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 appropriate treatment failures. In about half of these patients new lesions continue to appear, and one must assume that either therapy has been inadvertently 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 which are genetically atopic, have developed an itchscratch cycle and will continue to scratch indefinitely except 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 consequent 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 epidemiological 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. But it is quite effective and reliably 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.
Both lindane and permethrin are used in a similar manner. Patients and all of their close contacts are instructed to bathe and then apply a thin layer of the medication over the entire body. This is left in place for 8 to 12 hours, at which time the patient bathes a second time. Clothing or bed linen used prior to or during therapy is washed with soap and water in a normal fashion. No special attention is needed for furniture and other inanimate objects. A single treatment carried out in this manner will result in clearing of 80% of patients. As mentioned above, an additional 10% will require retreatment because of problems with the medical compliance or reinfection. The remaining 10% will require the additional use of steroids and antihistamines in order to break up the itch-scratch cycle.