive for as long as 3 days away from the host. A 10% sulfur in petrolatum ointment, which is over the counter, has been used for many years. It is safe to use in small infants. It is messy, requires repeat applications, and is less effective than more recently introduced agents. Benzoyl benzoate is moderately effective but not available in the US. Lindane (Kwell) lotion is no longer widely used because of concerns about neurotoxicity, and may be difficult to obtain. It should not be used in children or pregnant women. A 5% permethrin cream (Elimite) has recently been the topical treatment of choice. The cream is applied to the entire body except the face and washed off after 8 hours. A second treatment is applied after one week. Permethrin is non-toxic, and can be used in children as young as two months of age. It is not recommended for pregnant women. Recently Ivermectin, which has been widely used as a vermifuge, has been found to be highly effective and safe in treating scabies, in a single dose of 200 micrograms per kilogram. It is not currently FDA approved, however, for this use. Therefore it is prudent to continue to use permethrin cream as the first line treatment. Ivermectin can be considered in cases of treatment failure, with institutional epidemics where mass local treatment is impractical, or for massive infection in immunocompromized patients, the so-called Norwegian or crusted scabies.
1. Leme PA. Scabies and Pediculosis pubi, an Update of Treatment Regimens and General Review. Clinical Inf. Disease, 2007:44: s153-59.
Fawcett R. Invermectin Use in Scabies. Am. Fam. Physician, 2003: 68:1089-92.
Roger Boe MD