Differential Diagnosis—The differential diagnosis of erythroderma is broad and includes a drug eruption; Sézary syndrome; and pre-existing skin diseases, including psoriasis, atopic dermatitis, pityriasis rubra pilaris, pemphigus foliaceus, and bullous pemphigoid. Histopathology is critical to differentiate these diagnoses. Bullous pemphigoid and pemphigus foliaceus are immunobullous diseases that typically are positive for immunoglobulin deposition on DIF. In rare cases, scabies also can present with bullae and positive DIF test results.13
Treatment—First-line treatment of crusted scabies in the United States is permethrin cream 5%, followed by oral ivermectin 200 μg/kg.4,5,14,15 Other scabicides include topicals such as benzyl benzoate 10% to 25%; precipitated sulfur 2% to 10%; crotamiton 10%; malathion 0.5%; and lindane 1%.5 The association of neurotoxicity with lindane has considerably reduced the drug’s use.1
During treatment of scabies, it is important to isolate patients to mitigate the possibility of spread.4 Pruritus can persist for a few weeks after completion of therapy.5 Patients should be closely monitored to ensure that this symptom is secondary to skin inflammation and not incomplete treatment.
Treatment of crusted scabies may require repeated treatments to decrease the notable mite burden as well as the associated crusting and scale. Adding a keratolytic such as 5% to 10% salicylic acid in petrolatum to the treatment regimen may be useful for breaking up thick scale.5
Immunosuppression—With numerous immunomodulatory drugs for treating autoimmunity comes an increased risk for iatrogenic immunosuppression that may contribute to the development of crusted scabies.16 In a number of autoimmune diseases such as rheumatoid arthritis,17-19 psoriasis,20,21 pemphigus vulgaris,22 systemic lupus erythematosus,23 systemic sclerosis,22,24 bullous pemphigoid,25,26 and dermatomyositis,27 patients have developed crusted scabies secondary to treatment-related immunosuppression. These immunosuppressive therapies include systemic steroids,22-24,26-31 methotrexate,23 infliximab,18 adalimumab,21 toclizumab,19 and etanercept.20 In a case of drug-induced Stevens-Johnson syndrome, the patient developed crusted scabies during long-term use of oral steroids.22
Patients with a malignancy who are being treated with chemotherapy also can develop crusted scabies.28 Crusted scabies has even been associated with long-term topical steroid32-34 and topical calcineurin inhibitor use.16
Iatrogenic immunosuppression in our patient resulted from treatment of GPA with azathioprine, an immunosuppressive drug that acts as an antagonist of the breakdown of purines, leading to inhibition of DNA, RNA, and protein synthesis.35 On occasion, azathioprine can induce immunosuppression in the form of myelosuppression and resulting pancytopenia, as was the case with our patient.
Conclusion
Although scabies is designated as a neglected tropical disease by the World Health Organization, it still causes a notable burden worldwide, regardless of the economics. Our case highlights an unusual presentation of scabies as erythroderma in the setting of iatrogenic immunosuppression from azathioprine use. Dermatologists should consider crusted scabies in the differential diagnosis of erythroderma, especially in immunocompromised patients, to avoid delays in diagnosis and treatment. Immunosuppressive therapy is an important mainstay in the treatment of many conditions, but it is important to consider that these medications can place patients at an increased risk for rare opportunistic infections. Therefore, patients receiving such treatment should be closely monitored.