Scabies is caused by the mite Sarcoptes scabiei, an obligate human parasite. Patients present with a pruritic rash that is often worse in the night. Skin findings include papules, nodules, burrows, and vesiculopustules (Fig. 33-61). The distribution includes the interdigital spaces, wrists, ankles, waist, groin, and axillae. Pruritic nodules around the axillae, umbilicus, or on the penis and scrotum are highly suggestive of scabies. In children the head can also be involved. Look for burrows because these are pathognomonic of scabies and will be the best site to find mites.
For the most part, scabies is a clinical diagnosis based on the typical rash and the history. It is often helpful if other family members have pruritus and a similar rash. In cases in which the diagnosis is in question or there appears to
be multiple recurrences, the scraping is worthwhile to confirm the clinical impression. A dermatoscope or magnifying lens can be used to look for the small arrowhead-appearing mite at the end of a burrow. Scraping of active lesions can yield the identification of mites, eggs, or feces under the microscope (KOH or mineral oil can be used on the slide) (Fig. 33-62).
Crusted (Norwegian) scabies is a highly contagious form with a propensity for older adults and immunocompro-mised or physically debilitated patients (Fig. 33-63). Widespread, crusted lesions appear with thick, hyperkeratotic scales over the elbows, knees, palms, and soles. Infections may occur with thousands of mites at a time and can be especially problematic in nursing homes or assisted-living environments. Ivermectin is an oral treatment for resistant or crusted scabies with demonstrated safety and efficacy. Most studies used a single dose of ivermectin at 200 pg/kg (Strong and Johnstone, 2007).
Permethrin cream is applied from the neck down (include the head when involved) and rinsed off 8 to 14 hours later. Usually this is done overnight. Repeating the treatment in 1 to 2 weeks may increase the cure rate. Unfortunately, scabies resistance to permethrin is increasing. Antihistamines and midpotency steroid creams can be used for symptomatic relief of itching. It is important to note that pruritus may persist for 1 to 2 weeks after successful treatment because the dead mites and eggs still have antigenic qualities that may cause persistent inflammation. Environmental decontamination is a standard component of all therapies. Clothing, bed linens, and towels should be machine-washed in hot water. Clothing or other items (e.g., stuffed animals) that cannot be washed may be dry-cleaned or stored in bags for 1 week. All household or family members living in the infested home should be treated. Failure to treat all involved individuals often results in recurrences within the family.
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