Prototheca: An algae that “LICHENS” to a deep cutaneous mycosis

A 70-year-old man, described as unkempt, presented with a 4-week history of erythematous plaques with occasional drainage on the dorsal hand. The patient described a flesh colored nodule as the initial lesion. Past medical history was significant for chronic obstructive pulmonary disease requiring prolonged steroid therapy. Physical examination revealed violaceous boggy plaques on the dorsal left hand. Crusted and verrucous nodules were present on the surface of and adjacent to the plaques. A dermal mycosis was suspected. Skin biopsy demonstrated pseudoepitheliomatous hyperplasia, pandermal suppurative, necrotizing granulomatous inflammation and numerous round/ovoid, refractile basophilic bodies with septate forms and variable size (3 mm to large morula-like aggregates). The organisms were PAS positive, consistent with Prototheca species. Tissue cultures were negative. Cutaneous protothecosis is an uncommon indolent condition occurring primarily in immunocompromised individuals. The infective organism, Prototheca, is an achlorophyllic algae isolated in a variety of reservoirs including soil, water, animals and food items. The postulated mode of cutaneous infection is through traumatic inoculation. Prototheca are spherical unicellular nonbudding organisms ranging from 3 to 30 mm, consisting of thick-walled sporangia filled with multiple endospores. The organisms are GMS and PAS positive and can be cultured on Sabouraud dextrose agar. Treatment is with anti-fungal agents and/or surgical excision of the lesions. This case illustrates the similarities in clinical and pathologic findings between cutaneous protothecosis and deep mycosis. In our case, tissue cultures were negative and, therefore, the microscopic findings on tissue biopsy were essential to the final diagnosis. Thus, protothecosis must be considered when working up a lesion that is suspicious for a more common fungal infection.