Mites can live their entire life cycles in the skin of humans, causing chronic infection. The female mite burrows into the skin to lay her eggs, from which larvae emerge to copulate and then reburrow under the skin.
Transmission of scabies occurs through skin or sexual contact. The adult mite can survive without a human host for only 2 to 3 days.
Signs and symptoms
Typically, scabies causes itching that intensifies at night. Characteristic lesions take many forms but are usually excoriated and may appear as erythematous nodules.
Burrows are threadlike lesions about ¾? (2 cm) long and generally occur between fingers, on flexor surfaces of the wrists, on elbows, in axillary folds, at the waistline, on nipples in females, and on genitalia in males. In infants, the burrows may appear on the head and neck.
Intense scratching can lead to severe excoriation and secondary bacterial infection. Itching may become generalized secondary to sensitization.
Superficial scraping and examination, under a low-power microscope, of material that has been expressed from a burrow may reveal the mite, ova, or mite feces. However, excoriation or inflammation of the burrow can make such identification difficult.
If diagnostic tests offer no positive identification of the mite and if scabies is still suspected (for example, close contacts of the patient also report itching), skin clearing that occurs after a therapeutic trial of a pediculicide confirms the diagnosis.
Treatment of scabies generally involves applying a pediculicide—permethrin cream or lindane lotion—in a thin layer over the entire skin surface. The pediculicide is left on for 8 to 12 hours. To make certain that all areas have been treated, this application should be repeated in about 1 week.
Other treatments include crotamiton cream, y-benzene hexachloride, and benzyl benzoate. Widespread bacterial infections require a systemic antibiotic such as ivermectin.
Persistent pruritus (from mite sensitization or contact dermatitis) may develop from repeated use of a pediculicide rather than from continued infection. An antipruritic emollient or topical steroid can reduce itching; an intralesional steroid may resolve erythematous nodules.
- Instruct the adult patient to apply permethrin cream or lindane lotion at bedtime from the neck down, covering the entire body. The cream or lotion should be washed off in 8 to 12 hours. Contaminated clothing and linens must be washed in hot water or dry-cleaned.
- Tell the patient not to apply lindane lotion if skin is raw or inflamed. Advise him to immediately report any skin irritation or hypersensitivity reaction, to stop using the drug, and to wash it off thoroughly.
- Suggest that family members, sexual partners, and other close contacts of the patient be checked for possible symptoms and be treated, if necessary.
- If a hospitalized patient has scabies, take steps to prevent transmission to other patients: Practice good hand-washing technique or wear gloves when touching the patient, observe wound and skin precautions for 24 hours after treatment with a pediculicide, gas autoclave blood pressure cuffs before using them on other patients, isolate linens until the patient is noninfectious, and thoroughly disinfect the patient’s room after discharge.