Case Letter

Crusted Demodicosis in an Immunocompetent Patient

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The 2 species of Demodex that cause disease in humans each behave distinctively: D folliculorum, with a cigar-shaped body, favors superficial hair follicles; D brevis, a smaller form, burrows deeper into skin where it feeds on the pilosebaceous unit.1 Colonization occurs through direct skin-skin contact that begins as early as infancy and becomes more common with age due to development of sebaceous glands, the main source of nourishment for the mites.2

Demodicosis is classified as primary and secondary. In a prospective study of patients with clinical findings of demodicosis, Akilov et al1 discovered that the 2 forms can be differentiated by skin distribution, seasonality, mite species, and preexisting dermatoses. Primary demodicosis is categorized by sudden onset of symptoms on healthy skin, usually the face. Secondary demodicosis develops progressively in patients with preexisting skin disease, such as rosacea, and can have a broader distribution, involving the face and trunk.2 Clinical manifestations of demodicosis are broad and include pruritic papulopustular, nodulocystic, crusted, and abscesslike lesions.5

Most cases of demodicosis reported in the literature are associated with either local or systemic immunosuppression.6-8 In a case report, an otherwise immunocompetent child developed facial demodicosis after local immunosuppression from chronic use of 2 topical steroid agents.9

Demodex infestation can be diagnosed using a variety of methods, including standardized skin surface biopsy, punch biopsy, and potassium hydroxide analysis. Standardized skin surface biopsy is the preferred method to diagnose demodicosis because it is noninvasive and samples the superficial follicle where Demodex mites typically reside. Diagnosis is made by identifying 5 or more Demodex mites in a low-power field or more than 5 mites per square centimeter in standardized skin surface biopsy.2 Other potential diagnostic tools reported in the literature include dermoscopy and confocal laser scanning microscopy.10,11

There is no standard therapeutic regimen for demodicosis because evidence-based trials regarding the efficacy of treatments are lacking. Oral ivermectin 200 µg/kg in a single dose is considered the preferred treatment; it can be combined with oral erythromycin, topical permethrin, or topical metronidazole.5-7,9

Our case is unique, as crusted demodicosis developed in an immunocompetent adult. Demodicosis usually causes severe eruptions in immunocompromised persons, with only 1 case report detailing a papulopustular rash in an immunocompetent adult.12,13

The pathogenesis of demodicosis remains unclear. Many mechanisms have been hypothesized to play a role in its pathogenesis, including mechanical obstruction of hair follicles, hypersensitivity reaction to Demodex mites, immune dysregulation, and a foreign-body granulomatous reaction to the skeleton of the mite.2,3 Our patient’s particular infestation could have been caused by an exuberant reaction to Demodex; however, it is likely that many factors played a role in his disease process to cause an increase in mite density and subsequent manifestations of disease.

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