Crusted Scabies Presenting as Erythroderma in a Patient With Iatrogenic Immunosuppression for Treatment of Granulomatosis With Polyangiitis
The diagnosis of scabies can be difficult when the infection presents as erythroderma. Crusted scabies is a severe form of scabies caused by cutaneous ectoparasitic infection by the mite Sarcoptes scabiei var hominis. Crusted scabies most commonly occurs in patients with underlying immunosuppression from acquired infection or subsequent to solid organ or bone marrow transplantation. We present a rare case of a patient with granulomatosis with polyangiitis (GPA) who developed azathioprine-induced myelosuppression and subsequent erythrodermic crusted scabies. It is critical to maintain a broad differential when patients present with erythroderma, especially in the setting of medication-induced immunosuppression for the treatment of autoimmune disease.
Practice Points
- Crusted scabies is a highly contagious, severe cutaneous ectoparasitic infection that can present atypically in the form of erythroderma.
- Immunomodulatory drugs for the treatment of autoimmune disease can predispose patients to infection, including ectoparasitic infection.
- Dermatologists should be familiar with the full scope of the clinical presentations of scabies and should especially consider this condition in the differential diagnosis of patients who present in an immunosuppressed state.
The patient was treated with permethrin cream 5% and oral ivermectin 200 μg/kg on day 1 and day 10. The prednisone dosage was increased from 10 mg/d to 50 mg/d and tapered over 2 weeks to treat the symptomatic rash and GPA. He remains on maintenance rituximab for GPA, without recurrence of scabies.
Comment
Pathogenesis—As an obligate parasite, S scabiei spends its entire life cycle within the host. Impregnated female mites burrow into the epidermis after mating and lay eggs daily for 1 to 2 months. Eggs hatch 2 or 3 days later. Larvae then migrate to the skin surface; burrow into the stratum corneum, where they mature into adults; and then mate on the skin surface.1,4
Clinical Presentation and Sequelae—Typically, scabies presents 2 to 6 weeks after initial exposure with generalized and intense itching and inflammatory pruritic papules on the finger webs, wrists, elbows, axillae, buttocks, umbilicus, genitalia, and areolae.1 Burrows are specific for scabies but may not always be present. Often, there are nonspecific secondary lesions, including excoriations, dermatitis, and impetiginization.
Complications of scabies can be severe, with initial colonization and infection of the skin resulting in impetigo and cellulitis. Systematic sequelae from local skin infection include post-streptococcal glomerulonephritis, rheumatic fever, and sepsis. Mortality from sepsis in scabies can be high.3,5
Classic Crusted Scabies and Other Variants—Crusted scabies presents with psoriasiform hyperkeratotic plaques involving the hands and feet with potential nail involvement that can become more generalized.1 Alterations in CD4+ T-cell function have been implicated in the development of crusted scabies, in which an excessive helper T cell (TH2) response is elicited against the ectoparasite, which may help explain the intense pruritus of scabies.6 Occasionally, iatrogenic immunosuppression contributes to development of crusted scabies,1 as was the case with our patient. However, it is rare for crusted scabies to present with erythroderma.7
Other atypical presentations of scabies include a seborrheic dermatitis–like presentation in infants, nodular lesions in the groin and axillae in more chronic scabies, and vesicles or bullous lesions.1
Diagnosis—Identification of mites, eggs, or feces is necessary for definitive diagnosis of scabies.8 These materials can be obtained through skin scrapings with mineral oil and observed under light microscopy or direct dermoscopy. Multiple scrapings on many lesions should be performed because failure to identify mites can be common and does not rule out scabies. Dermoscopic examination of active lesions under low power also can be helpful, given that identification of dark brown triangular structures can correspond to visualization of the pigmented anterior section of the mite.9-11 A skin biopsy can help identify mites, but histopathology often shows a nonspecific hypersensitivity reaction.12 Therefore, empiric treatment often is necessary.
