Cutaneous protothecosis.

Prototheca species are an achlorophyllic algae that cause infections primarily in immunocompromised individuals. At least one-half of infectious cases are cutaneous. Because protothecosis is seldom suspected clinically, patients may be subjected to various treatment modalities for extended periods without satisfactory results. Cutaneous protothecosis shares similar clinical and pathologic findings with deep tissue fungal mycoses. The typical presentation occurs most commonly on the face and extremities as erythematous plaques, nodules, or superficial ulcers. Prototheca spp are spherical, unicellular, nonbudding organisms that are sometimes noted on routine hematoxylin-eosin staining but are best visualized with periodic acid-Schiff and Gomori methenamine-silver histochemical stains. Although protothecosis can be diagnosed on biopsy, culture of the organism on a medium such as Sabouraud dextrose agar is required for definitive diagnosis. Treatment may require a combination of surgical excision and antifungal agents. Therefore, cutaneous protothecosis should be considered in a lesion that appears suspicious for the more-common fungal infections.