Pruritus disappears quickly after treatment in most individuals. No new lesions develop, and all evidence of infestation ordinarily disappears within 7 to 14 days. Approximately 20% of patients, however, are apparent treatment failures. In about half of these patients new lesions continue to appear, and one must assume that either therapy has been inadequately carried out or that reinfection has occurred. Such patients and all of their contacts should be retreated. In the other half, no new lesions develop, but itching and scratching persist at the site of old lesions. These individuals, most of whom are genetically atopic, have developed an itch­scratch cycle and will continue to scratch indefinitely unless topical steroids and antihistamines are used to break up the cycle.

Scabies, if left untreated, persists for years. During this time there is a gradual resolution of old lesions and subsequent development of new lesions. This prolonged course accounts for the colloquial name of the disease" the 7-year itch."

Pathogenesis

Scabies is due to an infestation with the human variety of the mite Sarcoptes scabiei. The female of the species burrows within the stratum corneum, depositing eggs that over a 3-week period mature into adult mites. The adult mite is just at the threshold of visibility and can sometimes be recognized as a tiny red or brown-red dot at the end of an intact burrow.

Transmission of the disease ordinarily depends on direct person-to-person contact. In a small percentage of cases, however, the contagion occurs through the use of shared clothing or bed linen. Infestation occurs in individuals at all ages and from all socioeconomic groups, but for unknown reasons it rarely develops in blacks. Once present, the disease spreads by scratching; mites and eggs are transferred from one location to another via fingernail contamination.

Early in the course of infestation there is little in the way of host inflammatory reaction, but after several weeks, allergic sensitization with accompanying pruritus and inflammation takes place. The role of immunologic response in the resolution of scabies has not been adequately studied, but the occurrence of epidemics at 20- to 30-year intervals suggests that the development and waning of herd immunity may be important from an epidemiologic standpoint.

Therapy

For many years, lindane (gamma-benzene), sold under the trade names Kwell and Scabene, has been the treatment of choice for scabetic infestations. It is quite effective and relatively cheap, but systemic absorption does occur and is associated with the potential problem of neurotoxicity. Expression of this potential problem only occurred when lindane was used for infants, but for this reason its use has been generally supplanted by 5% permethrin (Elimite). Crotamiton (Eurax) and precipitated sulfur have also been used historically, but neither is as effective as lindane or permethrin.

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