Concurrent Scabies Incognito and Crusted Scabies With Scalp Lesions Masquerading as Erythrodermic Dermatitis

I read with interest the excellent report by Tolkachjhov et al¹ that describes the delay in diagnosis of a scabies infestation in a 90-year-old woman. Her condition mimicked dermatitis not only on clinical presentation but also on evaluation of skin biopsies. In addition to erythroderma, she had hyperkeratotic crusted plaques on her groin, extremities, face, and scalp. The suspicion of an atypical presentation of scabies prompted her astute physicians to perform a mineral oil preparation of skin lesion scrapings that established the correct diagnosis when mites, eggs, and feces were observed; a skin biopsy was repeated, which also confirmed the scabies infestation when numerous mites were observed in the stratum corneum.1The term scabies surrepticius was coined in 2017 to provide a comprehensive designation of the multiple different atypical morphologic presentations of scabies mite infestation.² Scabies masquarding as either bullous pemphigoid,² malar erythema of systemic lupus erythematosus,³ nodules of prurigo nodularis,³ and pityriasis rosea4 have recently been reported as scabies surrepticius. Tolkachjhov et al’s1 nonagenarian patient had several mite-associated features that masqueraded as other dermatoses; these included scabies incognito, crusted hyperkeratotic plaques, and lesions affecting the scalp and face–all three of these manifestations are subtypes of scabies surrepticius^.2Scabies incognito refers to the altered classic presentation and lesion morphology of the mite infestation in individuals who have received topical or systemic corticosteroid treatment of their scabies;⁵ Tolkachjov et al’s¹ patient was receiving not only systemic corticosteroids but also mycophenolate mofetil. Their patient also had crusted hyperkeratotic plaques;¹ the crusted (Norwegian) scabies variant of scabies surrepticius often occurs in immunosuppressed individuals.6 Finally, the scalp is an uncommon site for scabies involvement; however, similar to the reported woman,1 scalp mite infestation of scabies surrepticius is more prevalent in patients with either congenital or acquired deficiency of their immune system.⁷In conclusion, patients who experience the classic presentation of scabies are often characterized by finger web burrows; however, the unifying term scabies surrepticius can be used to categorize individuals who develop atypical subtypes of mite infestations. In contrast to scabies in an immunocompetent person, scabies surrepticius not only masquerades as other dermatoses but also appears on the scalp and face—especially in immunosuppressed patients. A mineral oil preparation of skin scrapings or a skin biopsy or both may be helpful in establishing the correct diagnosis of scabies surrepticius in patients with suspected or established dermatoses that are recalcitrant to therapy.