Crusted Scabies and Multiple Dosages of Ivermectin

A 61-year-old man was referred from the oncology department for a generalized, highly pruritic eruption of 8 weeks’ duration. Three months earlier, he had successfully received an allogeneic bone marrow transplant to treat acute myelocytic leukemia; he was currently on immunesuppressant medications. The eruption began on his chest and gradually spread over his torso and both extremities. Topical triamcinolone and permethrin provided only minimal relief. Of note, his wife also complained of a pruritic rash over this same time frame. On physical examination, several erythematous hyperkeratotic plaques were noted on his upper trunk and upper thighs (Figure 1). Several erythematous papules and excoriations were present on his wife’s abdomen and upper and lower extremities. Microscopic evaluation of skin scrapings from the patient using a mineral oil preparation revealed Sarcoptes scabiei mites. After the patient failed therapy with appropriately applied topical permethrin, he was prescribed oral ivermectin 15 mg (0.2 mg/kg/dose) every week for 2 weeks. His wife was instructed to see her primary care provider for treatment. The couple was also counseled about how to treat their environment.

Two weeks later, the patient showed neither clinical nor symptomatic improvement. An additional oral dose of ivermectin was prescribed. Despite this third dose, he had no clinical response. The Centers for Disease Control and Prevention (CDC) were consulted for alternative treatment options. Per their recommendation, the patient was treated with a 7-day course of daily oral ivermectin at 0.2 mg/kg/dose. After only the third dose of this regimen, the patient was remarkably improved; the skin eruption was resolved by the seventh dose.

Crusted scabies is rare, usually associated with underlying immunodeficiency, and historically, has a 5-year mortality up to 50%. Risk factors causing immunosuppression in patients with crusted scabies include heavy ethanol use, past leprosy, heavy kava use, and diabetes, among others.¹ Very few cases of crusted scabies in patients who have undergone bone marrow transplantation have been previously described in the literature, so we believe this is an uncommon finding.

Ivermectin is a semisynthetic macrocyclic lactone oral antibiotic. By disrupting the function of a class of ligand-gated chloride ion channels, it causes persistent opening of the channels and death of the mite.² The serum half-life of ivermectin is 18 hours; elimination occurs through hepatic metabolism and renal excretion of inactive metabolites.³

Although crusted scabies is typically recalcitrant to topical therapy, it generally responds to the 2-dose oral ivermectin regimen initially prescribed to our patient.⁴ Previous publications^1,5 and the CDC recommend various regimens for the treatment of severe crusted scabies, including topical permethrin 5% applied every 2 to 3 days for 12 weeks with oral ivermectin (200 μg/kg/dose) taken with food, administered as either a 3-dose regimen (days 1, 2, and 8), a 5-dose regimen (days 1, 2, 8, 9, and 15), or